My MySQL tips

ProxySQL Firewalling

ProxySQL_firewall_small Not long ago we had an internal discussion about security and how to enforce a stricter set of rules to prevent malicious acts, and block other undesired queries.

ProxySQL comes up as a possible tool that could help us in achieving what we were looking for. Last year I had written how to use ProxySQL to stop a single query.

That approach may be good for few queries and as temporary solution. But what can we do when we really want to use ProxySQL as an SQL-based firewall? And more importantly, how to do it right?

First of all, let us define what “right” can be in this context.

For right I mean an approach that will allow us to have rules matching as specific as possible, and impacting the production system as least as possible.

To make this clearer, let us assume I have 3 schemas:

Shakila

World

Windmills

I want to have my firewall block/allow SQL access independently by each schema, user, eventually by source, and so on.

There are a few case where this is not realistic, like in SaaS setups where each schema represents a customer. In this case, the application will have exactly the same kind of SQL just pointing to different schemas depending the customer.

Using ProxySQL

Anyhow… ProxySQL allows you to manage query firewalling in a very simple and efficient way using the query rules.

In the mysql_query_rules table we can define a lot of important things and one of this is, to set our SQL firewall.

How?

Let us take a look to the mysql_query_rules table:

rule_id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
    active INT CHECK (active IN (0,1)) NOT NULL DEFAULT 0,
    username VARCHAR,
    schemaname VARCHAR,
    flagIN INT NOT NULL DEFAULT 0,
    client_addr VARCHAR,
    proxy_addr VARCHAR,
    proxy_port INT,
    digest VARCHAR,
    match_digest VARCHAR,
    match_pattern VARCHAR,
    negate_match_pattern INT CHECK (negate_match_pattern IN (0,1)) NOT NULL DEFAULT 0,
    re_modifiers VARCHAR DEFAULT 'CASELESS',
    flagOUT INT,
    replace_pattern VARCHAR,
    destination_hostgroup INT DEFAULT NULL,
    cache_ttl INT CHECK(cache_ttl > 0),
    reconnect INT CHECK (reconnect IN (0,1)) DEFAULT NULL,
    timeout INT UNSIGNED,
    retries INT CHECK (retries>=0 AND retries <=1000),
    delay INT UNSIGNED,
    next_query_flagIN INT UNSIGNED,
    mirror_flagOUT INT UNSIGNED,
    mirror_hostgroup INT UNSIGNED,
    error_msg VARCHAR,
    OK_msg VARCHAR,
    sticky_conn INT CHECK (sticky_conn IN (0,1)),
    multiplex INT CHECK (multiplex IN (0,1,2)),
    log INT CHECK (log IN (0,1)),
    apply INT CHECK(apply IN (0,1)) NOT NULL DEFAULT 0,
    comment VARCHAR)

We can define rules around almost everything: connection source, port, destination IP/Port, user, schema, SQL text, and any combination of them.

Given we may have a quite large set of queries to manage, I prefer to logically create “areas” around which add the rules to manage SQL access.

For instance, I may decide to allow a specific set of SELECTs to my schema windmills but nothing more.

Given that, I allocate the set of rule ids from 100 to 1100 to my schema, and add my rules in 3 groups.

The exception that will bypass the firewall
The blocking rule(s) [the firewall]
The managing rules (post processing like sharding and so on)

There is a simple thing to keep in mind when you design rules for firewalling.
Do you need post processing of the query or not?
In the case you DON’T need post processing, the rule can simply apply and exit the QueryProcessor. That is probably the most common scenario and read/write split can be define in the exception rules assigning to the rule the desired HostGroup.

While if you need post-processing the rule MUST have apply=0 and the FLAGOUT must be define. That will allow you to have additional actions once the query is beyond the firewall.

An example is in case of sharding, where you need to process the sharding key/comment or whatever.

I will use the simple Firewall scenario, given this is the topic of the current article.

The rules

Let us start with the easy one, set 2, the blocking rule:

1
2
3

insert into mysql_query_rules (rule_id,username,schemaname,match_digest,error_msg,active,apply) 
values(1000,'pxc_test','windmills','.',
'You cannot pass.....I am a servant of the Secret Fire, wielder of the flame of Anor,. You cannot pass.',1, 1);

In this query rule, I had defined the following:

User connecting
Schema name
Any query
Message to report
Rule_id

That rule will block ANY query that will try to access the schema windmills from application user pxc_test.

Now in set 1, I will add all the rules I want let pass, will report here one only but all can be found in github here.

insert into mysql_query_rules (rule_id,proxy_port,username,destination_hostgroup,
schemaname,active,retries,apply,flagout,match_digest) 
values(101,6033,'pxc_test',52,'windmills',1,3,1,1000,
'SELECT wmillAUTOINC\.id,wmillAUTOINC\.millid,wmillAUTOINC\.location 
FROM wmillAUTOINC WHERE wmillAUTOINC\.millid=.* and wmillAUTOINC\.active=.*');

That is quite simple and straightforward but there is an important element that you must note.
In this rule, apply must have value of =1 always, to allow the query rule to bypass without further delay the firewall.

(Side Note: if you need post-processing, the flagout needs to have a value (like flagout=1000) and apply must be =0. That will allow the query to jump to the set 3, the managing rules.)

This is it, ProxySQL will go to the managing rules as soon as it finds a matching rule that will allow the application to access my database/schema, or it will exit if apply=1.

A graph can help to understand better:

Rule set 3 will have the standard query rules to manage what to do with the incoming connection like sharding or redirecting SELECT FOR UPDATE and so on:

insert into mysql_query_rules (rule_id,proxy_port,schemaname,username,
destination_hostgroup,active,retries,match_digest,apply,flagin) 
values(1040,6033,'windmills','pxc_test',50,1,3,'^SELECT.*FOR UPDATE',1,1000);

Please note the presence of the flagin which matches the flagout above.

Setting rules, sometimes thousands of them can be very confusing. It is very important to plan correctly what should be in as excluding rule and what not. Do not rush, take your time and identify the queries you need to manage carefully.

Once more proxySQL can help us. Querying the table stats_mysql_query_digest will tell us exactly what queries were sent to ProxySQL, ie:

1	(admin@127.0.0.1) [main]>select hostgroup,schemaname,digest,digest_text,count_star from stats_mysql_query_digest where schemaname='windmills' order by count_star desc;

The above query shows us all the queries hitting the windmills schema. From there we can decide which queries we want to pass and which not.

 
>select hostgroup,schemaname,digest,digest_text,count_star 
from stats_mysql_query_digest where schemaname='windmills' order by count_star desc  limit 1\G
 
*************************** 1. row ***************************
hostgroup: 50
schemaname: windmills
digest: 0x18CA8FF2C9C53276
digest_text: SHOW GLOBAL STATUS
count_star: 141

Once we have our set done (check on github for an example), we are ready to check how our firewall works.

By default, I suggest you to keep all the exceptions (in set 1) with active=0, just to test the firewall.

For instance, my application will generate the following exception:

com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException:
 You cannot pass.....I am a servant of the Secret Fire, wielder of the flame of Anor,. You cannot pass.
	at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
	at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
	at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
	at com.mysql.jdbc.Util.getInstance(Util.java:386)
	at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1054)
	at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:4187)
	at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:4119)
	at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2570)
	at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2731)
	at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2809)
	at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2758)
	at com.mysql.jdbc.StatementImpl.executeQuery(StatementImpl.java:1612)
	at net.tc.stresstool.statistics.providers.MySQLStatus.getStatus(MySQLStatus.java:48)
	at net.tc.stresstool.statistics.providers.MySQLSuper.collectStatistics(MySQLSuper.java:92)
	at net.tc.stresstool.statistics.StatCollector.collectStatistics(StatCollector.java:258)
	at net.tc.stresstool.StressTool.<init>(StressTool.java:198)
	at net.tc.stresstool.StressTool.main(StressTool.java:282)

Activating the rules, will instead allow your application to work as usual.

What the impact will be?

First,let us define the baseline, running the application without any rule blocking but only the r/w split (set 3).

Queries/sec global

Using two application servers:

Server A: Total Execution time = 213

Server B: Total Execution time = 209

Queries/sec per server

As we can see queries are almost equally distributed.

QueryProcessor time taken/Query processed total

All queries are processed by QueryProcessor in ~148ms AVG (total)

QueryProcessor efficiency per query.

Single query cost is in nanoseconds avg 10 us.

Use match_digest

Once defined the baseline we can go ahead and activate all the rules using the match_digest.
Run the same tests again and… :

Queries/sec global

Using two application servers:

Server A: Total Execution time = 207

Server B: Total Execution time = 204

First of all, we can notice that the execution time did not increase. This is mainly because we have CPU cycles to use in the ProxySQL box.

Queries/sec per server

Here we have a bit of unbalance. We will investigate that in a separate session, but all in all, time/effort sounds ok.

QueryProcessor time taken/Query processed total

Here we have the first thing to notice. Comparing this to the baseline we had defined, we can see that using the rules as match_digest had significantly increase the execution time to 458ms.

QueryProcessor efficiency per query.

Notice that also if we are in the range of nanoseconds, the cost of processing the query is now 3 times that of the baseline. Not too much but if you add a stone to another stone and another stone and another stone … you end up building a huge wall.

So, what to do? Up to now we had seen that managing the firewall with ProxySQL is easy and it can be set at very detailed level, but the cost may not be what we expect it to be.

What can be done? Use DIGEST instead.

The secret is to not use match_digest, which implies interpretation of the string, but to use the DIGEST of the query, which is calculated ahead and remains constant for that query.

Let us see what will be if we run the same load using DIGEST in the MYSQL_QUERY_RULES table.

Queries/sec global

Using two application servers:

Server A: Total Execution time = 213

Server B: Total Execution time = 209

No, this is not an issue with cut and paste. I had more or less the same execution time as if without rules, at the seconds; different millisecond though.

Queries/sec per server

Again, here some unbalance, but minor thing.

QueryProcessor time taken/Query processed total

And we go down as we should to 61ms as execution of all queries. Note that we improve the efficiency of the Query Processor from 148ms AVG to 61ms AVG.

Why? Because our rules using the DIGEST also have the instructions for read/write split, so request can exit the Query Processor with all the information required at this stage; more efficient.

QueryProcessor efficiency per query.

Finally using the DIGEST, the cost for query drops to 4us which is … LOW!

That’s it! ProxySQL using the DIGEST field from mysql_query_rules, will perform much better, given that it will not need to analyze the whole SQL string with regular expression, but it will just match the DIGEST.

Conclusions

ProxySQL can be effectively used as an SQL firewall, but some best practices should be taken in to consideration.

First of all, try to use specific rules, and be specific on what should be filtered/allowed. Use filter by schema or user or IP/port or combination of them.

Always try to avoid match_digest and use digest instead. That will allow ProxySQL to bypass the call to the regularExp lib and it will be by far more efficient.

Use stats_mysql_query_digest to identify the correct DIGEST.

Regarding this, it would be nice to have an GUI interface that will allow us to manage these rules; that would make the usage of the ProxySQL much easier, and the maintenance/creation of rule_chains friendlier.

ProxySQL PXC Single Writer Mode and auto failover, re-bootstrap

Overview

ProxySQL had been adopted as solution for HA in place of HAProxy in Percona PXC package. writing
The new solution has a lot of advantages and provide an unbelievable flexibility we did not had before. But when talking about HA and PXC there was still a gap.

As already discussed in my previous article “ProxySQL and Percona XtraDB Cluster (Galera) Integration”(https://www.percona.com/blog/2016/09/15/proxysql-percona-cluster-galera-integration/),
ProxySQL is working great when using multi-master approach, but trying to have it in single Node writer, was not possible unless using creative (nice way to say wrong) solutions like what I was covering in “ProxySQL and PXC using Replication HostGroup” in the same article.

In the following months, I had few emails and requests from colleagues and externals reporting me that some time that specific solution was not working or was not covering this or that case. No matter if I had mark the related feature active_failover as experimental, apparently that in conjunction of Replication HG was deployed as solution.

What was wrong with that? Well first of all that was only a test, an experimental feature, as such I was not going to maintain it or fix bugs. But secondly and more important, it was based on a wrong concept. ProxySQL is on-top-of MySQL as reverse proxy. The specific feature was breaking the main idea and was actually acting on the PXC nodes changing their status (READ_ONLY). That should not happen and any action or operation should involve only the ProxySQL node(s)

Why this blog

This blog will describe what I had implemented in the script as solution (not experimental), for the case in which we need to use PXC in Single Writer Mode (SWM). I will illustrate the new feature in PXC and the additional concept of Failover Host Group (FHG).

After few months, I realized I have to change the script and either remove the experimental feature or redesign it. But something was missed in PXC to allow me (us) to correctly enable the SWM. Actually at the beginning I was looking to have it fully integrated in ProxySQL, and I had several discussion with Percona colleagues, and Rene’, about this. But at the end the PXC new internal features had not be enough to guarantee ProxySQL to be able to identify and manage most dangerous cases. Let us see what had being implemented in PXC and what was still missed such that I had to put my hands on the script code again.

Good .. but not good enough When in the need to have PXC in SWM, we need to have ONE server in the HG receiving the WRITES, and as many as we like in the HG(s) receiving the READS.

What this imply is that ProxySQL either need to perform this operation using Replication HG (abusing of the READ_ONY flag), or we need a way to:

Identify which node(s) can be a WRITER candidate in place of
Identify/assign the correct relevance and priority to each candidate
Have a unique view to recognize the nodes in the cluster and their segment membership Identify as much as possible any reason(s) for which a node cannot be used as writer

What we were discussing internally, was to have a solution that would allow us to see the cluster status no matter which node would answer us, and should also be able to provide us which node could become the next writer. This last should be eventually evaluated against custom settings/preferences.

PXC 5.7.19 come with a new feature pxc_cluster_view, a table that reside in performance_schema and that can be easily access simply doing :

(pxc_test@192.168.1.11) [(none)]>select * FROM performance_schema.pxc_cluster_view ORDER BY SEGMENT,LOCAL_INDEX;
+-----------+--------------------------------------+--------+-------------+---------+
| HOST_NAME | UUID                                 | STATUS | LOCAL_INDEX | SEGMENT |
+-----------+--------------------------------------+--------+-------------+---------+
| node1     | 05b5554f-be34-11e7-aa92-5e038abbae35 | SYNCED |           0 |       1 |
| node2     | 63870cc3-af5d-11e7-a0db-463be8426737 | SYNCED |           1 |       1 |
| node3     | 666ad5a0-af5d-11e7-9b39-2726e5de8eb1 | SYNCED |           2 |       1 |
| node6     | 7540bdca-b267-11e7-bae9-464c3d263470 | SYNCED |           3 |       2 |
| node5     | ab551483-b267-11e7-a1a1-9be1826f877f | SYNCED |           4 |       2 |
| node4     | e24ebd01-af5d-11e7-86f0-42c8ceb6886c | SYNCED |           5 |       2 |
+-----------+--------------------------------------+--------+-------------+---------+

As you can see the table is reporting us the list of ALL nodes ordered by Segment and LOCAL_INDEX. Why I had use that sort? Because this order will give me, in descending order, the priority for which PXC see the nodes. As we know segment is assigned and is part of the customization/design we have to take care when deploying the cluster. Local_Index, instead, is an auto calculated value against the UUID assigned internally by the cluster. As such while this is a good indicator internally, is totally useless for us in case we want to “customize” the priority.
But this is not all, as already covered in my previously mentioned article, PXC/Galera nodes can be in a state that will prevent the proper access to them, without having ProxySQL realizing it … or PXC itself .

One very simple example is the following:

(root@localhost) [(none)]>set global wsrep_reject_queries=ALL; 
2017-11-06T19:21:14.533407Z 738828 [Note] WSREP: Rejecting client queries due to manual setting

(pxc_test@192.168.1.231) [(none)]>select now();select * FROM performance_schema.pxc_cluster_view ORDER BY SEGMENT,LOCAL_INDEX;
| 2017-11-06 14:22:19 |
| HOST_NAME | UUID                                 | STATUS | LOCAL_INDEX | SEGMENT |
| node1     | 05b5554f-be34-11e7-aa92-5e038abbae35 | SYNCED |           0 |       1 | ← still looks good
| node2     | 63870cc3-af5d-11e7-a0db-463be8426737 | SYNCED |           1 |       1 |
| node3     | 666ad5a0-af5d-11e7-9b39-2726e5de8eb1 | SYNCED |           2 |       1 |
| node6     | 7540bdca-b267-11e7-bae9-464c3d263470 | SYNCED |           3 |       2 |
| node5     | ab551483-b267-11e7-a1a1-9be1826f877f | SYNCED |           4 |       2 |
| node4     | e24ebd01-af5d-11e7-86f0-42c8ceb6886c | SYNCED |           5 |       2 |

No need to say that by default the Vanilla ProxySQL version will not catch that either, so Bingo! Or to be more correct BOOOM!

Your cluster is doomed, because by mistake or on purpose something not traced had stop your production Write node.
So what else/more do we need, to have not only an healty PXC cluster but also the chance to perform a nice failover when using SWM?

Well of course the script covering the different node state, and that my script was already doing it.
We need to have the chance to indicate a list of preferred node, with priority. After we can decide to go for the priority as it comes from pxc_cluster_view if for any reasons our choice will not work. Or even go for a different segment as last resource but still respecting the pxc_cluster_view.

To fix the first and second points, I had first look to what had been done in ProxySQL for group replication. Group Replication has a dedicated table with a lot of additional information:

writer_hostgroup
backup_writer_hostgroup
reader_hostgroup
offline_hostgroup
active
max_writers
writer_is_also_reader
Max_transactions_behind

This could make sense for Group replication, also if I consider it a bit redundant, but for us and PXC, that would be too much and no needed given I can resolve to have what I need in a more elegant way.

Keep in mind that when Rene’ had implement the HostGroup approach he had put the foundations of an unbelievable flexibility with elegance and consistency.
Everything in ProxySQL revolve around HG, we do not have servers (well we do as part of) we have HG, our rules point to HG, queries are directed to HG servers are incidentally mention as part of HG.
So why not take the benefit of this? Afterall I had already used the special group 9000 to manage edge cases of maintenance and offline.

The only problem was, do I have all the information I need there? Let us see the content of mysql_servers table:

hostgroup_id
hostname
port
status
weight
compression
max_connections
max_replication_lag
use_ssl
max_latency_ms
comment

To generate a list of candidates WRITER for PXC, I need exactly that nothing less or more.
The new mysql_servers table will looks like:

SELECT hostgroup_id,hostname,port,STATUS,weight FROM runtime_mysql_servers ORDER BY hostgroup_id,weight DESC;
+--------------+---------------+------+--------+---------+
| hostgroup_id | hostname      | port | STATUS | weight  |
+--------------+---------------+------+--------+---------+
| 50           | 192.168.1.205 | 3306 | ONLINE | 1000000 |<-- writer
| 52           | 192.168.1.21  | 3306 | ONLINE | 1000000 |
| 52           | 192.168.1.233 | 3306 | ONLINE | 10      |
| 52           | 192.168.1.205 | 3306 | ONLINE | 1       |
| 52           | 192.168.1.22  | 3306 | ONLINE | 1       |
| 52           | 192.168.1.23  | 3306 | ONLINE | 1       |
| 52           | 192.168.1.231 | 3306 | ONLINE | 1       |
| 8050         | 192.168.1.205 | 3306 | ONLINE | 1000000 |<-- Candidates with weight
| 8050         | 192.168.1.231 | 3306 | ONLINE | 10000   |<--
| 8050         | 192.168.1.22  | 3306 | ONLINE | 100000  |<--
+--------------+---------------+------+--------+---------+

Now that we had established we have what we need, let us test, the solution.

letusdoit

Implement and test SWM and HA with galera_check.pl
As previously discuss the new script can efficiently manage the SWM and HA, using 3 different methods of failover:

Use special HG8000 to set preferences
Use pxc_cluster_view inside same segment
Last resource pxc_cluster_view in ANY segment

What we will test is:

Controlled failover for maintenance
Server cannot get queries
Server crash

Failover methods:

Use provided list of servers
Use pxc_cluster_view for same segment
Use pxc_cluster_view for all segment

We will have:

6 nodes
2 segments
2 HG (with read/write split)

We will test it as we did for https://www.percona.com/blog/2016/09/15/proxysql-percona-cluster-galera-integration/ with Sysbench commands:

Prepare
sysbench --test=/usr/share/doc/sysbench/tests/db/oltp.lua --mysql-host=192.168.1.11 --mysql-port=6033 --mysql-user=pxc_test --mysql-password=test --mysql-db=test_galera --db-driver=mysql --oltp-tables-count=50 --oltp-tablesize=50000 --num-threads=10 --report-interval=10  prepare
 
Write
sysbench --test=/usr/share/doc/sysbench/tests/db/oltp.lua --mysql-host=192.168.1.11 --mysql-port=6033 --mysql-user=pxc_test --mysql-password=test --mysql-db=test_galera --db-driver=mysql --oltp-tables-count=50 --oltp-tablesize=50000 --max-requests=0 --max-time=9000 --oltp-point-selects=5 --oltp-read-only=off --oltp-dist-type=uniform --oltp-reconnect-mode=transaction --oltp-skip-trx=off --num-threads=10 --report-interval=10 --mysql-ignore-errors=all run
 
Read only
sysbench --test=/usr/share/doc/sysbench/tests/db/oltp.lua --mysql-host=192.168.1.11 --mysql-port=6033 --mysql-user=pxc_test --mysql-password=test --mysql-db=test_galera --db-driver=mysql --oltp-tables-count=50 --oltp-tablesize=50000 --max-requests=0 --max-time=9000 --oltp-point-selects=5 --oltp-read-only=on --num-threads=10 --oltp-reconnect-mode=query --oltp-skip-trx=on --report-interval=10 --mysql-ignore-errors=all run

Set it up:

UPDATE global_variables SET Variable_Value=0 WHERE Variable_name='mysql-hostgroup_manager_verbose';

Please take a moment to note the above setting.
IF you omit ProxySQL will verbosely report host-groups changes, I suggest to put it to 0 unless you are trying to debug “something” or your logs will become enormous soon.

INSERT INTO mysql_users (username,password,active,default_hostgroup,default_schema,transaction_persistent) VALUES ('pxc_test','test',1,50,'mysql',1);
LOAD MYSQL USERS TO RUNTIME;SAVE MYSQL USERS TO DISK;

Below the entries to populate add your servers:

DELETE FROM mysql_servers WHERE hostgroup_id IN (50,52);
 
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.1.21',50,3306,1000000,2000);
 
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.1.205',52,3306,1,2000);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.1.21',52,3306,1000000,2000);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.1.231',52,3306,1,2000);
 
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.1.22',52,3306,1,2000);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.1.23',52,3306,1,2000);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.1.233',52,3306,10,2000);

Here the section with the list of WRITER candidates:

INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.1.205',8050,3306,1000000,2000);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.1.231',8050,3306,100000,2000);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.1.22',8050,3306,100000,2000);
 
LOAD MYSQL SERVERS TO RUNTIME; SAVE MYSQL SERVERS TO DISK;

Finally query rules to have read/write split:

DELETE FROM mysql_query_rules WHERE rule_id IN (40,41,45,46,80,81);
INSERT INTO mysql_query_rules (rule_id,proxy_port,username,destination_hostgroup,active,retries,match_digest,apply) VALUES(4,6033,'pxc_test',50,1,3,'.',1);
INSERT INTO mysql_query_rules (rule_id,proxy_port,username,destination_hostgroup,active,retries,match_digest,apply) VALUES(40,6033,'pxc_test',50,1,3,'^SELECT.*FOR UPDATE',1);
INSERT INTO mysql_query_rules (rule_id,proxy_port,username,destination_hostgroup,active,retries,match_digest,apply) VALUES(41,6033,'pxc_test',52,1,3,'^SELECT ',1); 
 
LOAD MYSQL QUERY RULES TO RUNTIME;SAVE MYSQL QUERY RULES TO DISK;

Time to test our SWM and failover.

Clone the script from git in your preferred directory, I will put it in opt for now.

git clone https://github.com/Tusamarco/proxy_sql_tools
chmod +x  /opt/proxy_sql_tools/galera_check.pl

Add to the script to the scheduler

DELETE FROM scheduler WHERE id=10;
INSERT  INTO scheduler (id,active,interval_ms,filename,arg1) VALUES (10,0,2000,"/opt/proxy_sql_tools/galera_check.pl","-u=admin -p=admin -h=127.0.0.1 -H=50:W,52:R -P=6032 --execution_time=0 --retry_down=0 --retry_up=0 --main_segment=1 --debug=0  --log=/var/lib/proxysql/galeraLog --active_failover=1");
LOAD SCHEDULER TO RUNTIME;SAVE SCHEDULER TO DISK;

Make it active:

UPDATE scheduler SET active=1 WHERE id=10; LOAD scheduler TO run;

If you had left the --execution_time=1 the script will constantly print the time it takes to execute in the log:

2017/11/10 09:03:44.439:[INFO] END EXECUTION Total Time(ms):391

I suggest you to initially keep it on so you will see what is actually happening.

Time to perform the first test.

Controlled failover for maintenance

redflag When you want to do a CONTROLLED failover, the best way to do with minimal impact is to manually add the second node with very low weight, and after remove the node you need to work on. I will show you how to do this safely.

Add the new new write node to the writer HG:

INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.1.205',50,3306,1,2000);
LOAD mysql servers TO run;

You will have something like this:

+---------+-----------+---------------+----------+--------+----------+----------+--------+---------+---------+-----------------+-----------------+------------+
| weight  | hostgroup | srv_host      | srv_port | STATUS | ConnUsed | ConnFree | ConnOK | ConnERR | Queries | Bytes_data_sent | Bytes_data_recv | Latency_us |
+---------+-----------+---------------+----------+--------+----------+----------+--------+---------+---------+-----------------+-----------------+------------+
| 1000000 | 50        | 192.168.1.21  | 3306     | ONLINE | 10       | 0        | 35863  | 0       | 1137903 | 75147779        | 1527773047     | 6794       |
| 1       | 50        | 192.168.1.205 | 3306     | ONLINE | 0        | 0        | 0      | 0       | 0       | 0               | 0              | 8236       |
| 1000    | 52        | 192.168.1.233 | 3306     | ONLINE | 8        | 1        | 33153  | 0       | 1747722 | 82574272        | 3919114554     | 9304       |
| 1000    | 52        | 192.168.1.231 | 3306     | ONLINE | 3        | 6        | 15206  | 0       | 1469312 | 69597188        | 3340444198     | 2989       |
| 1000    | 52        | 192.168.1.23  | 3306     | ONLINE | 6        | 3        | 36690  | 4       | 2046599 | 96797464        | 4606494587     | 10261      |
| 1000    | 52        | 192.168.1.22  | 3306     | ONLINE | 4        | 5        | 36778  | 262     | 2152217 | 101807337       | 4846638759      | 2108       |
| 10      | 52        | 192.168.1.21  | 3306     | ONLINE | 0        | 1        | 302    | 0       | 21960   | 1039596         | 49257671       | 6794       |
| 1000    | 52        | 192.168.1.205 | 3306     | ONLINE | 3        | 6        | 31355  | 0       | 2180310 | 103198644       | 4917528180      | 8236       |
+---------+-----------+---------------+----------+--------+----------+----------+--------+---------+---------+-----------------+-----------------+------------+

With the added host handling very few connections because the weight.

At this point you just need to connect to the node you want to put in maintenance and set it desync:

SET global wsrep_desync=1;

Once desync the script will put the server in OFFLINE_SOFT for you, and the server will allow the running connection to complete while the new ones will go to the node we had just insert.
No errors no service interruption at all:

2017/11/10 09:31:41.967:[WARN]  Move node:192.168.1.21;3306;50;3010 SQL: UPDATE mysql_servers SET status='OFFLINE_SOFT' WHERE hostgroup_id=50 AND hostname='192.168.1.21' AND port='3306'
2017/11/10 09:31:41.978:[WARN]  Move node:192.168.1.21;3306;52;3010 SQL: UPDATE mysql_servers SET status='OFFLINE_SOFT' WHERE hostgroup_id=52 AND hostname='192.168.1.21' AND port='3306'

Once maintenance is done, just reverse the wsrep_desync and the script will put the node back to the pool.
redflag If you need to stop the mysql server on that node and you do not want it automatically back, remember to remove the entry from the host group 50 (the Writer one) or it will be automatically placed back in production as active.

DELETE FROM mysql_servers WHERE hostname=”192.168.1.21” AND port=3306 AND hostgroup_id=50;
LOAD mysql servers TO run;

Remember at the and to remove the temporary node from the Writer HG.

Real failover

Now a real fail over and we will test it using wsrep_rejectqueries to trigger the issue, but that can be any other issue.

In this test, given there is no other node accepting the writes in the HG, we should expect some connections error, but what we want is to have the failover in very short time.
Keep in mind we have define these as possible failover candidates:

+--------------+---------------+------+--------+---------+
| hostgroup_id | hostname      | port | STATUS | weight  |
+--------------+---------------+------+--------+---------+
| 8050         | 192.168.1.205 | 3306 | ONLINE | 1000000 |
| 8050         | 192.168.1.22  | 3306 | ONLINE | 100000  |
| 8050         | 192.168.1.231 | 3306 | ONLINE | 100000  |
+--------------+---------------+------+--------+---------+
 
While the pxc_cluster_view sees the following:
 
+-----------+--------------------------------------+--------+-------------+---------+
| HOST_NAME | UUID                                 | STATUS | LOCAL_INDEX | SEGMENT |
+-----------+--------------------------------------+--------+-------------+---------+
| node1     | 05b5554f-be34-11e7-aa92-5e038abbae35 | SYNCED |           0 |       1 |
| node2     | 63870cc3-af5d-11e7-a0db-463be8426737 | SYNCED |           2 |       1 |
| node3     | 666ad5a0-af5d-11e7-9b39-2726e5de8eb1 | SYNCED |           3 |       1 |
| node5     | 07f55a00-c57a-11e7-bd7d-8a568ca96345 | SYNCED |           1 |       2 |
| node4     | 6c185b87-c57a-11e7-b2a6-8f095c523cd3 | SYNCED |           4 |       2 |
| node6     | 7540bdca-b267-11e7-bae9-464c3d263470 | SYNCED |           5 |       2 |
+-----------+--------------------------------------+--------+-------------+---------+
 
FOR better understanding also FOR the following tests this IS the mapping, node name <> IP:
+-----------+---------------+
| HOST_NAME | IP            |
+-----------+---------------+
| node1     | 192.168.1.205 | 
| node2     | 192.168.1.21  | 
| node3     | 192.168.1.231 | 
| node5     | 192.168.1.23  | 
| node4     | 192.168.1.22  | 
| node6     | 192.168.1.233 | 
+-----------+---------------+

My Servers layout is:

 
+--------------+---------------+------+--------+---------+
| hostgroup_id | hostname      | port | STATUS | weight  |
+--------------+---------------+------+--------+---------+
| 50           | 192.168.1.205 | 3306 | ONLINE | 1000000 | ← writer
| 52           | 192.168.1.205 | 3306 | ONLINE | 1000    |
| 52           | 192.168.1.22  | 3306 | ONLINE | 1000    |
| 52           | 192.168.1.23  | 3306 | ONLINE | 1000    |
| 52           | 192.168.1.231 | 3306 | ONLINE | 1000    |
| 52           | 192.168.1.233 | 3306 | ONLINE | 1000    |
| 52           | 192.168.1.21  | 3306 | ONLINE | 10      |
| 8050         | 192.168.1.205 | 3306 | ONLINE | 1000000 |
| 8050         | 192.168.1.22  | 3306 | ONLINE | 100000  |
| 8050         | 192.168.1.231 | 3306 | ONLINE | 100000  |
+--------------+---------------+------+--------+---------+

The first candidate is

192.168.1.22

which is node4 and stay in a DIFFERENT segment than the current writer. As such the first election should go to the other node

192.168.1.231

which is Node3 and reside in the same segment.
Let us see what happens with the active-failover=1:
Do:

set global wsrep_reject_queries=all;

On the current writer.

The script will identify the Writer cannot accept queries and will take action:

2017/11/13 10:51:52.856:[WARN]  Move node:192.168.1.205;3306;50;3001 SQL: UPDATE mysql_servers SET hostgroup_id=9050 WHERE hostgroup_id=50 AND hostname='192.168.1.205' AND port='3306'
2017/11/13 10:51:52.861:[WARN]  Move node:192.168.1.205;3306;52;3001 SQL: UPDATE mysql_servers SET hostgroup_id=9052 WHERE hostgroup_id=52 AND hostname='192.168.1.205' AND port='3306'
2017/11/13 10:51:55.038:[WARN] Fail-over in action Using Method = 1
2017/11/13 10:51:55.038:[INFO] Special Backup - Group found! I am electing a node to writer following the indications
 This Node Try to become the new WRITER for HG 50 Server details: 192.168.1.231:3306:HG8050
2017/11/13 10:51:55.038:[INFO] This Node Try to become a WRITER promoting to HG 50192.168.1.231:3306:HG8050
2017/11/13 10:51:55.083:[WARN]  Move node:192.168.1.231:33061000002000 SQL:INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.1.231',50,3306,100000,2000);

It will also analyze the failover candidates and it will promote one node if present. Also as expected it will choose the one in the same segment.

What happen at the applications?

Writes
[ 2010s ] thds: 10 tps: 33.40 qps: 503.33 (r/w/o: 302.92/127.31/73.10) lat (ms,95%): 434.83 err/s: 0.00 reconn/s: 0.00
[ 2020s ] thds: 10 tps: 38.30 qps: 575.27 (r/w/o: 344.68/147.59/82.99) lat (ms,95%): 376.49 err/s: 0.00 reconn/s: 0.00
[ 2030s ] thds: 10 tps: 35.80 qps: 537.20 (r/w/o: 322.40/137.90/76.90) lat (ms,95%): 419.45 err/s: 0.00 reconn/s: 0.00
[ 2040s ] thds: 10 tps: 32.70 qps: 489.34 (r/w/o: 293.53/125.71/70.11) lat (ms,95%): 427.07 err/s: 0.00 reconn/s: 0.00
[ 2050s ] thds: 10 tps: 30.60 qps: 456.43 (r/w/o: 273.62/117.61/65.20) lat (ms,95%): 493.24 err/s: 0.00 reconn/s: 0.00
[ 2060s ] thds: 10 tps: 19.20 qps: 298.36 (r/w/o: 181.18/74.99/42.19) lat (ms,95%): 657.93 err/s: 0.00 reconn/s: 30.50 <-- here 
[ 2070s ] thds: 10 tps: 23.40 qps: 345.80 (r/w/o: 206.30/89.10/50.40) lat (ms,95%): 634.66 err/s: 0.00 reconn/s: 0.00
[ 2080s ] thds: 10 tps: 27.20 qps: 414.93 (r/w/o: 249.72/106.71/58.50) lat (ms,95%): 612.21 err/s: 0.00 reconn/s: 0.00
 
Reads
[ 2000s ] thds: 30 tps: 256.50 qps: 2309.91 (r/w/o: 2309.91/0.00/0.00) lat (ms,95%): 189.93 err/s: 0.00 reconn/s: 0.00
[ 2010s ] thds: 30 tps: 250.27 qps: 2254.26 (r/w/o: 2254.26/0.00/0.00) lat (ms,95%): 200.47 err/s: 0.00 reconn/s: 0.00
[ 2020s ] thds: 30 tps: 256.29 qps: 2305.30 (r/w/o: 2305.30/0.00/0.00) lat (ms,95%): 186.54 err/s: 0.00 reconn/s: 0.00
[ 2030s ] thds: 30 tps: 241.65 qps: 2176.28 (r/w/o: 2176.28/0.00/0.00) lat (ms,95%): 193.38 err/s: 0.00 reconn/s: 0.00
[ 2040s ] thds: 30 tps: 217.88 qps: 1956.96 (r/w/o: 1956.96/0.00/0.00) lat (ms,95%): 215.44 err/s: 0.00 reconn/s: 0.00
[ 2050s ] thds: 30 tps: 277.70 qps: 2504.09 (r/w/o: 2504.09/0.00/0.00) lat (ms,95%): 196.89 err/s: 0.00 reconn/s: 0.00
[ 2060s ] thds: 30 tps: 207.37 qps: 1863.52 (r/w/o: 1863.52/0.00/0.00) lat (ms,95%): 240.02 err/s: 0.00 reconn/s: 0.00
[ 2070s ] thds: 30 tps: 199.44 qps: 1795.70 (r/w/o: 1795.70/0.00/0.00) lat (ms,95%): 244.38 err/s: 0.00 reconn/s: 0.00

The moment of the server lock the application had a hiccup for the write as expected, but the time was very limited and the difference in WRITE transactions was only of 75-89/sec against the normally processed ~120/sec, as such only ~50 missed during the incident.Reads 0 loss.

What if there is no other server in same segment?

My new servers layout is:

+--------------+---------------+------+--------+---------+
| hostgroup_id | hostname      | port | STATUS | weight  |
+--------------+---------------+------+--------+---------+
| 50           | 192.168.1.231 | 3306 | ONLINE | 100000  | <--- new 
| 52           | 192.168.1.22  | 3306 | ONLINE | 1000    |
| 52           | 192.168.1.23  | 3306 | ONLINE | 1000    |
| 52           | 192.168.1.231 | 3306 | ONLINE | 1000    |
| 52           | 192.168.1.233 | 3306 | ONLINE | 1000    |
| 52           | 192.168.1.21  | 3306 | ONLINE | 10      |
| 8050         | 192.168.1.205 | 3306 | ONLINE | 1000000 |
| 8050         | 192.168.1.22  | 3306 | ONLINE | 100000  |
| 8050         | 192.168.1.231 | 3306 | ONLINE | 100000  |
| 9052         | 192.168.1.205 | 3306 | ONLINE | 1000    | <--- set in special group
+--------------+---------------+------+--------+---------+

Let us put down the writer again.

At this point the only available node is the one on the other segment.
As expected the script will identify the failed node and will take action:

2017/11/13 12:15:18.535:[WARN]  Move node:192.168.1.231;3306;50;3001 SQL: UPDATE mysql_servers SET hostgroup_id=9050 WHERE hostgroup_id=50 AND hostname='192.168.1.231' AND port='3306'
2017/11/13 12:15:18.543:[WARN]  Move node:192.168.1.231;3306;52;3001 SQL: UPDATE mysql_servers SET hostgroup_id=9052 WHERE hostgroup_id=52 AND hostname='192.168.1.231' AND port='3306'
2017/11/13 12:15:20.646:[WARN] Fail-over in action Using Method = 1
2017/11/13 12:15:20.646:[INFO] Special Backup - Group found! I am electing a node to writer following the indications
 This Node Try to become the new WRITER for HG 50 Server details: 192.168.1.22:3306:HG8050
2017/11/13 12:15:20.646:[INFO] This Node Try to become a WRITER promoting to HG 50192.168.1.22:3306:HG8050
2017/11/13 12:15:20.660:[WARN]  Move node:192.168.1.22:33061000002000 SQL:INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.1.22',50,3306,100000,2000);

This because when using automatic_failover=1 I assume YOU KNOW what you are doing in using a failover group. As such while I still use the segments for internal priority, I am not filtering out the node(s) in a different segment, as if using automatic_failover=2.

Same minimal impact on TPS:

Writes
[ 7040s ] thds: 10 tps: 26.39 qps: 395.02 (r/w/o: 236.85/101.48/56.69) lat (ms,95%): 569.67 err/s: 0.00 reconn/s: 0.00
[ 7050s ] thds: 10 tps: 26.81 qps: 405.20 (r/w/o: 243.16/104.73/57.31) lat (ms,95%): 623.33 err/s: 0.00 reconn/s: 0.00
[ 7060s ] thds: 10 tps: 18.80 qps: 276.65 (r/w/o: 167.27/68.29/41.09) lat (ms,95%): 634.66 err/s: 0.00 reconn/s: 23.70
[ 7070s ] thds: 10 tps: 18.90 qps: 292.87 (r/w/o: 177.78/73.99/41.10) lat (ms,95%): 5813.24 err/s: 0.00 reconn/s: 2.80
[ 7080s ] thds: 10 tps: 24.00 qps: 356.84 (r/w/o: 213.42/91.71/51.71) lat (ms,95%): 623.33 err/s: 0.00 reconn/s: 0.00
[ 7090s ] thds: 10 tps: 20.60 qps: 312.27 (r/w/o: 187.58/79.69/45.00) lat (ms,95%): 759.88 err/s: 0.00 reconn/s: 0.00
[ 7100s ] thds: 10 tps: 19.80 qps: 299.33 (r/w/o: 179.02/78.41/41.90) lat (ms,95%): 787.74 err/s: 0.00 reconn/s: 0.00
[ 7110s ] thds: 10 tps: 26.70 qps: 398.33 (r/w/o: 239.12/101.51/57.70) lat (ms,95%): 669.89 err/s: 0.00 reconn/s: 0.00</pre>
 
Reads
[ 7040s ] thds: 30 tps: 207.90 qps: 1870.43 (r/w/o: 1870.43/0.00/0.00) lat (ms,95%): 227.40 err/s: 0.00 reconn/s: 0.00
[ 7050s ] thds: 30 tps: 198.88 qps: 1790.44 (r/w/o: 1790.44/0.00/0.00) lat (ms,95%): 240.02 err/s: 0.00 reconn/s: 0.00
[ 7060s ] thds: 30 tps: 165.52 qps: 1488.05 (r/w/o: 1488.05/0.00/0.00) lat (ms,95%): 282.25 err/s: 0.00 reconn/s: 0.00
[ 7070s ] thds: 30 tps: 186.87 qps: 1681.46 (r/w/o: 1681.46/0.00/0.00) lat (ms,95%): 267.41 err/s: 0.00 reconn/s: 0.00
[ 7080s ] thds: 30 tps: 178.24 qps: 1607.05 (r/w/o: 1607.05/0.00/0.00) lat (ms,95%): 267.41 err/s: 0.00 reconn/s: 0.00
[ 7090s ] thds: 30 tps: 167.67 qps: 1506.13 (r/w/o: 1506.13/0.00/0.00) lat (ms,95%): 282.25 err/s: 0.00 reconn/s: 0.00
[ 7100s ] thds: 30 tps: 170.50 qps: 1535.70 (r/w/o: 1535.70/0.00/0.00) lat (ms,95%): 287.38 err/s: 0.00 reconn/s: 0.00
[ 7110s ] thds: 30 tps: 175.52 qps: 1580.38 (r/w/o: 1580.38/0.00/0.00) lat (ms,95%): 267.41 err/s: 0.00 reconn/s: 0.00
 
Servers layout
+--------------+---------------+------+--------+---------+
| hostgroup_id | hostname      | port | STATUS | weight  |
+--------------+---------------+------+--------+---------+
| 50           | 192.168.1.22  | 3306 | ONLINE | 100000  |<-- new writer
| 52           | 192.168.1.22  | 3306 | ONLINE | 1000    |
| 52           | 192.168.1.23  | 3306 | ONLINE | 1000    |
| 52           | 192.168.1.233 | 3306 | ONLINE | 1000    |
| 52           | 192.168.1.21  | 3306 | ONLINE | 10      |
| 8050         | 192.168.1.205 | 3306 | ONLINE | 1000000 |
| 8050         | 192.168.1.22  | 3306 | ONLINE | 100000  |
| 8050         | 192.168.1.231 | 3306 | ONLINE | 100000  |
| 9052         | 192.168.1.205 | 3306 | ONLINE | 1000    |<-- node in special hg
| 9052         | 192.168.1.231 | 3306 | ONLINE | 1000    |<-- “
+--------------+---------------+------+--------+---------+

At this point as security feature also if you put back the failed nodes, the script WILL NOT fail back. But I am working on this feature request to implement th eoption to eventually perform fail-back.
As for today if you do:

Set global wsrep_reject_queries=none

On both servers, and you will have:

+--------------+---------------+------+--------+---------+
| hostgroup_id | hostname      | port | STATUS | weight  |
+--------------+---------------+------+--------+---------+
| 50           | 192.168.1.22  | 3306 | ONLINE | 100000  |
| 52           | 192.168.1.205 | 3306 | ONLINE | 1000    |<-- back but READ
| 52           | 192.168.1.22  | 3306 | ONLINE | 1000    |
| 52           | 192.168.1.23  | 3306 | ONLINE | 1000    |
| 52           | 192.168.1.231 | 3306 | ONLINE | 1000    |<-- back but READ 
| 52           | 192.168.1.233 | 3306 | ONLINE | 1000    |
| 52           | 192.168.1.21  | 3306 | ONLINE | 10      |
| 8050         | 192.168.1.205 | 3306 | ONLINE | 1000000 |
| 8050         | 192.168.1.22  | 3306 | ONLINE | 100000  |
| 8050         | 192.168.1.231 | 3306 | ONLINE | 100000  |
+--------------+---------------+------+--------+---------+

To fail back you need to manually trigger it.
Connect to the current writer and set it to wsrep_reject_queries = ALL
The script will move back as Writer the node with Higher weight in the 8000 HG

>+--------------+---------------+------+--------+---------+
| hostgroup_id | hostname      | port | STATUS | weight  |
+--------------+---------------+------+--------+---------+
| 50           | 192.168.1.205 | 3306 | ONLINE | 1000000 |<-- back as writer
| 52           | 192.168.1.205 | 3306 | ONLINE | 1000    |
| 52           | 192.168.1.23  | 3306 | ONLINE | 1000    |
| 52           | 192.168.1.231 | 3306 | ONLINE | 1000    |
| 52           | 192.168.1.233 | 3306 | ONLINE | 1000    |
| 52           | 192.168.1.21  | 3306 | ONLINE | 10      |
| 8050         | 192.168.1.205 | 3306 | ONLINE | 1000000 |
| 8050         | 192.168.1.22  | 3306 | ONLINE | 100000  |
| 8050         | 192.168.1.231 | 3306 | ONLINE | 100000  |
| 9052         | 192.168.1.22  | 3306 | ONLINE | 1000    |<-- set as special group
+--------------+---------------+------+--------+---------+

Server crash

In case of server crash the behaviour will be exactly the same with the difference that the node will be elected immediately, no intermediate step for the HG shift.
Killing the mysqld with kill -9 (SIGKILL) will result in the following:

writes
[ 240s ] thds: 10 tps: 42.80 qps: 642.78 (r/w/o: 385.99/166.00/90.80) lat (ms,95%): 331.91 err/s: 0.00 reconn/s: 0.00
[ 250s ] thds: 10 tps: 40.80 qps: 603.37 (r/w/o: 361.74/155.92/85.71) lat (ms,95%): 363.18 err/s: 0.00 reconn/s: 0.00
[ 260s ] thds: 10 tps: 39.60 qps: 600.79 (r/w/o: 361.20/156.20/83.40) lat (ms,95%): 376.49 err/s: 0.00 reconn/s: 0.00
[ 270s ] thds: 10 tps: 35.00 qps: 541.33 (r/w/o: 324.82/141.91/74.60) lat (ms,95%): 356.70 err/s: 0.00 reconn/s: 1.00<--
[ 280s ] thds: 10 tps: 14.80 qps: 216.47 (r/w/o: 130.78/54.89/30.80) lat (ms,95%): 7615.89 err/s: 0.00 reconn/s: 0.00
[ 290s ] thds: 10 tps: 23.10 qps: 347.64 (r/w/o: 208.42/91.21/48.00) lat (ms,95%): 682.06 err/s: 0.00 reconn/s: 0.00</pre><br>
 
reads
[ 230s ] thds: 30 tps: 253.27 qps: 2281.51 (r/w/o: 2281.51/0.00/0.00) lat (ms,95%): 193.38 err/s: 0.00 reconn/s: 0.00
[ 240s ] thds: 30 tps: 229.80 qps: 2066.20 (r/w/o: 2066.20/0.00/0.00) lat (ms,95%): 215.44 err/s: 0.00 reconn/s: 0.00
[ 250s ] thds: 30 tps: 217.51 qps: 1957.59 (r/w/o: 1957.59/0.00/0.00) lat (ms,95%): 240.02 err/s: 0.00 reconn/s: 0.00
[ 260s ] thds: 30 tps: 207.89 qps: 1872.62 (r/w/o: 1872.62/0.00/0.00) lat (ms,95%): 240.02 err/s: 0.00 reconn/s: 0.00
[ 270s ] thds: 30 tps: 200.18 qps: 1802.03 (r/w/o: 1802.03/0.00/0.00) lat (ms,95%): 253.35 err/s: 0.00 reconn/s: 0.00<--
[ 280s ] thds: 30 tps: 199.33 qps: 1791.80 (r/w/o: 1791.80/0.00/0.00) lat (ms,95%): 240.02 err/s: 0.00 reconn/s: 0.00
[ 290s ] thds: 30 tps: 187.49 qps: 1689.78 (r/w/o: 1689.78/0.00/0.00) lat (ms,95%): 257.95 err/s: 0.00 reconn/s: 0.00
 
Script Log:
2017/11/16 10:02:10.923:[ERROR] Cannot connect TO DBI:mysql:host=192.168.1.205;port=3306 AS monitor
2017/11/16 10:02:11.179:[WARN] Fail-over IN action USING Method = 1
2017/11/16 10:02:11.179:[INFO] Special Backup - GROUP found! I am electing a node TO writer following the indications
 This Node Try TO become the new WRITER FOR HG 50 Server details: 192.168.1.231:3306:HG8050
2017/11/16 10:02:11.179:[INFO] This Node Try TO become a WRITER promoting TO HG 50 192.168.1.231:3306:HG 8050
2017/11/16 10:02:11.234:[WARN]  Move node:192.168.1.231:33061000002000 SQL:INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.1.231',50,3306,100000,2000);
2017/11/16 10:02:13.190:[ERROR] Cannot connect TO DBI:mysql:host=192.168.1.205;port=3306 AS monitor
 
Server layout:
(admin@127.0.0.1) [main]>select hostgroup_id,hostname,port,STATUS,weight FROM runtime_mysql_servers ORDER BY hostgroup_id,weight DESC;
+--------------+---------------+------+---------+---------+
| hostgroup_id | hostname      | port | STATUS  | weight  |
+--------------+---------------+------+---------+---------+
| 50           | 192.168.1.231 | 3306 | ONLINE  | 100000  |<-- new writer
| 52           | 192.168.1.205 | 3306 | SHUNNED | 1000    |<--shunned server
| 52           | 192.168.1.22  | 3306 | ONLINE  | 1000    |
| 52           | 192.168.1.23  | 3306 | ONLINE  | 1000    |
| 52           | 192.168.1.231 | 3306 | ONLINE  | 1000    |
| 52           | 192.168.1.233 | 3306 | ONLINE  | 1000    |
| 52           | 192.168.1.21  | 3306 | ONLINE  | 10      |
| 8050         | 192.168.1.205 | 3306 | SHUNNED | 1000000 |
| 8050         | 192.168.1.22  | 3306 | ONLINE  | 100000  |
| 8050         | 192.168.1.231 | 3306 | ONLINE  | 100000  |
+--------------+---------------+------+---------+---------+

Using Auto_failover=2|3

This is going to be exactly the same behaviour not matter what is the failover method choose.
The most flexible and controlled one is to use the 8000 HG with the identification of the list of servers prioritize by weight.

The use of pxc_cluster_view, will give you an pseudo automatic way to set the same priority.
But choosing that method will NOT guarantee you that the node you want will take the traffic given is based the internal Galera decision.

Conclusions

While the galera_check script was initially conceptualize to work with multiple writers, the inclusion of pxc_cluste_view and the addition of the special Host Group 8000, had provide the needed base for me to develop a sustainable feature to support single writer and failover.

I have to remember you that the preferred solution is and remain to use multiple writers with different weights, given that solution provide internal failover in Proxysql.
The use of active_failover can significantly reduce the amount of service interruption caused by a possible single writer node crash, still it is an external script and is not a effective as internal balancing base on weight.

By design the script at the moment is NOT supporting the failback, but I already have a feature request on this.
Once implemented this feature will allow a node that had being removed from the active pool, to recover his state once in a healthy state again.
Another very important aspect is the way ProxySQL will manage the configuration cross node using the cluster option.

This will be described in the next article.

More blogs (shorter) will come on how to use the script and ProxySQL Cluster.

Reference

https://github.com/Tusamarco/proxy_sql_tools
https://www.percona.com/blog/2016/09/15/proxysql-percona-cluster-galera-integration/

How ProxySQL deal with schema (and schemaname)

I think that very often we are so focus in analyzing internals, or specific behaviours/situations/anomalies that we tend to forget the simple things.

It happened to me that last week a couple of customers raise the same question: "How ProxySQL manage the default schema, or the ones declared inside a FROM/JOIN?"
I was a bit surprise because I was given that for granted, and my first thought was, 'well read the documentation', but then I realize we do not have a clear section in the documentation about this.

Given that and also because I realize I had not done a full and extensive test on how the SCHEMA is actually managed.
I decide to do a simple set of tests and write down few lines.

This blog is to answer that very simple question:"How ProxySQL manage the default schema, or the ones declared inside a FROM/JOIN?"
The blog is split in two parts, part 1 simple declaration and summary of what happen. Part 2 all the details and tests, in case you want to see them.

Schemaname and ProxySQL

In ProxySQL we can specify the schema in two different places and with different scope.

In the mysql_user table as property of the USER, where it will represent the DEFAULT schema.
In the mysql_query_rules as filter for which a query can be capture. The filter is valid only for the explicitly set default SCHEMA, (ie with -D mysql command line or USE). ProxySQL will NOT analyze the FROM SQL clausole. Given the limitation in the above point 2, it is not 100% safe to trust the SCHEMANAME as filter unless you are 200% sure the code do not contains commands to change default schema like USE.

On the other hand if I want to filter by a schemaname (in general) it is safer and more flexible to use regular expression and query_rules, as I will show later.

For the scope of this article I want to answer these simple sub-questions:

How ProxySQL differes from MySQL in managing explicit default schema declaration? does it respect the -D or USE
How proxy respect/follow security agains schema
How schemaname filter acts in the query rules?
How can I transparently redirect using schema name?

To test the above I have created:

two servers:

Master

Slave

two schemas:

world

City

Country

myworld

CityM

CountryM

three users:

uallworld, can access all the schemas (including test)

uworld, can access world in write/read on Master, read on slave. Can access myworld in read on slave.

umyworld, can access myworld in write/read on Master, read on slave. Can access world in read on slave.

Queries used during the tests:

    select database();
    update world.City set Population=10500000 where ID=1024;
    update world.Country set LifeExpectancy=62.5 where Code='IND';
    update myworld.CityM set Population=10500001 where ID=1024;
    update myworld.CountryM set LifeExpectancy=0 where Code='IND';
    Select * from world.City order by Population DESC limit 5 ;
    Select * from myworld.CityM order by Population DESC limit 5 ;
    Select City.*, Country.name, Country.LifeExpectancy from world.City as City

        join world.Country as Country on City.CountryCode=Country.code order by Population DESC limit 5; 
    Select City.*, Country.name, Country.LifeExpectancy from myworld.CityM as City

        join myworld.CountryM as Country on City.CountryCode=Country.code order by Population DESC limit 5; 
    Select City.*, Country.name, Country.LifeExpectancy from world.City as City

        join myworld.CountryM as Country on City.CountryCode=Country.code order by Population DESC limit 5;

To setup the environment see instructions at Annex 1.

Short story

How ProxySQL differes from MySQL in managing explicit default schema declaration? Does it respect the -D or USE?
MySQL and ProxySQL will behave the same when passing the default schema, setting it as default.
MySQL
mysql -uuallworld -ptest -h192.168.1.107 -P 3306 -D test

ProxySQL
mysql -uuallworld -ptest -h127.0.0.1 -P 6033 -D test

If a default schema is set in ProxySQL the schema coming from command line or connection (like in java:"connUrl=jdbc:mysql://192.168.1.50:6033/test"), will override the ProxySQL default.
In case a default schema is not pass during the connection MySQL and ProxySQL will differs on how the behave:
MySQL will set the current schema to NULL. It is to be noted that MySQL accept a NULL schema when u connect but then once SET it with USE you cannot set it back to NULL.
ProxySQL will set it as the one declared default in the mysql_user table. If no schema is declared as default, ProxySQL will elect information_schema as the default. In short ProxySQL cannot have a default schema set to NULL.
How proxy respect/follow security agains schema
MySQL
mysql -uuworld -ptest -h192.168.1.107 -P 3306 -D test ERROR 1044 (42000):
Access denied for user 'uworld'@'%' to database 'test'

ProxySQL
mysql -uuworld -ptest -h127.0.0.1 -P 6033 -D test
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 483902 Server version: 5.5.30 (ProxySQL)

On connection the behaviour is different between MySQL and ProxySQL.
Why? Because while you directly connect to MySQL, when you connect to ProxySQL you do not actually touch the final server.

ProxySQL is NOT a forward proxy, but a reverse proxy, so its scope is act as an intermediary for its associated servers to be contacted by any client. Backend connection that will serve your client is establish at the monent you actually send a query, which will be comunicated to the relevent host group.

As such when you connect, you only open a connection to the ProxySQL. While issue a query will USE a connection to the backend and if the user do not have the right grants an error will be returned.
But ProxySQL will not known until you submit the query and it can decide where this query should go (which HG to point to).
mysql> select database(); ERROR 1044 (42000): Access denied for user 'uworld'@'%' to database 'test'

Aside from this all the GRANTS defined in MySQL are transparent and followed by ProxySQL
How schemaname filter acts in the query rules?

In MySQL we can easily change the default schema with USE , this action is fully supported by ProxySQL.
But it may have some side effects when using "schemaname" as filter in the query_rules.

If you define a rule that include the default schemaname and the default schema is changed with USE, the rule will not apply, and unpredictable results may happen.
To prevent that ProxySQL has another option in mysql_user "schema_locked" which will prevent the schema to be changed by USE.

This feature is present but not fully implemented, and after a brief conversation with Rene (https://github.com/sysown/proxysql/issues/1133), I hope it will be soon.

Given that, when designing Query rules using the Default schema, you must to take in consideration the possibility to have the application or user changing the default schema and invalidating that rule.
how can I transparently redirect using schema name?

This is not a Schema feature, more one of the things that in ProxySQL are quite easy to set, while close to be impossible if done in plain MySQL.
When connecting directly with MySQL there is no option for you to "play" with GRANTS and schema such that you will transparently allow a user to do an action on a schema/server pair
and another on a different schema/server pair.
When using ProxySQL to filter by schemaname is quite trivial.
For instance assuming we have 3 users one is admin of the platform which include 2 separate schemas (or more), each user can access one schema for write (but that can be table as well),
and a final slave with reporting information, where all the users that needs to read from other schema except their own can read cross schemas. While all the select not cross schema mus still got to the Master;
This is not so uncommon, actually with few variant is exactly what one of the customer I spoke last week needs to do.
Now in MySQL this is impossible while in ProxySQL is just a matter of 3 rules:
insert into mysql_query_rules (rule_id,username,destination_hostgroup,active,retries,match_digest,apply) values(10,'uworld',10,1,3,'^SELECT.*FOR UPDATE',1);
insert into mysql_query_rules (rule_id,username,destination_hostgroup,active,retries,match_digest,apply,FlagOUT,FlagIN) values(11,'uworld',10,1,3,'^SELECT ',0,50,0);
insert into mysql_query_rules (rule_id,username,destination_hostgroup,active,retries,match_digest,apply,FlagOUT,FlagIN) values(12,'uworld',11,1,3,'\smyworld.',1,50,50);
Simply applying the rules above will allow the application to transparently access the data from different servers without errors,
following the GRANTS given on the MySQL side. This for ONE user (uworld), but it can eventually extended to any, and the rule chain is minimal just 2 rules, so minimal overhead also with high traffic.

Summarizing

ProxySQL is following the MySQL model to access the schema, in most parts. There are a couple of differences though.

ProxySQL will require to set a default schema when connecting, implicitly or explicitly.

ProxySQL will not return an error at connection time, if a user is not authorized to connect to the given schema. Error will raise at the first query, moment when ProxySQL will actually establish the connection.

Finally using ProxySQL, will allow administrator to play with GRANTS and HG/servers to provide transparent access to data in a more granular way, choosing an HG where user may have read (or other specific) access, against one where user is not allow at all .

Long Story ... click me

Annex 1

Create environment for test

create schema myworld;
create table myworld.CityM like City;
create table myworld.CountryM like Country;
create table myworld.CountryLanguageM like CountryLanguage;
insert into  myworld.CityM select * from City;
insert into  myworld.CountryM select * from Country;
insert into  myworld.CountryLanguageM select * from CountryLanguage;
 
delete from mysql_users where username like '%world';
insert into mysql_users (username,password,active,default_hostgroup,default_schema,transaction_persistent) values ('uworld','test',1,10,'world',1);
insert into mysql_users (username,password,active,default_hostgroup,default_schema,transaction_persistent) values ('umyworld','test',1,10,'myworld',1);
insert into mysql_users (username,password,active,default_hostgroup,default_schema,transaction_persistent) values ('uallworld','test',1,10,'test',1);
 
LOAD MYSQL USERS TO RUNTIME;SAVE MYSQL USERS TO DISK;
 
delete from mysql_servers where hostgroup_id in (10,11,20,21);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,comment) VALUES ('192.168.1.107',10,3306,100,'master');
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,comment) VALUES ('192.168.1.109',11,3307,100,'slave');
 
LOAD MYSQL SERVERS TO RUNTIME; SAVE MYSQL SERVERS TO DISK;
 
delete from mysql_replication_hostgroups;
INSERT INTO mysql_replication_hostgroups VALUES (10,11,'world-myworld replication hgroup');
LOAD MYSQL SERVERS TO RUNTIME; SAVE MYSQL SERVERS TO DISK;
 
DO NOT RUN AT THE beginning this is for test 3 !!
delete from mysql_query_rules where rule_id in (10,11,12,13,14,15);
insert into mysql_query_rules (rule_id,username,schemaname,destination_hostgroup,active,retries,match_digest,apply)

  values(10,'uworld','world',10,1,3,'^SELECT.*FOR UPDATE',1);
insert into mysql_query_rules (rule_id,username,schemaname,destination_hostgroup,active,retries,match_digest,apply)

  values(11,'uworld','world',11,1,3,'^SELECT ',1);
LOAD MYSQL QUERY RULES TO RUNTIME;SAVE MYSQL QUERY RULES TO DISK;
 
DO NOT RUN AT THE beginning this is for test 4 !!
delete from mysql_query_rules where rule_id in (10,11,12,13,14,15);
Let see what we need and how to do it:
1) user(s) uworld & umyworld need to go to their default schema on Master for Writes.
2) user(s) uworld & umyworld should go to their default schema on  master for direct reads
3) user(s) uworld & umyworld should go to the slave for reads when the other schema is used
 
To do this we will need the following rules:
insert into mysql_query_rules (rule_id,username,destination_hostgroup,active,retries,match_digest,apply) values(10,'uworld',10,1,3,'^SELECT.*FOR UPDATE',1);
insert into mysql_query_rules (rule_id,username,destination_hostgroup,active,retries,match_digest,apply,FlagOUT,FlagIN) values(11,'uworld',10,1,3,'^SELECT ',0,50,0); 
insert into mysql_query_rules (rule_id,username,destination_hostgroup,active,retries,match_digest,apply,FlagOUT,FlagIN) values(12,'uworld',11,1,3,'myworld.',1,50,50);
 
 
insert into mysql_query_rules (rule_id,username,destination_hostgroup,active,retries,match_digest,apply) values(13,'umyworld',10,1,3,'^SELECT.*FOR UPDATE',1);
insert into mysql_query_rules (rule_id,username,destination_hostgroup,active,retries,match_digest,apply,FlagOUT,FlagIN) values(14,'umyworld',10,1,3,'^SELECT ',0,50,0); 
insert into mysql_query_rules (rule_id,username,destination_hostgroup,active,retries,match_digest,apply,FlagOUT,FlagIN) values(15,'umyworld',11,1,3,'\sworld.',1,50,50);
 
LOAD MYSQL QUERY RULES TO RUNTIME;SAVE MYSQL QUERY RULES TO DISK;
 
GRANTS
-------
grant all on *.* to uallworld@'%' identified by 'test';
grant all on world.* to uworld@'%' identified by 'test';     
grant all on myworld.* to umyworld@'%' identified by 'test';   
 
on the slave
REVOKE ALL ON *.*  FROM 'uworld'@'%';
REVOKE ALL ON *.*  FROM 'umyworld'@'%';
grant select on myworld.* to uworld@'%' identified by 'test';     
grant select on world.* to umyworld@'%' identified by 'test';
 
 
To monitor what is happening
---------------------------------
watch -n 1 'mysql -h 127.0.0.1 -P 6032 -uadmin -padmin -t -e "select * from stats_mysql_connection_pool where hostgroup

< 30 order by hostgroup,srv_host desc;" -e " select srv_host,command,avg(time_ms), count(ThreadID) from

stats_mysql_processlist group by srv_host,command;" -e "select * from stats_mysql_users;";

mysql -h 127.0.0.1 -P 6032 -uadmin -padmin -t -e "select * from stats_mysql_global "|egrep -i  "(mirror|memory|stmt)"'  
 
select active,hits,destination_hostgroup, mysql_query_rules.rule_id, match_digest, match_pattern,

   replace_pattern, cache_ttl, apply,flagIn,flagOUT FROM mysql_query_rules

   NATURAL JOIN stats.stats_mysql_query_rules where destination_hostgroup < 30  ORDER BY mysql_query_rules.rule_id; 
select * from stats_mysql_query_digest;
select * from stats_mysql_query_digest_reset;

How ProxySQL deal with schema (and schemaname) Long story

This article is the spin-off of the article How ProxySQL deal with schema (and schemaname)

1 How ProxySQL differes from MySQL in managing explicit default schema declaration? does it respect the -D or USE

MySQL and ProxySQL will behave the same when passing the default schema, setting it as default.

      MySQL
        mysql -uuallworld -ptest -h192.168.1.107 -P 3306 -D test
        select database();
        +------------+
        | database() |
        +------------+
        | test       |
        +------------+
 
        SHOW TABLES;
        +-----------------+
        | Tables_in_test  |
        +-----------------+
        | rewrite_tbtest  |
        | rewrite_tbtest1 |
        +-----------------+
 
      ProxySQL
        mysql -uuallworld -ptest -h127.0.0.1 -P 6033 -D test
        select database();
        +------------+
        | database() |
        +------------+
        | test       |
        +------------+
 
        SHOW TABLES;
        +-----------------+
        | Tables_in_test  |
        +-----------------+
        | rewrite_tbtest  |
        | rewrite_tbtest1 |
        +-----------------+

Passing a different schema from the default one in ProxySQL will override the ProxySQL default:

        MySQL
         ./mysql-3306 -uuallworld -ptest -D mysql
        (uallworld@localhost) [mysql]>select database();
        +------------+
        | database() |
        +------------+
        | mysql      |
        +------------+
        ProxySQL
        [root@rms2 server57S]# ./mysql-3306 -uuallworld -ptest -h192.168.1.50 -P6033 -D mysql
 
        (This email address is being protected from spambots. You need JavaScript enabled to view it.) [mysql]>select database();
        +------------+
        | database() |
        +------------+
        | mysql      |
        +------------+

Connecting without DEFAULT Schema

      MySQL 
        mysql -uuallworld -ptest -h192.168.1.107 -P 3306
        mysql> select database();
          +------------+
          | database() |
          +------------+
          | NULL       |
          +------------+
 
      ProxySQL 
        mysql -uuallworld -ptest -h127.0.0.1 -P 6033
        mysql> select database();
        +------------+
        | database() |
        +------------+
        | test       |
        +------------+

The last is different between MySQL and ProxySQL.

By default ProxySQL will connect the user to his default schema.

Not only, if the default schema is not define:

      +-----------+----------+--------+---------+-------------------+----------------+---------------+------------------------+--------------+---------+----------+-----------------+
      | username  | password | active | use_ssl | default_hostgroup | default_schema | schema_locked | transaction_persistent | fast_forward | backend | frontend | max_connections |
      +-----------+----------+--------+---------+-------------------+----------------+---------------+------------------------+--------------+---------+----------+-----------------+
      | uallworld | test     | 1      | 0       | 10                | NULL           | 0             | 1                      | 0            | 1       | 1        | 10000           |
 
      ProxySQL will point the connection to information_schema.
          mysql> select database();
          +--------------------+
          | database()         |
          +--------------------+
          | information_schema |
          +--------------------+

2 How proxy respect/follow security agains schema

      MySQL
      mysql -uuworld -ptest -h192.168.1.107 -P 3306 -D test
        mysql: [Warning] Using a password on the command line interface can be insecure.
        ERROR 1044 (42000): Access denied for user 'uworld'@'%' to database 'test'
 
      ProxySQL
      mysql -uuworld -ptest -h127.0.0.1 -P 6033 -D test
        mysql: [Warning] Using a password on the command line interface can be insecure.
        Welcome to the MySQL monitor.  Commands end with ; or \g.
        Your MySQL connection id is 483902
        Server version: 5.5.30 (ProxySQL)

And no warnings.

Why?

Because while you directly connect to MySQL, when you connect to ProxySQL you do not actually touch the final server.

You only open a connection to the ProxySQL, issue a query will open a connection and if the user do not have the right grants an error will be returned.

Let us see.

just issuing the command:

1
2
3

        mysql> select database();
        ERROR 1044 (42000): Access denied for user 'uworld'@'%' to database 'test'

As soon as you try to query the MySQL server you got an error.

3 How schemaname filter acts in the query rules?

In MySQL we can easily change the default schema with USE , this action is fully supported by ProxySQL.
But it may have some side effects when using "schemaname" as filter in the query_rules.
If you define a rule that include the default schemaname and the default schema is changed with USE, the rule will not apply, and unpredictable results may happen.
To prevent that ProxySQL has another option in mysql_user "schema_locked" which will prevent the schema to be changed by USE.
This feature is present but not fully implemented, and after a brief conversation with Rene (https://github.com/sysown/proxysql/issues/1133).
For now my advice is to DO NOT use schemaname as filter in the query_rules.

Below the full test case:

    mysql> select active,hits,destination_hostgroup, mysql_query_rules.rule_id, match_digest, match_pattern, replace_pattern, cache_ttl, apply,flagIn,flagOUT FROM mysql_query_rules NATURAL JOIN stats.stats_mysql_query_rules where destination_hostgroup < 30  ORDER BY mysql_query_rules.rule_id;
      +--------+------+-----------------------+---------+---------------------+---------------+-----------------+-----------+-------+--------+---------+
      | active | hits | destination_hostgroup | rule_id | match_digest        | match_pattern | replace_pattern | cache_ttl | apply | flagIN | flagOUT |
      +--------+------+-----------------------+---------+---------------------+---------------+-----------------+-----------+-------+--------+---------+
      | 1      | 0    | 10                    | 10      | ^SELECT.*FOR UPDATE | NULL          | NULL            | NULL      | 1     | 0      | NULL    |
      | 1      | 0    | 11                    | 11      | ^SELECT             | NULL          | NULL            | NULL      | 1     | 0      | NULL    |
      +--------+------+-----------------------+---------+---------------------+---------------+-----------------+-----------+-------+--------+---------+
      2 rows in set (0.00 sec)
 
 
      (This email address is being protected from spambots. You need JavaScript enabled to view it.) [world]>Select * from world.City order by Population DESC limit 5 ;
      +------+-----------------+-------------+--------------+------------+
      | ID   | Name            | CountryCode | District     | Population |
      +------+-----------------+-------------+--------------+------------+
      | 1024 | Mumbai (Bombay) | IND         | Maharashtra  |   10500000 |
      | 2331 | Seoul           | KOR         | Seoul        |    9981619 |
      |  206 | São Paulo       | BRA         | São Paulo    |    9968485 |
      | 1890 | Shanghai        | CHN         | Shanghai     |    9696300 |
      |  939 | Jakarta         | IDN         | Jakarta Raya |    9604900 |
      +------+-----------------+-------------+--------------+------------+
      5 rows in set (0.01 sec)
 
 
 
      mysql> select active,hits,destination_hostgroup, mysql_query_rules.rule_id, match_digest, match_pattern, replace_pattern, cache_ttl, apply,flagIn,flagOUT FROM mysql_query_rules NATURAL JOIN stats.stats_mysql_query_rules where destination_hostgroup < 30  ORDER BY mysql_query_rules.rule_id;
      +--------+------+-----------------------+---------+---------------------+---------------+-----------------+-----------+-------+--------+---------+
      | active | hits | destination_hostgroup | rule_id | match_digest        | match_pattern | replace_pattern | cache_ttl | apply | flagIN | flagOUT |
      +--------+------+-----------------------+---------+---------------------+---------------+-----------------+-----------+-------+--------+---------+
      | 1      | 0    | 10                    | 10      | ^SELECT.*FOR UPDATE | NULL          | NULL            | NULL      | 1     | 0      | NULL    |
      | 1      | 1    | 11                    | 11      | ^SELECT             | NULL          | NULL            | NULL      | 1     | 0      | NULL    | <---
      +--------+------+-----------------------+---------+---------------------+---------------+-----------------+-----------+-------+--------+---------+
      2 rows in set (0.00 sec)
 
      mysql> select * from stats_mysql_query_digest;
      +-----------+------------+----------+--------------------+-----------------------------------------------------------+------------+------------+------------+----------+----------+----------+
      | hostgroup | schemaname | username | digest             | digest_text                                               | count_star | first_seen | last_seen  | sum_time | min_time | max_time |
      +-----------+------------+----------+--------------------+-----------------------------------------------------------+------------+------------+------------+----------+----------+----------+
      | 10        | world      | uworld   | 0x594F2C744B698066 | select USER()                                             | 1          | 1502789864 | 1502789864 | 0        | 0        | 0        |
      | 10        | world      | umyworld | 0x134B0F1E837A0B8B | Select * from world.City order by Population DESC limit ? | 1          | 1502789849 | 1502789849 | 10264    | 10264    | 10264    |
      | 10        | world      | uworld   | 0x02033E45904D3DF0 | show databases                                            | 1          | 1502789863 | 1502789863 | 1750     | 1750     | 1750     |
      | 10        | world      | umyworld | 0x594F2C744B698066 | select USER()                                             | 1          | 1502789745 | 1502789745 | 0        | 0        | 0        |
      | 10        | world      | uworld   | 0x226CD90D52A2BA0B | select @@version_comment limit ?                          | 1          | 1502789863 | 1502789863 | 0        | 0        | 0        |
      | 10        | world      | uworld   | 0x99531AEFF718C501 | show tables                                               | 1          | 1502789863 | 1502789863 | 470      | 470      | 470      |
      | 10        | world      | umyworld | 0x226CD90D52A2BA0B | select @@version_comment limit ?                          | 1          | 1502789745 | 1502789745 | 0        | 0        | 0        |
      | 10        | world      | umyworld | 0x99531AEFF718C501 | show tables                                               | 1          | 1502789745 | 1502789745 | 7427     | 7427     | 7427     |
      | 11        | world      | uworld   | 0x134B0F1E837A0B8B | Select * from world.City order by Population DESC limit ? | 1          | 1502789866 | 1502789866 | 3349     | 3349     | 3349     | <---
      | 10        | world      | umyworld | 0x02033E45904D3DF0 | show databases                                            | 1          | 1502789745 | 1502789745 | 615      | 615      | 615      |
      +-----------+------------+----------+--------------------+-----------------------------------------------------------+------------+------------+------------+----------+----------+----------+
      10 rows in set (0.00 sec)
 
      (This email address is being protected from spambots. You need JavaScript enabled to view it.) [world]>USE information_schema;
      Reading table information for completion of table and column names
      You can turn off this feature to get a quicker startup with -A
 
      Database changed
      (This email address is being protected from spambots. You need JavaScript enabled to view it.) [information_schema]>Select * from world.City order by Population DESC limit 5 ;
      +------+-----------------+-------------+--------------+------------+
      | ID   | Name            | CountryCode | District     | Population |
      +------+-----------------+-------------+--------------+------------+
      | 1024 | Mumbai (Bombay) | IND         | Maharashtra  |   10500000 |
      | 2331 | Seoul           | KOR         | Seoul        |    9981619 |
      |  206 | São Paulo       | BRA         | São Paulo    |    9968485 |
      | 1890 | Shanghai        | CHN         | Shanghai     |    9696300 |
      |  939 | Jakarta         | IDN         | Jakarta Raya |    9604900 |
      +------+-----------------+-------------+--------------+------------+
      5 rows in set (0.01 sec)
 
 
 
 
      mysql> select * from stats_mysql_query_digest;
      +-----------+--------------------+----------+--------------------+-----------------------------------------------------------+------------+------------+------------+----------+----------+----------+
      | hostgroup | schemaname         | username | digest             | digest_text                                               | count_star | first_seen | last_seen  | sum_time | min_time | max_time |
      +-----------+--------------------+----------+--------------------+-----------------------------------------------------------+------------+------------+------------+----------+----------+----------+
      | 10        | information_schema | uworld   | 0x99531AEFF718C501 | show tables                                               | 1          | 1502789929 | 1502789929 | 540      | 540      | 540      |
      | 10        | information_schema | uworld   | 0x02033E45904D3DF0 | show databases                                            | 1          | 1502789929 | 1502789929 | 897      | 897      | 897      |
      | 11        | world              | uworld   | 0x620B328FE9D6D71A | SELECT DATABASE()                                         | 1          | 1502789929 | 1502789929 | 537      | 537      | 537      |
      | 10        | world              | uworld   | 0x594F2C744B698066 | select USER()                                             | 1          | 1502789864 | 1502789864 | 0        | 0        | 0        |
      | 10        | world              | umyworld | 0x134B0F1E837A0B8B | Select * from world.City order by Population DESC limit ? | 1          | 1502789849 | 1502789849 | 10264    | 10264    | 10264    |
      | 10        | world              | uworld   | 0x02033E45904D3DF0 | show databases                                            | 1          | 1502789863 | 1502789863 | 1750     | 1750     | 1750     |
      | 10        | world              | umyworld | 0x594F2C744B698066 | select USER()                                             | 1          | 1502789745 | 1502789745 | 0        | 0        | 0        |
      | 10        | world              | uworld   | 0x226CD90D52A2BA0B | select @@version_comment limit ?                          | 1          | 1502789863 | 1502789863 | 0        | 0        | 0        |
      | 10        | world              | uworld   | 0x99531AEFF718C501 | show tables                                               | 1          | 1502789863 | 1502789863 | 470      | 470      | 470      |
      | 10        | world              | umyworld | 0x226CD90D52A2BA0B | select @@version_comment limit ?                          | 1          | 1502789745 | 1502789745 | 0        | 0        | 0        |
      | 10        | world              | umyworld | 0x99531AEFF718C501 | show tables                                               | 1          | 1502789745 | 1502789745 | 7427     | 7427     | 7427     |
      | 10        | information_schema | uworld   | 0x134B0F1E837A0B8B | Select * from world.City order by Population DESC limit ? | 1          | 1502789933 | 1502789933 | 3707     | 3707     | 3707     | <---
      | 11        | world              | uworld   | 0x134B0F1E837A0B8B | Select * from world.City order by Population DESC limit ? | 1          | 1502789866 | 1502789866 | 3349     | 3349     | 3349     | <---
      | 10        | world              | umyworld | 0x02033E45904D3DF0 | show databases                                            | 1          | 1502789745 | 1502789745 | 615      | 615      | 615      |
      +-----------+--------------------+----------+--------------------+-----------------------------------------------------------+------------+------------+------------+----------+----------+----------+
      14 rows in set (0.01 sec)
 
      mysql> select active,hits,destination_hostgroup, mysql_query_rules.rule_id, match_digest, match_pattern, replace_pattern, cache_ttl, apply,flagIn,flagOUT FROM mysql_query_rules NATURAL JOIN stats.stats_mysql_query_rules where destination_hostgroup < 30  ORDER BY mysql_query_rules.rule_id;
      +--------+------+-----------------------+---------+---------------------+---------------+-----------------+-----------+-------+--------+---------+
      | active | hits | destination_hostgroup | rule_id | match_digest        | match_pattern | replace_pattern | cache_ttl | apply | flagIN | flagOUT |
      +--------+------+-----------------------+---------+---------------------+---------------+-----------------+-----------+-------+--------+---------+
      | 1      | 0    | 10                    | 10      | ^SELECT.*FOR UPDATE | NULL          | NULL            | NULL      | 1     | 0      | NULL    |
      | 1      | 2    | 11                    | 11      | ^SELECT             | NULL          | NULL            | NULL      | 1     | 0      | NULL    |
      +--------+------+-----------------------+---------+---------------------+---------------+-----------------+-----------+-------+--------+---------+
      2 rows in set (0.00 sec)

4 how can I transparently redirect using schema name?

This is not a Schema feature, more one of the things that in ProxySQL are quite easy to set, while close to be impossible if done in plain MySQL.
When connecting directly with MySQL there is no option for you to "play" with GRANTS and schema such that you will transparently allow a user to do an action on a schema/server pair and another on a different schema/server pair.
When using ProxySQL to filter by schemaname is quite trivial.

For instance assuming we have 3 users one is admin of the platform which include 2 separate schemas (or more), each user can access one schema for write (but that can be table as well), and a final slave with reporting information, where all the users that needs to read from other schema except their own can read cross schemas.
While all the select not cross schema mus still got to the Master.
This is not so uncommon, actually with few variant is exactly what one of the customer I spoke last week needs to do.

Let see what we need and how to do it:

user(s) uworld & umyworld need to go to their default schema on Master for Writes.
user(s) uworld & umyworld should go to their default schema on master for direct reads
user(s) uworld & umyworld should go to the slave for reads when the other schema is used

To do this we will need the following rules:

        insert into mysql_query_rules (rule_id,username,destination_hostgroup,active,retries,match_digest,apply) values(10,'uworld',10,1,3,'^SELECT.*FOR UPDATE',1);
        insert into mysql_query_rules (rule_id,username,destination_hostgroup,active,retries,match_digest,apply,FlagOUT,FlagIN) values(11,'uworld',10,1,3,'^SELECT ',0,50,0); 
        insert into mysql_query_rules (rule_id,username,destination_hostgroup,active,retries,match_digest,apply,FlagOUT,FlagIN) values(12,'uworld',11,1,3,'\smyworld.',1,50,50);
 
 
        insert into mysql_query_rules (rule_id,username,destination_hostgroup,active,retries,match_digest,apply) values(13,'umyworld',10,1,3,'^SELECT.*FOR UPDATE',1);
        insert into mysql_query_rules (rule_id,username,destination_hostgroup,active,retries,match_digest,apply,FlagOUT,FlagIN) values(14,'umyworld',10,1,3,'^SELECT ',0,50,0); 
        insert into mysql_query_rules (rule_id,username,destination_hostgroup,active,retries,match_digest,apply,FlagOUT,FlagIN) values(15,'umyworld',11,1,3,'\sworld.',1,50,50);

        LOAD MYSQL QUERY RULES TO RUNTIME;SAVE MYSQL QUERY RULES TO DISK;

To check the behaviour:

        select database();
        update world.City set Population=10500000 where ID=1024;
        update world.Country set LifeExpectancy=62.5 where Code='IND';
        update myworld.CityM set Population=10500001 where ID=1024;
        update myworld.CountryM set LifeExpectancy=0 where Code='IND';
 
        select database();
        Select * from world.City order by Population DESC limit 5 ;
        Select * from myworld.CityM order by Population DESC limit 5 ;
 
        Select City.*, Country.name, Country.LifeExpectancy from world.City as City join world.Country as Country on City.CountryCode=Country.code order by Population DESC limit 5; 
        Select City.*, Country.name, Country.LifeExpectancy from myworld.CityM as City join myworld.CountryM as Country on City.CountryCode=Country.code order by Population DESC limit 5; 
 
        Select City.*, Country.name, Country.LifeExpectancy from world.City as City join myworld.CountryM as Country on City.CountryCode=Country.code order by Population DESC limit 5;

Once I run the test (queries above):

        mysql> select active,hits,destination_hostgroup, mysql_query_rules.rule_id, match_digest, match_pattern, replace_pattern, cache_ttl, apply,flagIn,flagOUT FROM mysql_query_rules NATURAL JOIN stats.stats_mysql_query_rules where destination_hostgroup < 30  ORDER BY mysql_query_rules.rule_id;
        +--------+------+-----------------------+---------+---------------------+---------------+-----------------+-----------+-------+--------+---------+
        | active | hits | destination_hostgroup | rule_id | match_digest        | match_pattern | replace_pattern | cache_ttl | apply | flagIN | flagOUT |
        +--------+------+-----------------------+---------+---------------------+---------------+-----------------+-----------+-------+--------+---------+
        | 1      | 0    | 10                    | 10      | ^SELECT.*FOR UPDATE | NULL          | NULL            | NULL      | 1     | 0      | NULL    |
        | 1      | 5    | 10                    | 11      | ^SELECT             | NULL          | NULL            | NULL      | 0     | 0      | 50      | <-- 5 selects in total OK
        | 1      | 3    | 11                    | 12      | myworld.            | NULL          | NULL            | NULL      | 1     | 50     | 50      | <-- 3 contains term myworld OK
        | 1      | 0    | 10                    | 13      | ^SELECT.*FOR UPDATE | NULL          | NULL            | NULL      | 1     | 0      | NULL    |
        | 1      | 0    | 10                    | 14      | ^SELECT             | NULL          | NULL            | NULL      | 0     | 0      | 50      |
        | 1      | 0    | 11                    | 15      | world.              | NULL          | NULL            | NULL      | 1     | 50     | 50      |
        +--------+------+-----------------------+---------+---------------------+---------------+-----------------+-----------+-------+--------+---------+
        6 rows in set (0.00 sec)
 
 
 
        mysql> select * from stats_mysql_query_digest;                                                                                                                                         
        +-----------+------------+----------+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------+------------+------------+----------+----------+----------+
        | hostgroup | schemaname | username | digest             | digest_text                                                                                                                                                                       | count_star | first_seen | last_seen  | sum_time | min_time | max_time |
        +-----------+------------+----------+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------+------------+------------+----------+----------+----------+
        | 11        | world      | uworld   | 0x921512ADAF79D0FF | Select City.*, Country.name, Country.LifeExpectancy from world.City as City join myworld.CountryM as Country on City.CountryCode=Country.code order by Population DESC limit ?    | 1          | 1502718358 | 1502718358 | 6531     | 6531     | 6531     |
        | 11        | world      | uworld   | 0xE9D26001526F2618 | Select City.*, Country.name, Country.LifeExpectancy from myworld.CityM as City join myworld.CountryM as Country on City.CountryCode=Country.code order by Population DESC limit ? | 1          | 1502718358 | 1502718358 | 6573     | 6573     | 6573     |
        | 10        | world      | uworld   | 0xE846287B5A6B3945 | Select City.*, Country.name, Country.LifeExpectancy from world.City as City join world.Country as Country on City.CountryCode=Country.code order by Population DESC limit ?       | 1          | 1502718358 | 1502718358 | 3181     | 3181     | 3181     |
        | 11        | world      | uworld   | 0x55FFF888F4642D3A | Select * from myworld.CityM order by Population DESC limit ?                                                                                                                      | 1          | 1502718358 | 1502718358 | 7753     | 7753     | 7753     |
        | 10        | world      | uworld   | 0x26DB674419D1E979 | update myworld.CountryM set LifeExpectancy=? where Code=?                                                                                                                         | 1          | 1502718358 | 1502718358 | 257      | 257      | 257      |
        | 10        | world      | uworld   | 0x056615DE2CFD8C8E | update myworld.CityM set Population=? where ID=?                                                                                                                                  | 1          | 1502718358 | 1502718358 | 235      | 235      | 235      |
        | 10        | world      | uworld   | 0x134B0F1E837A0B8B | Select * from world.City order by Population DESC limit ?                                                                                                                         | 1          | 1502718358 | 1502718358 | 3262     | 3262     | 3262     |
        | 10        | world      | uworld   | 0x7A15CC342D54452D | update world.Country set LifeExpectancy=?.? where Code=?                                                                                                                          | 1          | 1502718358 | 1502718358 | 319      | 319      | 319      |
        | 10        | world      | uworld   | 0x500E6F01B02078B6 | update world.City set Population=? where ID=?                                                                                                                                     | 1          | 1502718358 | 1502718358 | 970      | 970      | 970      |
        +-----------+------------+----------+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+------------+------------+------------+----------+----------+----------+

As we can see with this simple set of rules my uworld user was able to perform exactly as expected and able to access the schema from the designated HG.

All the selects with schema "myworld" were redirect to HG 11.

Results details:

        (This email address is being protected from spambots. You need JavaScript enabled to view it.) [(none)]>update world.City set Population=10500000 where ID=1024;
        Query OK, 1 row affected (0.01 sec)
        Rows matched: 1  Changed: 0  Warnings: 0
 
        (This email address is being protected from spambots. You need JavaScript enabled to view it.) [(none)]>update world.Country set LifeExpectancy=62.5 where Code='IND';
        Query OK, 1 row affected (0.00 sec)
        Rows matched: 1  Changed: 0  Warnings: 0
 
        (This email address is being protected from spambots. You need JavaScript enabled to view it.) [(none)]>update myworld.CityM set Population=10500001 where ID=1024;      <--- I am NOT managing this but I could if needed
        ERROR 1142 (42000): UPDATE command denied to user 'uworld'@'192.168.1.50' for table 'CityM'
        (This email address is being protected from spambots. You need JavaScript enabled to view it.) [(none)]>update myworld.CountryM set LifeExpectancy=0 where Code='IND';   <--- I am NOT managing this but I could if needed
        ERROR 1142 (42000): UPDATE command denied to user 'uworld'@'192.168.1.50' for table 'CountryM'
        (This email address is being protected from spambots. You need JavaScript enabled to view it.) [(none)]>
 
        (This email address is being protected from spambots. You need JavaScript enabled to view it.) [(none)]>Select * from world.City order by Population DESC limit 5 ;
        +------+-----------------+-------------+--------------+------------+
        | ID   | Name            | CountryCode | District     | Population |
        +------+-----------------+-------------+--------------+------------+
        | 1024 | Mumbai (Bombay) | IND         | Maharashtra  |   10500000 |
        | 2331 | Seoul           | KOR         | Seoul        |    9981619 |
        |  206 | São Paulo       | BRA         | São Paulo    |    9968485 |
        | 1890 | Shanghai        | CHN         | Shanghai     |    9696300 |
        |  939 | Jakarta         | IDN         | Jakarta Raya |    9604900 |
        +------+-----------------+-------------+--------------+------------+
        5 rows in set (0.00 sec)
 
        (This email address is being protected from spambots. You need JavaScript enabled to view it.) [(none)]>Select * from myworld.CityM order by Population DESC limit 5 ;
        +------+-----------------+-------------+--------------+------------+
        | ID   | Name            | CountryCode | District     | Population |
        +------+-----------------+-------------+--------------+------------+
        | 1024 | Mumbai (Bombay) | IND         | Maharashtra  |   10500001 |
        | 2331 | Seoul           | KOR         | Seoul        |    9981619 |
        |  206 | São Paulo       | BRA         | São Paulo    |    9968485 |
        | 1890 | Shanghai        | CHN         | Shanghai     |    9696300 |
        |  939 | Jakarta         | IDN         | Jakarta Raya |    9604900 |
        +------+-----------------+-------------+--------------+------------+
        5 rows in set (0.01 sec)
 
        (This email address is being protected from spambots. You need JavaScript enabled to view it.) [(none)]>
        (This email address is being protected from spambots. You need JavaScript enabled to view it.) [(none)]>Select City.*, Country.name, Country.LifeExpectancy from world.City as City join world.Country as Country on City.CountryCode=Country.code order by Population DESC limit 5; 
        +------+-----------------+-------------+--------------+------------+-------------+----------------+
        | ID   | Name            | CountryCode | District     | Population | name        | LifeExpectancy |
        +------+-----------------+-------------+--------------+------------+-------------+----------------+
        | 1024 | Mumbai (Bombay) | IND         | Maharashtra  |   10500000 | India       |           62.5 |
        | 2331 | Seoul           | KOR         | Seoul        |    9981619 | South Korea |           74.4 |
        |  206 | São Paulo       | BRA         | São Paulo    |    9968485 | Brazil      |           62.9 |
        | 1890 | Shanghai        | CHN         | Shanghai     |    9696300 | China       |           71.4 |
        |  939 | Jakarta         | IDN         | Jakarta Raya |    9604900 | Indonesia   |           68.0 |
        +------+-----------------+-------------+--------------+------------+-------------+----------------+
        5 rows in set (0.01 sec)
 
        (This email address is being protected from spambots. You need JavaScript enabled to view it.) [(none)]>Select City.*, Country.name, Country.LifeExpectancy from myworld.CityM as City join myworld.CountryM as Country on City.CountryCode=Country.code order by Population DESC limit 5; 
        +------+-----------------+-------------+--------------+------------+-------------+----------------+
        | ID   | Name            | CountryCode | District     | Population | name        | LifeExpectancy |
        +------+-----------------+-------------+--------------+------------+-------------+----------------+
        | 1024 | Mumbai (Bombay) | IND         | Maharashtra  |   10500001 | India       |            0.0 |
        | 2331 | Seoul           | KOR         | Seoul        |    9981619 | South Korea |           74.4 |
        |  206 | São Paulo       | BRA         | São Paulo    |    9968485 | Brazil      |           62.9 |
        | 1890 | Shanghai        | CHN         | Shanghai     |    9696300 | China       |           71.4 |
        |  939 | Jakarta         | IDN         | Jakarta Raya |    9604900 | Indonesia   |           68.0 |
        +------+-----------------+-------------+--------------+------------+-------------+----------------+
        5 rows in set (0.00 sec)
 
        (This email address is being protected from spambots. You need JavaScript enabled to view it.) [(none)]>
        (This email address is being protected from spambots. You need JavaScript enabled to view it.) [(none)]>Select City.*, Country.name, Country.LifeExpectancy from world.City as City join myworld.CountryM as Country on City.CountryCode=Country.code order by Population DESC limit 5; 
        +------+-----------------+-------------+--------------+------------+-------------+----------------+
        | ID   | Name            | CountryCode | District     | Population | name        | LifeExpectancy |
        +------+-----------------+-------------+--------------+------------+-------------+----------------+
        | 1024 | Mumbai (Bombay) | IND         | Maharashtra  |   10500000 | India       |            0.0 |
        | 2331 | Seoul           | KOR         | Seoul        |    9981619 | South Korea |           74.4 |
        |  206 | São Paulo       | BRA         | São Paulo    |    9968485 | Brazil      |           62.9 |
        | 1890 | Shanghai        | CHN         | Shanghai     |    9696300 | China       |           71.4 |
        |  939 | Jakarta         | IDN         | Jakarta Raya |    9604900 | Indonesia   |           68.0 |
        +------+-----------------+-------------+--------------+------------+-------------+----------------+
        5 rows in set (0.00 sec)

Obviosuly doing it on the other user/schema work exacty the same.

Back to main article

Sweet and sour can become bitter

Recently Fred published a post (http://lefred.be/content/mysql-group-replication-is-sweet-but-can-be-sour-if-you-misunderstand-it) in which he was stating, I had publish my blog (http://www.tusacentral.net/joomla/index.php/mysql-blogs/191-group-replication-sweet-a-sour.html) which contains few “misunderstanding”.

All the people that knows me, also know I have no problem to admit if I do a mistake, at the end doing mistake is a way of learning and be afraid of the ones who don’t do them.

But at the same time, I need to have proof of it. As I provide proof of what I wrote with numbers and tests (all available on github).

Let us put down the basis of the discussion with facts, not personal speculation or assertions.

Facts:

1) From MySQL official documentations (https://dev.mysql.com/doc/refman/5.7/en/group-replication-use-cases.html)

Group Replication enables you to create fault-tolerant systems with redundancy by replicating the system state throughout a set of servers. Consequently, even if some of the servers fail, as long it is not all or a majority, the system is still available, and all it could have degraded performance or scalability, it is still available. Server failures are isolated and independent. They are tracked by a group membership service which relies on a distributed failure detector that is able to signal when any servers leave the group, either voluntarily or due to an unexpected halt. There is a distributed recovery procedure to ensure that when servers join the group they are brought up to date automatically. There is no need for server fail-over, and the multi-master update everywhere nature ensures that not even updates are blocked in the event of a single server failure. Therefore MySQL Group Replication guarantees that the database service is continuously available.

2) Still from MySQL (https://dev.mysql.com/worklog/task/?id=9838) Group Replication introduced flow control to avoid excessive buffering and to maintain group members reasonably close to one another. For several reasons, it's more efficient to keep buffering low but, as mentioned before, it is not a requirement that members are kept in sync for replication to work: once a slave becomes delayed it will just increase the number of pending transactions in the relay log.

The flow control mechanism enters the scene to bound the back-log members can accumulate, in terms of transactions to certify and transactions to apply. Once those bounds are exceeded, and while they remain exceeded, the flow-control mechanism limits the throughput of writer members to adjust to the capacity of the slowest members of the group. By imposing conservative bounds it tries to make sure all members are able to keep-up and can return safely to within the bounds defined by the user.

So given the above, I, as “standard” user, read that as “when using MySQL and GR, and setting (eventually) the Flow Control correctly, I will achieve to have a data platform that is continuously available given the use of GR”.

Cool, right? So what I was doing in my tests? Well two main things, one is to see if I can use GR for scaling reads as I was doing (and hundreds of customers as well) with PXC, the other is to see if in case of crash of my Master, I can safely fail-over another slave.

This doesn’t mean I am comparing the way the two product works. I cannot care less at this stage, as I am sure most of the customer will not care. What they care is what they can do SAFELY, and with what. All the mambo-jambo about the technical details (how much sync or async I can be) is not their concern.

So the point was and is… Given I am used to product X can I move to product Y and if so how and what I should be aware of?

Once more I was trying to answer to the question “If GR and InnoDB cluster has to work as alternative to other (virtually) synchronous replication mechanism, what change or shift our customers must consider if they want move from one to the other.”

The outcome of the tests is pointing to answer that, period.

Let us clarify two things more:

I am perfectly aware (Fred talking to you) that GR use a different mechanism for FC, and that the numbers set in the group_replication_flow_control_certifier_threshold/ group_replication_flow_control_applier_threshold are then use to calculate the Quota. Still they are user threshold, and Specifies the number of waiting transactions in the applier/certifier queue that trigger flow control which are connected to the final statements reported above: By imposing conservative bounds it tries to make sure all members are able to keep-up and can return safely to within the bounds defined by the user.
Bound that as for the manual can go from 0 to 2147483648.
As such setting it to 25 (I did try also 1000 or more with even worse results) is perfectly legit and I have some issues in considering it a mistake.

I was measuring the lag with the only tools MySQL/Oracle had give us, in the article I said I had used:
“select @last_rec:=SUBSTRING_INDEX(SUBSTRING_INDEX(SUBSTRING_INDEX( Received_transaction_set,':',-2),':',1),'-',-1) last_received FROM performance_schema.replication_connection_status WHERE Channel_name = 'group_replication_applier'; select (@last_rec - @last_exec) as real_lag"

Which use the only information available at the moment regarding the incoming and the current executed transactions.
Fred says that is not valid because the certification and This means that a transaction listed on last_received_transaction_set is certified and queued to apply on this member. On the other members it may already be certified/applied or soon be.
I agree that it may not be perfect .. and I should have said: is for sure the last apply or soon to be given the certification on the master a node can know about.
But here we are not talking of 1 or 10 entries, but in most cases lag of hundreds or thousands entries. So also if this is not perfect and I can miss in a way or another a couple or entries because still processing the certification, I still think it is a valid way to evaluate the lag given the dimensions we are talking about.

BTW if this is not … well please help us and provide the right way to calculate the REAL lag between each node in a GR cluster and the writing master, with the most accurate precision.

About the comment of the node dimension, well thanks and yes you are right here I forgot to put the details.

The four nodes are VMS on separated hosts, so the gr1/2 where hosted on Host1 while gr3/4 hosted on the another host. Nothing else running on the hosts while test was running. Applications and monitor where running on a 3 host. The exact dimension is not really relevant given all of them were the same.

I want to say that I can setup the perfect environment for GR or PXC or NDB or whatever you are testing and showing how cool that is.

But I think we should consider that real life is far to be like that, and that we must see/test is how a product is behaving in adverse conditions, or if not adverse challenging.

Given all the above, in my opinion the tests clearly shown that also if the Flow Control mechanism in GR is the coolest thing ever conceptualize, at the end it does not what it is suppose to, no matter what.

Was I setting the thresholds wrong? Well not sure about that, given the results I got.

If I have 4 nodes and 2 of them (so … no majority) are lagging behind of hundreds or even thousands of entries, while my threshold is in the order of hundreds or less, this means that the cool mechanism is not doing his job, period.

One think is write down whatever about this or that but another is doing tests and provide numbers and evidences, not just words.

As said in a comment (not published in the Fred blog) I am more than happy to perform more tests and do them in conjunctions with anyone else.

Then if I am wrong I am wrong… but until I have different evidence, just saying a car can fly because it has very nice and colorful wings, doesn’t make it fly.

chitty-1

MySQL Blogs

Using ProxySQL

How?

The rules

What the impact will be?

Queries/sec global

Queries/sec per server

QueryProcessor time taken/Query processed total

QueryProcessor efficiency per query.

Use match_digest

Queries/sec global

Queries/sec per server

QueryProcessor time taken/Query processed total

QueryProcessor efficiency per query.

What can be done? Use DIGEST instead.

Queries/sec global

Queries/sec per server

QueryProcessor time taken/Query processed total

QueryProcessor efficiency per query.

Conclusions

Overview

Why this blog

Now that we had established we have what we need, let us test, the solution.

Controlled failover for maintenance

What if there is no other server in same segment?

Server crash

Conclusions

Reference

Schemaname and ProxySQL

Short story

Summarizing

Annex 1

1 How ProxySQL differes from MySQL in managing explicit default schema declaration? does it respect the -D or USE

2 How proxy respect/follow security agains schema

3 How schemaname filter acts in the query rules?

4 how can I transparently redirect using schema name?

More Articles ...

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