Automatic Failover With MariaDB Monitor

The MariaDB Monitor is not only capable of monitoring the state of a MariaDB master-slave cluster but is also capable of performing failover and switchover. In addition, in some circumstances it is capable of rejoining a master that has gone down and later reappears.

Note that the failover (and switchover and rejoin) functionality is only supported in conjunction with GTID-based replication and initially only for simple topologies, that is, 1 master and several slaves.

The failover, switchover and rejoin functionality are inherent parts of the MariaDB Monitor, but neither automatic failover nor automatic rejoin are enabled by default.

The following examples have been written with the assumption that there are four servers - server1, server2, server3 and server4 - of which server1 is the initial master and the other servers are slaves. In addition there is a monitor called TheMonitor that monitors those servers.

Somewhat simplified, the MaxScale configuration file would look like:

[server1]
type=server
address=192.168.121.51
port=3306
protocol=MariaDBBackend

[server2]
...

[server3]
...

[server4]
...

[TheMonitor]
type=monitor
module=mariadbmon
servers=server1,server2,server3,server4
...

Manual Failover

If everything is in order, the state of the cluster will look something like this:

$ maxctrl list servers
┌─────────┬─────────────────┬──────┬─────────────┬─────────────────┐
│ Server  │ Address         │ Port │ Connections │ State           │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server1 │ 192.168.121.51  │ 3306 │ 0           │ Master, Running │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server2 │ 192.168.121.190 │ 3306 │ 0           │ Slave, Running  │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server3 │ 192.168.121.112 │ 3306 │ 0           │ Slave, Running  │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server4 │ 192.168.121.201 │ 3306 │ 0           │ Slave, Running  │
└─────────┴─────────────────┴──────┴─────────────┴─────────────────┘

If the master now for any reason goes down, then the cluster state will look like this:

$ maxctrl list servers
┌─────────┬─────────────────┬──────┬─────────────┬────────────────┐
│ Server  │ Address         │ Port │ Connections │ State          │
├─────────┼─────────────────┼──────┼─────────────┼────────────────┤
│ server1 │ 192.168.121.51  │ 3306 │ 0           │ Down           │
├─────────┼─────────────────┼──────┼─────────────┼────────────────┤
│ server2 │ 192.168.121.190 │ 3306 │ 0           │ Slave, Running │
├─────────┼─────────────────┼──────┼─────────────┼────────────────┤
│ server3 │ 192.168.121.112 │ 3306 │ 0           │ Slave, Running │
├─────────┼─────────────────┼──────┼─────────────┼────────────────┤
│ server4 │ 192.168.121.201 │ 3306 │ 0           │ Slave, Running │
└─────────┴─────────────────┴──────┴─────────────┴────────────────┘

Note that the status for server1 is Down.

Since failover is by default not enabled, the failover mechanism must be invoked manually:

$ maxctrl call command mariadbmon failover TheMonitor
OK

There are quite a few arguments, so let's look at each one separately call command indicates that it is a module command that is to be invoked, mariadbmon indicates the module whose command we want to invoke (that is the MariaDB Monitor), failover is the command we want to invoke, and TheMonitor is the first and only argument to that command, the name of the monitor as specified in the configuration file.

The MariaDB Monitor will now autonomously deduce which slave is the most appropriate one to be promoted to master, promote it to master and modify the other slaves accordingly.

If we now check the cluster state we will see that one of the remaining slaves has been made into master.

$ maxctrl list servers
┌─────────┬─────────────────┬──────┬─────────────┬─────────────────┐
│ Server  │ Address         │ Port │ Connections │ State           │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server1 │ 192.168.121.51  │ 3306 │ 0           │ Down            │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server2 │ 192.168.121.190 │ 3306 │ 0           │ Master, Running │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server3 │ 192.168.121.112 │ 3306 │ 0           │ Slave, Running  │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server4 │ 192.168.121.201 │ 3306 │ 0           │ Slave, Running  │
└─────────┴─────────────────┴──────┴─────────────┴─────────────────┘

If server1 now reappears, it will not be rejoined to the cluster, as shown by the following output:

$ maxctrl list servers
┌─────────┬─────────────────┬──────┬─────────────┬─────────────────┐
│ Server  │ Address         │ Port │ Connections │ State           │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server1 │ 192.168.121.51  │ 3306 │ 0           │ Running         │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server2 │ 192.168.121.190 │ 3306 │ 0           │ Master, Running │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server3 │ 192.168.121.112 │ 3306 │ 0           │ Slave, Running  │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server4 │ 192.168.121.201 │ 3306 │ 0           │ Slave, Running  │
└─────────┴─────────────────┴──────┴─────────────┴─────────────────┘

Had auto_rejoin=true been specified in the monitor section, then an attempt to rejoin server1 would have been made.

In MaxScale 2.2.1, rejoining cannot be initiated manually, but in a subsequent version a command to that effect will be provided.

Automatic Failover

To enable automatic failover, simply add auto_failover=true to the monitor section in the configuration file.

[TheMonitor]
type=monitor
module=mariadbmon
servers=server1,server2,server3,server4
auto_failover=true
...

When everything is running fine, the cluster state looks like follows:

$ maxctrl list servers
┌─────────┬─────────────────┬──────┬─────────────┬─────────────────┐
│ Server  │ Address         │ Port │ Connections │ State           │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server1 │ 192.168.121.51  │ 3306 │ 0           │ Master, Running │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server2 │ 192.168.121.190 │ 3306 │ 0           │ Slave, Running  │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server3 │ 192.168.121.112 │ 3306 │ 0           │ Slave, Running  │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server4 │ 192.168.121.201 │ 3306 │ 0           │ Slave, Running  │
└─────────┴─────────────────┴──────┴─────────────┴─────────────────┘

If server1 now goes down, failover will automatically be performed and an existing slave promoted to new master.

$ maxctrl list servers
┌─────────┬─────────────────┬──────┬─────────────┬────────────────────────┐
│ Server  │ Address         │ Port │ Connections │ State                  │
├─────────┼─────────────────┼──────┼─────────────┼────────────────────────┤
│ server1 │ 192.168.121.51  │ 3306 │ 0           │ Down                   │
├─────────┼─────────────────┼──────┼─────────────┼────────────────────────┤
│ server2 │ 192.168.121.190 │ 3306 │ 0           │ Master, Slave, Running │
├─────────┼─────────────────┼──────┼─────────────┼────────────────────────┤
│ server3 │ 192.168.121.112 │ 3306 │ 0           │ Slave, Running         │
├─────────┼─────────────────┼──────┼─────────────┼────────────────────────┤
│ server4 │ 192.168.121.201 │ 3306 │ 0           │ Slave, Running         │
└─────────┴─────────────────┴──────┴─────────────┴────────────────────────┘

If you are continuously monitoring the server states, you may notice for a brief period that the state of server1 is Down and the state of server2 is still Slave, Running.

Rejoin

To enable automatic rejoin, simply add auto_rejoin=true to the monitor secion in the configuration file.

[TheMonitor]
type=monitor
module=mariadbmon
servers=server1,server2,server3,server4
auto_rejoin=true
...

When automatic rejoin is enabled, the MariaDB Monitor will attempt to rejoin a failed master as a slave, if it reappears.

When everything is running fine, the cluster state looks like follows:

$ maxctrl list servers
┌─────────┬─────────────────┬──────┬─────────────┬─────────────────┐
│ Server  │ Address         │ Port │ Connections │ State           │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server1 │ 192.168.121.51  │ 3306 │ 0           │ Master, Running │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server2 │ 192.168.121.190 │ 3306 │ 0           │ Slave, Running  │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server3 │ 192.168.121.112 │ 3306 │ 0           │ Slave, Running  │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server4 │ 192.168.121.201 │ 3306 │ 0           │ Slave, Running  │
└─────────┴─────────────────┴──────┴─────────────┴─────────────────┘

Assuming auto_failover=true has been specified in the configuration file, when server1 goes down for some reason, failover will be performed and we end up with the following cluster state:

$ maxctrl list servers
┌─────────┬─────────────────┬──────┬─────────────┬─────────────────┐
│ Server  │ Address         │ Port │ Connections │ State           │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server1 │ 192.168.121.51  │ 3306 │ 0           │ Down            │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server2 │ 192.168.121.190 │ 3306 │ 0           │ Master, Running │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server3 │ 192.168.121.112 │ 3306 │ 0           │ Slave, Running  │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server4 │ 192.168.121.201 │ 3306 │ 0           │ Slave, Running  │
└─────────┴─────────────────┴──────┴─────────────┴─────────────────┘

If server1 now reappears, the MariaDB Monitor will detect that and attempt to rejoin the old master as a slave.

Whether rejoining will succeed depends upon the actual state of the old master. For instance, if the old master was modified and the changes had not been replicated to the new master, before the old master went down, then automatic rejoin will not be possible.

If rejoining can be performed, then the cluster state will end up looking like:

$ maxctrl list servers
┌─────────┬─────────────────┬──────┬─────────────┬─────────────────┐
│ Server  │ Address         │ Port │ Connections │ State           │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server1 │ 192.168.121.51  │ 3306 │ 0           │ Slave, Running  │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server2 │ 192.168.121.190 │ 3306 │ 0           │ Master, Running │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server3 │ 192.168.121.112 │ 3306 │ 0           │ Slave, Running  │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server4 │ 192.168.121.201 │ 3306 │ 0           │ Slave, Running  │
└─────────┴─────────────────┴──────┴─────────────┴─────────────────┘

Switchover

Switchover is for cases when you explicitly want to move the master role from one server to another.

If we continue from the cluster state at the end of the previous example and want to make server1 master again, then we must issue the following command:

$ maxctrl call command mariadbmon switchover TheMonitor server1 server2
OK

There are quite a few arguments, so let's look at each one separately call command indicates that it is a module command that is to be invoked, mariadbmon indicates the module whose command we want to invoke, switchover is the command we want to invoke, and TheMonitor is the first argument to the command, the name of the monitor as specified in the configuration file, server1 is the second argument to the command, the name of the server we want to make into master, and server2 is the third argument to the command, the name of the current master.

If the command executes successfully, we will end up with the following cluster state:

$ maxctrl list servers
┌─────────┬─────────────────┬──────┬─────────────┬─────────────────┐
│ Server  │ Address         │ Port │ Connections │ State           │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server1 │ 192.168.121.51  │ 3306 │ 0           │ Master, Running │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server2 │ 192.168.121.190 │ 3306 │ 0           │ Slave, Running  │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server3 │ 192.168.121.112 │ 3306 │ 0           │ Slave, Running  │
├─────────┼─────────────────┼──────┼─────────────┼─────────────────┤
│ server4 │ 192.168.121.201 │ 3306 │ 0           │ Slave, Running  │
└─────────┴─────────────────┴──────┴─────────────┴─────────────────┘

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