Back to part 1

Now that we had seen how to setup our environment with PXC8 is time to see how our setup will behave and what can be done in case of issues.

We will now analyse the following common situations:
- Put a node in maintenance or remove a node and insert it back
- node crash
- network failure
- proxysql node crash

The following tests are done using a java application connecting using straight JDBC connection and no connection pooling. I choose to use that more than a benchmarking tool such as sysbench because I was looking for something as close to a real application more than a benchmark. Especially I was interested in having dependencies cross requests and real data retrieve and error management.

As soon as we start the schedule script, the script checks the nodes and identify that we have writer_is_also_reader=0 so it removes the entry for the current writer

2020/07/03 06:43:50.130:[INFO] Writer is also reader disabled removing node from reader Hostgroup 192.168.1.5;3306;201;5000 Retry #1
2020/07/03 06:43:50.138:[WARN] DELETE node:192.168.1.5;3306;201;5000 SQL: DELETE from mysql_servers WHERE hostgroup_id=201 AND hostname='192.168.1.5' AND port='3306'
2020/07/03 06:43:50.143:[INFO] END EXECUTION Total Time(ms):277.705907821655

Test 1 Put a node in maintenance

PXC has a very useful feature pxc_maint_mode to deal with maintenance and to notify applications and midlevel architectural blocks (such as ProxySQL) that a node is going to be under maintenance.

With pxc_maint_mode you can specifies the maintenance mode for taking a node down without adjusting settings in ProxySQL.

The following values are available:

• DISABLED: This is the default state that tells ProxySQL to route traffic to the node as usual.
• SHUTDOWN: This state is set automatically when you initiate node shutdown.
• MAINTENANCE: You can manually change to this state if you need to perform maintenance on a node without shutting it down.

The First test is to put a reader in maintenance and put it back.
current scenario in ProxySQL:

+--------+--------------+-------------+--------+----------+----------+--------+---------+-------------+
| weight | hostgroup_id | srv_host    | status | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed |
+--------+--------------+-------------+--------+----------+----------+--------+---------+-------------+
| 10000  | 200          | 192.168.1.5 | ONLINE | 2        | 3        | 5      | 0	| 5           |
| 10000  | 201          | 192.168.1.7 | ONLINE | 2        | 15       | 17     | 0	| 17          |
| 10000  | 201          | 192.168.1.6 | ONLINE | 0        | 12       | 12     | 0	| 12          |
| 998    | 8200         | 192.168.1.7 | ONLINE | 0        | 0        | 0      | 0	| 0           |
| 999    | 8200         | 192.168.1.6 | ONLINE | 0        | 0        | 0      | 0.      | 0           |
| 1000   | 8200         | 192.168.1.5 | ONLINE | 0        | 0        | 0      | 0	| 0           |
| 1000   | 8201         | 192.168.1.7 | ONLINE | 0        | 0        | 0      | 0	| 0           |
| 1000   | 8201         | 192.168.1.6 | ONLINE | 0        | 0        | 0      | 0	| 0           |
| 1000   | 8201         | 192.168.1.5 | ONLINE | 0        | 0        | 0      | 0	| 0           |
+--------+--------------+-------------+--------+----------+----------+--------+---------+-------------+

Putting down the 192.168.1.6 node:

(root localhost) [(none)]>set global pxc_maint_mode=maintenance;
Query OK, 0 rows affected (10.00 sec)

+--------+--------------+-------------+--------------+----------+----------+--------+---------+-------------+
| weight | hostgroup_id | srv_host    | status       | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed |
+--------+--------------+-------------+--------------+----------+----------+--------+---------+-------------+
| 10000  | 200          | 192.168.1.5 | ONLINE       | 0        | 5        | 5      | 0       | 5           |
| 10000  | 201          | 192.168.1.7 | ONLINE       | 1        | 15       | 17     | 0       | 17          |
| 10000  | 201          | 192.168.1.6 | OFFLINE_SOFT | 0        | 0        | 12     | 0       | 12          | 
| 998    | 8200         | 192.168.1.7 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 999    | 8200         | 192.168.1.6 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8200         | 192.168.1.5 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.7 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.6 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.5 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
+--------+--------------+-------------+--------------+----------+----------+--------+---------+-------------+

Node is in OFFLINE_SOFT as such it will allow any existing connection to finish while not accepting any new.
We can wait for running connection to end, then do whatever kind of maintenance we need without affecting our production.

Once done we can move the node back:

[(none)]>set global pxc_maint_mode=disabled;
Query OK, 0 rows affected (0.00 sec)

In the script log we will see that the node is identify as ready to be put back:

2020/07/03 06:58:23.871:[INFO] Evaluate nodes state 192.168.1.6;3306;201;1000 Retry #1
2020/07/03 06:58:23.882:[WARN] Move node:192.168.1.6;3306;201;1000 SQL: UPDATE mysql_servers SET status='ONLINE' WHERE hostgroup_id=201 AND hostname='192.168.1.6' AND port='3306'
2020/07/03 06:58:23.887:[INFO] END EXECUTION Total Time(ms):265.045881271362

 In all this we never had a moment of service interruption.

The above test was the easy one.

Let us now see what happens if we put the writer in maintenance.

This is a much more impacting action, given the node is accepting write transactions and is in single mode.
Ler us put the writer 192.168.1.5 in maintenance:

set global pxc_maint_mode=maintenance;
Query OK, 0 rows affected (10.00 sec)

 And in few seconds:

+--------+--------------+-------------+--------------+----------+----------+--------+---------+-------------+
| weight | hostgroup_id | srv_host    | status       | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed |
+--------+--------------+-------------+--------------+----------+----------+--------+---------+-------------+
| 999    | 200          | 192.168.1.6 | ONLINE       | 3        | 2        | 5      | 0       | 5           |
| 10000  | 200          | 192.168.1.5 | OFFLINE_SOFT | 0        | 0        | 5      | 0       | 5           |
| 10000  | 201          | 192.168.1.7 | ONLINE       | 4        | 12       | 17     | 0       | 17          |
| 998    | 8200         | 192.168.1.7 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 999    | 8200         | 192.168.1.6 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8200         | 192.168.1.5 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.7 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.6 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.5 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
+--------+--------------+-------------+--------------+----------+----------+--------+---------+-------------+

 What happened?

In the script log:

...
2020/07/03 08:11:22.110:[INFO] END EXECUTION Total Time(ms):231.402158737183
2020/07/03 08:11:24.299:[WARN] PXC maintenance on single writer, is asking for failover. Fail-over in action Using Method = 1
2020/07/03 08:11:24.307:[WARN] Move node:192.168.1.5;3306;200;3020 SQL: UPDATE mysql_servers SET status='OFFLINE_SOFT' WHERE hostgroup_id=200 AND hostname='192.168.1.5' AND port='3306'
2020/07/03 08:11:24.313:[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 200 Server details: 192.168.1.6:3306:HG8200
2020/07/03 08:11:24.313:[INFO] This Node Try to become a WRITER promoting to HG 200 192.168.1.6:3306:HG 8200
2020/07/03 08:11:24.313:[WARN] DELETE from writer group as: SQL:DELETE from mysql_servers where hostgroup_id in (200,9200) AND STATUS = 'ONLINE'
2020/07/03 08:11:24.720:[WARN] Move node:192.168.1.6:33069992000 SQL:INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.1.6',200,3306,999,2000);
2020/07/03 08:11:24.720:[WARN] !!!! FAILOVER !!!!!
 Cluster was without WRITER I have try to restore service promoting a node
2020/07/03 08:11:24.720:[INFO] END EXECUTION Total Time(ms):685.911178588867
...

The script identify the need to shift production to another node.
It set the current writer as offline in the staging environment and identify which node from the special group 8000 is the appropriate replacement.

Then push all the changes to runtime
All is done in ~700 ms, so the whole process takes less then a second and the production was not impacted. 

Test 2 writer node crash

Note: In the text below MySQL is set to work on CEST while system is EDT.

This is of course a much more impacting scenario, we need to keep in to account not only the the situation of the node in ProxySQL, but the need for PXC to rebuild the Primary view getting quorum etc..

Initial picture:

+--------+--------------+-------------+--------------+----------+----------+--------+---------+-------------+
| weight | hostgroup_id | srv_host    | status       | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed |
+--------+--------------+-------------+--------------+----------+----------+--------+---------+-------------+
| 999    | 200          | 192.168.1.6 | ONLINE       | 3        | 2        | 5      | 0       | 5           |
| 10000  | 201          | 192.168.1.7 | ONLINE       | 4        | 12       | 17     | 0       | 17          |
| 998    | 8200         | 192.168.1.7 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 999    | 8200         | 192.168.1.6 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8200         | 192.168.1.5 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.7 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.6 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.5 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
+--------+--------------+-------------+--------------+----------+----------+--------+---------+-------------+/pre>

Abruptly killing the writer with kill -9 signal:

[root@galera1h1n6 gal8_2]# ps aux|grep gal8_2
root 5265 0.0 0.0 113452 876 ? S Jun26 0:00 /bin/sh /opt/mysql_templates/PXC8/bin/mysqld_safe --defaults-file=/opt/mysql_instances/gal8_2/my.cnf
mysql 8745 7.7 72.9 9355104 5771476 ? Sl Jun26 882:54 /opt/mysql_templates/PXC8/bin/mysqld

PXC cluster start to identify the issue:

2020-07-04T10:05:41.011531Z 0 [Note] [MY-000000] [Galera] (27260297, 'tcp://192.168.1.5:4567') turning message relay requesting on, nonlive peers: tcp://192.168.1.6:4567

And script as well:

2020/07/04 06:05:41.000:[ERROR] Cannot connect to DBI:mysql:host=192.168.1.6;port=3306 as monitor
2020/07/04 06:05:41.001:[ERROR]  Node is not responding setting it as SHUNNED (ProxySQL bug - #2658)192.168.1.6:3306:HG200

2020/07/04 06:05:41.052:[WARN] PXC maintenance on single writer, is asking for failover. Fail-over in action Using Method = 1
2020/07/04 06:05:41.061:[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 200 Server details: 192.168.1.5:3306:HG8200
2020/07/04 06:05:41.062:[INFO] This Node Try to become a WRITER promoting to HG 200 192.168.1.5:3306:HG 8200
2020/07/04 06:05:41.062:[WARN]  DELETE from writer group as:  SQL:DELETE from mysql_servers where hostgroup_id in (200,9200) AND STATUS = 'ONLINE'

 As said there is also the need from the cluster to rebuild the cluster view and get a quorum (see below):

2020-07-04T10:05:45.685154Z 0 [Note] [MY-000000] [Galera] Current view of cluster as seen by this node
view (view_id(PRIM,27260297,16)
memb {
	27260297,1
	7e4d3144,1
	}
joined {
	}
left {
	}
partitioned {
	3eb94984,1
	}
)

As soon as the view is available the script can perform the failover:

2020/07/04 06:05:46.318:[WARN] Move node:192.168.1.5:330610002000 SQL:INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.1.5',200,3306,1000,2000);
2020/07/04 06:05:46.318:[WARN] !!!! FAILOVER !!!!!
 Cluster was without WRITER I have try to restore service promoting a node
2020/07/04 06:05:46.318:[INFO] END EXECUTION Total Time(ms):5551.42211914062

the whole exercise takes 5 seconds.

Which for a server crash is not bad at all.

In my case given the Java application was design to deal with minimal service interruption with retry loop, I did not had any error, but this depends on how you had wrote the application layer. 

+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+
| weight | hostgroup_id | srv_host    | status  | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed |
+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+
| 1000   | 200          | 192.168.1.5 | ONLINE  | 0        | 4        | 5      | 0       | 5           |
| 10000  | 201          | 192.168.1.7 | ONLINE  | 0        | 26       | 30     | 0       | 28          |
| 998    | 8200         | 192.168.1.7 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 999    | 8200         | 192.168.1.6 | SHUNNED | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8200         | 192.168.1.5 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.7 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.6 | SHUNNED | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.5 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+

Node also if SHUNNED in the special groups will be normally managed and in the need put ONLINE. 

Crash of a reader

 Let us now do the same with a reader

[root galera1h1n7 gal8_3]# date; kill -9 5259 8739
Sat Jul 4 06:43:46 EDT 2020

The script will fail to connect :

2020/07/04 06:43:46.923:[ERROR] Cannot connect to DBI:mysql:host=192.168.1.7;port=3306 as monitor
2020/07/04 06:43:46.923:[ERROR]  Node is not responding setting it as SHUNNED (ProxySQL bug - #2658)192.168.1.7:3306:HG8201

2020-07-04T10:43:47.998377Z 0 [Note] [MY-000000] [Galera] (27260297, 'tcp://192.168.1.5:4567') reconnecting to 7e4d3144 (tcp://192.168.1.7:4567), attempt 0

The node is internally SHUNNED by the script given not accessible, while waiting for ProxySQL to take action and shun the node. All reads request are managed by ProxySQL.
The final picture will be:

+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+
| weight | hostgroup_id | srv_host    | status  | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed |
+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+
| 1000   | 200          | 192.168.1.5 | ONLINE  | 0        | 5        | 6      | 0       | 5           |
| 10000  | 201          | 192.168.1.7 | SHUNNED | 0        | 0        | 30     | 98      | 28          |
| 1000   | 201          | 192.168.1.6 | ONLINE  | 1	   | 24       | 25     | 0	 | 25          |
| 998    | 8200         | 192.168.1.7 | SHUNNED | 0        | 0        | 0      | 0       | 0           |
| 999    | 8200         | 192.168.1.6 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8200         | 192.168.1.5 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.7 | SHUNNED | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.6 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.5 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+

 No service interruption from the read side thanks to ProxySQL.

Test 3 network failure

Network failures are always tricky, they can happen as a simple single instance, or for a large but unpredictable window of time.
Network can have a slow down first such that the node affect the whole cluster, then fully resolve themselves, leaving you with very limited data to understand what and why that happens.

Given that is important to try to take action to limit negative effects as much as possible.
That level of actions are above the scope of a scheduler Script, who is in charge only of the layout of the nodes.
Given that what it should do is not to solve the possible impact at PXC level but reduce the possible confusion in distributing the traffic with ProxySQL.

The initial picture is:

+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+
| weight | hostgroup_id | srv_host    | status  | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed |
+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+
| 1000   | 200          | 192.168.1.5 | ONLINE  | 0        | 5        | 6      | 0       | 5           |
| 10000  | 201          | 192.168.1.7 | SHUNNED | 0        | 0        | 30     | 98      | 28          |
| 1000   | 201          | 192.168.1.6 | ONLINE  | 1	   | 24       | 25     | 0	 | 25          |
| 998    | 8200         | 192.168.1.7 | SHUNNED | 0        | 0        | 0      | 0       | 0           |
| 999    | 8200         | 192.168.1.6 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8200         | 192.168.1.5 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.7 | SHUNNED | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.6 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.5 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+

On node 192.168.1.5 I will stop the network interface.

enp0s8: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
 inet 192.168.1.5 netmask 255.255.255.0 broadcast 192.168.1.255
[vagrant galera1h1n5 ~]$ date;sudo ifdown enp0s8
Sat Jul 4 13:20:32 EDT 2020
Device 'enp0s8' successfully disconnected.

As soon as I stop it, the PXC cluster identify the node is not reachable:

2020-07-04T17:20:35.980786Z 0 [Note] [MY-000000] [Galera] (7e4d3144, 'tcp://192.168.1.7:4567') connection to peer 27260297 with addr tcp://192.168.1.5:4567 timed out, no messages seen in PT3S (gmcast.peer_timeout)

The script is running and must timeout the attempt to connect (6 seconds), and fencing other process to start:

2020/07/04 13:20:36.729:[ERROR] Another process is running using the same HostGroup and settings,
 Or orphan pid file. check in /tmp/proxysql_galera_check_200_W_201_R.pid
2020/07/04 13:20:38.806:[ERROR] Another process is running using the same HostGroup and settings,
 Or orphan pid file. check in /tmp/proxysql_galera_check_200_W_201_R.pid

Finally script node connection time out and failover starts

2020/07/04 13:20:40.699:[ERROR] Cannot connect to DBI:mysql:host=192.168.1.5;port=3306;mysql_connect_timeout=6 as monitor
2020/07/04 13:20:40.699:[ERROR] Node is not responding setting it as SHUNNED (internally) (ProxySQL bug - #2658)192.168.1.5:3306:HG200
2020/07/04 13:20:40.804:[WARN] Fail-over in action Using Method = 1
2020/07/04 13:20:40.805:[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 200 Server details: 192.168.1.6:3306:HG8200
2020/07/04 13:20:40.805:[INFO] This Node Try to become a WRITER promoting to HG 200 192.168.1.6:3306:HG 8200
2020/07/04 13:20:40.805:[WARN] DELETE from writer group as: SQL:DELETE from mysql_servers where hostgroup_id in (200,9200) AND STATUS = 'ONLINE'
2020/07/04 13:20:42.622:[WARN] Move node:192.168.1.6:33069992000 SQL:INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.1.6',200,3306,999,2000);
2020/07/04 13:20:42.623:[WARN] !!!! FAILOVER !!!!!
 Cluster was without WRITER I have try to restore service promoting a node
2020/07/04 13:20:42.623:[INFO] END EXECUTION Total Time(ms):7983.0858707428

Script does failover due to network in 7 seconds, 10 seconds from network issue

final picture:

+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+
| weight | hostgroup_id | srv_host    | status  | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed |
+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+
| 999    | 200          | 192.168.1.6 | ONLINE  | 0        | 5        | 5      | 0       | 5           |
| 10000  | 201          | 192.168.1.7 | ONLINE  | 3        | 16       | 60     | 721     | 28          |
| 998    | 8200         | 192.168.1.7 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 999    | 8200         | 192.168.1.6 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8200         | 192.168.1.5 | SHUNNED | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.7 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.6 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.5 | SHUNNED | 0        | 0        | 0      | 0       | 0           |
+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+

Given the level of complexity a network failure brings, I think this is a pretty good results, also if it will be inevitable to have some application errors given the connection was on the fly, and for that there is no way to prevent, except re-run the whole transaction.

Test 4 ProxySQL node crash

 Last test is what happen if a proxysql node crash? We have set a VIP that we use to connect to ProxySQL (192.168.1.194), and set Keepalived to deal with the move of the VIP cross nodes will that be efficient enough?

Let us try killing one of the ProxySQL node.

Picture before:

+--------+--------------+-------------+--------+----------+----------+--------+---------+-------------+
| weight | hostgroup_id | srv_host    | status | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed |
+--------+--------------+-------------+--------+----------+----------+--------+---------+-------------+
| 999    | 200          | 192.168.1.6 | ONLINE | 3        | 1        | 4      | 0	    | 4           |
| 10000  | 201          | 192.168.1.7 | ONLINE | 3        | 7        | 10     | 0	    | 10          |
| 1000   | 201          | 192.168.1.5 | ONLINE | 0        | 2        | 2      | 0	    | 2           |
| 998    | 8200         | 192.168.1.7 | ONLINE | 0        | 0        | 0      | 0	    | 0           |
| 999    | 8200         | 192.168.1.6 | ONLINE | 0        | 0        | 0      | 0	    | 0           |
| 1000   | 8200         | 192.168.1.5 | ONLINE | 0        | 0        | 0      | 0	    | 0           |
| 1000   | 8201         | 192.168.1.7 | ONLINE | 0        | 0        | 0      | 0	    | 0           |
| 1000   | 8201         | 192.168.1.6 | ONLINE | 0        | 0        | 0      | 0	    | 0           |
| 1000   | 8201         | 192.168.1.5 | ONLINE | 0        | 0        | 0      | 0	    | 0           |
+--------+--------------+-------------+--------+----------+----------+--------+---------+-------------+

I connect on the current Proxy node that is in charge of the `vip and:

[root proxy1 proxysql]# date;kill -9 24890 24891
Sat Jul 4 14:02:29 EDT 2020

In the system log of the next in the chain Proxy server (different machine), Keepalived identify the node is down and swap the VIP:

Jul 4 14:02:31 proxy2 Keepalived_vrrp[6691]: VRRP_Instance(VI_01) forcing a new MASTER election
Jul 4 14:02:32 proxy2 Keepalived_vrrp[6691]: VRRP_Instance(VI_01) Transition to MASTER STATE
Jul 4 14:02:33 proxy2 Keepalived_vrrp[6691]: VRRP_Instance(VI_01) Entering MASTER STATE
Jul 4 14:02:33 proxy2 Keepalived_vrrp[6691]: VRRP_Instance(VI_01) setting protocol VIPs.
Jul 4 14:02:33 proxy2 Keepalived_vrrp[6691]: Sending gratuitous ARP on enp0s8 for 192.168.1.194
Jul 4 14:02:33 proxy2 Keepalived_vrrp[6691]: VRRP_Instance(VI_01) Sending/queueing gratuitous ARPs on enp0s8 for 192.168.1.194
Jul 4 14:02:33 proxy2 Keepalived_vrrp[6691]: Sending gratuitous ARP on enp0s8 for 192.168.1.194
Jul 4 14:02:33 proxy2 Keepalived_vrrp[6691]: Sending gratuitous ARP on enp0s8 for 192.168.1.194

Application identify few connection that were close unexpectedly and try to restore them:

20/07/04 14:02:33 ERROR [ACTIONS2]: ##### Connection was closed at server side unexpectedly. I will try to recover it
20/07/04 14:02:33 ERROR [ACTIONS2]: ##### Connection was closed at server side unexpectedly. I will try to recover it
20/07/04 14:02:33 ERROR [ACTIONS2]: ##### Connection was closed at server side unexpectedly. I will try to recover it
20/07/04 14:02:33 ERROR [ACTIONS2]: ##### Connection was closed at server side unexpectedly. I will try to recover it

So in few seconds ProxySQL on the Proxy2 node is up and running able to manage the traffic:

+--------+--------------+-------------+--------+----------+----------+--------+---------+-------------+
| weight | hostgroup_id | srv_host    | status | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed |
+--------+--------------+-------------+--------+----------+----------+--------+---------+-------------+
| 999    | 200          | 192.168.1.6 | ONLINE | 0        | 6        | 6      | 0	| 6           |
| 10000  | 201          | 192.168.1.7 | ONLINE | 3        | 29       | 32     | 0	| 32          |
| 1000   | 201          | 192.168.1.5 | ONLINE | 0        | 2        | 2      | 0	| 2           |
| 998    | 8200         | 192.168.1.7 | ONLINE | 0        | 0        | 0      | 0	| 0           |
| 999    | 8200         | 192.168.1.6 | ONLINE | 0        | 0        | 0      | 0	| 0           |
| 1000   | 8200         | 192.168.1.5 | ONLINE | 0        | 0        | 0      | 0	| 0           |
| 1000   | 8201         | 192.168.1.7 | ONLINE | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.6 | ONLINE | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.5 | ONLINE | 0        | 0        | 0      | 0	| 0           |
+--------+--------------+-------------+--------+----------+----------+--------+---------+-------------+ 

Full failover done in 4 Seconds !!!!

Also this to me sounds a quite good achievement.

Conclusions

We can achieve very highly customisable setup with the use of the scheduler+script in ProxySQL.

The flexibility we get is such that we can create a very high available solution without the need to accept compromises impose by native galera support.

All this without reducing the efficiency of the solution.
In terms of time we had (all times may have a max additional +2 seconds due the scheduler interval we had define):

• Maintenance on writer - All is done in ~700 ms no service interruption.
• Writer crash -  whole failover takes 5 seconds. It may have the need to retry transactions.
• Reader crash - no service interruption
• Network issue (with PXC cluster quorum) between 7 and 10 seconds
• ProxySQL node crash - 4 seconds to fail over another ProxySQL node

Of course applications must be design to be fault tolerant and retry the transactions if any problem raise, I would say this goes without saying and is in any programming Best Practices.
If you do not do that and fully rely on the data layer, better for you to review your developers team.

In real life, there are frequent cases where getting a running application to work correctly is strongly dependent on consistent write/read operations. This is no issue when using a single data node as a provider, but it becomes more concerning and challenging when adding additional nodes for high availability and/or read scaling.

In the MySQL dimension, I have already described it here in my blog Dirty Reads in High Availability Solution.

We go from the most loosely-coupled database clusters with primary-replica async replication, to the fully tightly-coupled database clusters with NDB Cluster (MySQL/Oracle).

Adding components like ProxySQL to the architecture can, from one side, help in improving high availability, and from the other, it can amplify and randomize the negative effect of a stale read. As such it is crucial to know how to correctly set up the environment to reduce the risk of stale reads, without reducing the high availability.

This article covers a simple HOW-TO for Percona XtraDB Cluster 8.0 (PXC) and ProxySQL, providing an easy to follow guide to obtain no stale reads, without the need to renounce at read, scaling or a high grade of HA thanks to PXC8.

The Architecture

The covered architecture is based on:

• PXC8 cluster compose by 3 nodes
• ProxySQL v2 node in a cluster to avoid a single point of failure
• Virtual IP with KeepAlived see here. If you prefer to use your already-existing load balancer, feel free to do so.
• N number of application nodes, referring to VIP

Installation

Install PXC8

Install ProxySQL

And finally, set the virtual IP as illustrated in the article mentioned above. It is now the time to do the first step towards the non-stale read solution.

Covering Stale Reads

With PXC, we can easily prevent stale reads by setting the parameter to one of the following values wsrep-sync-wait = 1 – 3 – 5 or 7 (default = 0).
We will see what changes in more detail in part 3 of the blog to be published soon.
For now, just set it to wsrep-sync-wait = 1 ;.

The cluster will ensure consistent reads no matter from which node you will write and read.

This is it. So simple!

ProxySQL Requirements

The second step is to be sure we set up our ProxySQL nodes to use:

• One writer a time to reduce the certification conflicts and Brutal Force Abort
• Avoid including the writer in the reader group
• Respect the order I am setting for failover in case of needs

Now here we have a problem; ProxySQL v2 comes with very interesting features like SSL Frontend/backend, support for AWS Aurora …and more. But it also comes with a very poor native PXC support. I have already raised this in my old article on February 19, 2019, and raised other issues with discussions and bug reports.

In short, we cannot trust ProxySQL for a few factors:

• The way it deals with the nodes failover/failback is not customizable
• The order of the nodes is not customizable
• As of this writing, the support to have the writer NOT working as a reader is broken

In the end, the reality is that in order to support PXC/Galera, the use of an external script using the scheduler is more flexible, solid, and trustworthy. As such, the decision is to ignore the native Galera support, and instead focus on the implementation of a more robust script.

For the scope of this article, I have reviewed, updated, and extended my old script.

Percona had also developed a Galera checker script that was part of the ProxySQL-Admin-Tools suite, but that now has been externalized and available in the PerconaLab GitHub.

Setting All Blocks

The setup for this specific case will be based on:

• Rules to perform read-write split.
• One host group to define the writer HG 200
• One host group to define the reader HG 201
• One host group to define candidate writers HG 8200
• One host group to define candidate readers HG 8201

The final architecture will look like this:

ProxySQL Nodes:

Node1 192.168.4.191 public ip
10.0.0.191 internal ip
Node1 192.168.4.192 public ip
10.0.0.192 internal ip
Node1 192.168.4.193 public ip
10.0.0.193 internal ip

VIP 192.168.4.194 public ip

PXC8 Nodes:

pxc1 10.0.0.22
pxc2 10.0.0.23
pxc3 10.0.0.33

Let us configure PXC8 first. Operation one is to create the users for ProxySQL and the script to access the PXC cluster for monitoring.

CREATE USER monitor@'10.0.%' IDENTIFIED BY '';
GRANT USAGE ON *.* TO monitor@'10.0.%';
GRANT SELECT ON performance_schema.* TO monitor@'10.0.%';

CREATE USER monitor@'10.0.%' IDENTIFIED BY '';
GRANT USAGE ON *.* TO monitor@'10.0.%';
GRANT SELECT ON performance_schema.* TO monitor@'10.0.%';

The second step is to configure ProxySQL as a cluster:

Add a user able to connect from remote. This is will require ProxySQL nodes to be restarted.

update global_variables set Variable_Value='admin:admin;cluster1:clusterpass'  where Variable_name='admin-admin_credentials';
SAVE ADMIN VARIABLES TO DISK;

systemctl restart proxysql.

On rotation, do all ProxySQL nodes.

The third part is to set the variables below.

Please note that the value for admin-cluster_mysql_servers_diffs_before_sync is not standard and is set to 1.

update global_variables set variable_value='cluster1' where variable_name='admin-cluster_username';
update global_variables set variable_value='clusterpass' where variable_name='admin-cluster_password';

update global_variables set variable_value=1 where variable_name='admin-cluster_mysql_servers_diffs_before_sync';
update global_variables set Variable_Value=0  where Variable_name='mysql-hostgroup_manager_verbose';
update global_variables set Variable_Value='true'  where Variable_name='mysql-query_digests_normalize_digest_text';
update global_variables set Variable_Value='8.0.19'  where Variable_name='mysql-server_version';
LOAD ADMIN VARIABLES TO RUN;SAVE ADMIN VARIABLES TO DISK;
LOAD MYSQL VARIABLES TO RUN;SAVE MYSQL VARIABLES TO DISK;

It is now time to define the ProxySQL cluster nodes:;

INSERT INTO proxysql_servers (hostname,port,weight,comment) VALUES('192.168.4.191',6032,100,'PRIMARY');
INSERT INTO proxysql_servers (hostname,port,weight,comment) VALUES('192.168.4.192',6032,100,'SECONDARY');
INSERT INTO proxysql_servers (hostname,port,weight,comment) VALUES('192.168.4.193',6032,100,'SECONDARY');
LOAD PROXYSQL SERVERS TO RUN;SAVE PROXYSQL SERVERS TO DISK;

Check the ProxySQL logs and you should see that the nodes are now linked: 2020-05-25 09:24:30 [INFO] Cluster: clustering with peer 192.168.4.192:6032 . Remote version: 2.1.0-159-g0bdaa0b . Self version: 2.1.0-159-g0bdaa0b 2020-05-25 09:24:30 [INFO] Cluster: clustering with peer 192.168.4.193:6032 . Remote version: 2.1.0-159-g0bdaa0b . Self version: 2.1.0-159-g0bdaa0b

Once this is done let us continue the setup, adding the PXC nodes and all the different host groups to manage the architecture:

delete from mysql_servers where hostgroup_id in (200,201);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections,comment) VALUES ('10.0.0.22',200,3306,10000,2000,'default writer');
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections,comment) VALUES ('10.0.0.23',201,3306,10000,2000,'reader');    
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections,comment) VALUES ('10.0.0.33',201,3306,10000,2000,'reader');        
LOAD MYSQL SERVERS TO RUNTIME; SAVE MYSQL SERVERS TO DISK;    
    
delete from mysql_servers where hostgroup_id in (8200,8201);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections,comment) VALUES ('10.0.0.22',8200,3306,1000,2000,'Writer preferred');
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections,comment) VALUES ('10.0.0.23',8200,3306,999,2000,'Second preferred');    
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections,comment) VALUES ('10.0.0.33',8200,3306,998,2000,'Thirdh and last in the list');      
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections,comment) VALUES ('10.0.0.22',8201,3306,1000,2000,'reader setting');
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections,comment) VALUES ('10.0.0.23',8201,3306,1000,2000,'reader setting');    
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections,comment) VALUES ('10.0.0.33',8201,3306,1000,2000,'reader setting');       
LOAD MYSQL SERVERS TO RUNTIME; SAVE MYSQL SERVERS TO DISK;

You can see that as mentioned we have two host groups to manage the cluster 8200 and 8201.

Those two host groups work as templates and they will change only by us manually.

The 8200 host group weight defines the order of the writers from higher to lower.

Given that node 10.0.0.22 with weight 1000 is the preferred writer.

At the moment of writing, I chose to NOT implement automatic fail-back.

I will illustrate later how to trigger that manually.

Once we have all the servers up, lets’ move on and create the users:

insert into mysql_users (username,password,active,default_hostgroup,default_schema,transaction_persistent,comment) values ('app_test2','test',1,200,'mysql',1,'application test user');
insert into mysql_users (username,password,active,default_hostgroup,default_schema,transaction_persistent,comment) values ('dba','dbapw',1,200,'mysql',1,'generic dba for application');
LOAD MYSQL USERS TO RUNTIME;SAVE MYSQL USERS TO DISK;

And the query rules to have Read/Write split:

insert into mysql_query_rules (rule_id,proxy_port,destination_hostgroup,active,retries,match_digest,apply) values(1040,6033,200,1,3,'^SELECT.*FOR UPDATE',1);
insert into mysql_query_rules (rule_id,proxy_port,destination_hostgroup,active,retries,match_digest,apply) values(1042,6033,201,1,3,'^SELECT.*$',1);
LOAD MYSQL QUERY RULES TO RUN;SAVE MYSQL QUERY RULES TO DISK;

The final step is to set the scheduler:

INSERT  INTO scheduler (id,active,interval_ms,filename,arg1) values (10,0,2000,"/var/lib/proxysql/galera_check.pl","-u=cluster1 -p=clusterpass -h=127.0.0.1 -H=200:W,201:R -P=6032  --main_segment=1 --debug=0  --log=/var/lib/proxysql/galeraLog --active_failover=1 --single_writer=1 --writer_is_also_reader=0");
LOAD SCHEDULER TO RUNTIME;SAVE SCHEDULER TO DISK;

 
Let analyze the script parameters:

The schedule ID. id: 10
As a best practice, always keep the scheduler script not active by default and enable it only when in the need. active: 0
Interval is how often the scheduler should execute the script; it needs to be often enough to reduce the time the service is in a degraded state, but not so often to be noisy. An interval of two seconds is normally a good start. interval_ms: 2000
The location of the script that must be set as executable filename: /var/lib/proxysql/galera_check.pl

Given the scheduler limitation to five arguments, we collapse all the parameters in one and let the script then parse them. arg1: -u=cluster1 -p=clusterpass -h=127.0.0.1 -H=200:W,201:R -P=6032 –retry_down=2 –retry_up=1 –main_segment=2 –debug=0 –log=/var/lib/proxysql/galeraLog –active_failover=1 –single_writer=1 –writer_is_also_reader=0

The parameters we pass here are:

The credential to connect to ProxySQL: -u=cluster1 -p=clusterpass -h=127.0.0.1 -P=6032
The host group definition: -H=200:W,201:R This setting is necessary because you can have multiple script running serving multiple clusters.
The retry settings are to reduce the risk of false positive, say a network hiccup or other momentary events against which you do not want to take action: –retry_down=2 –retry_up=1

Given the script is segment-aware, you need to declare the main segment that is serving the applications: –main_segment=1 
Log location/name the final name will be the combination of this plus the host groups (ie galeraLog_200_W_201_R.log ) : –log=/var/lib/proxysql/galeraLog
If script should deal with failover or not and what type (read documentation/help for details): –active_failover=1
If the script should support SINGLE writer (default recommended), or multiple writer nodes: –single_writer=1
Is (are) the writers also working as readers or fully write dedicated: –writer_is_also_reader=0
Once we are confident our settings are right, let us put the script in production: 

update scheduler set active=1 where id=10;
LOAD SCHEDULER TO RUNTIME;
 

Warning

One important thing to keep in mind is that ProxySQL scheduler IS NOT part of the cluster synchronization, as such we must manually configure that part on each node. Once the script runs, any change done inside ProxySQL to the mysql_server table will be kept in sync by the ProxySQL cluster. It is strongly recommended to not mix ProxySQL nodes in the cluster and sparse one, as this may cause unexpected behavior.

Conclusions

At this point, your PXC8 cluster architecture is fully running and will provide you with a very high level of HA and write isolation while preserving the read scaling capabilities.

In part two of this post, we will see the cluster in action and how it behaves in case of standard operations like backup or emergency cases like node crashes.

Continue in part 2