MariaDB MaxScale Administration Tutorial

The purpose of this tutorial is to introduce the MariaDB MaxScale Administrator to a few of the common administration tasks. This is intended to be an introduction for administrators who are new to MariaDB MaxScale and not a reference to all the tasks that may be performed.

Starting and Stopping MariaDB MaxScale

Systemd

Most modern operating systems support the Systemd interface.

Starting MaxScale:

systemctl start maxscale

Stopping MaxScale:

systemctl stop maxscale

The MaxScale service file is located in /lib/systemd/system/maxscale.service.

SysV

Legacy platforms should use the service interface to start MaxScale.

Starting MaxScale:

service maxscale start

Stopping MaxScale:

service maxscale stop

Additional command line arguments can be passed to MariaDB MaxScale with a configuration file placed at /etc/sysconfig/maxscale on RPM installations and /etc/default/maxscale file on DEB installations. Set the arguments in a variable called MAXSCALE_OPTIONS and remember to surround the arguments with quotes. The file should only contain environment variable declarations.

MAXSCALE_OPTIONS="--logdir=/home/maxscale/logs --piddir=/tmp --syslog=no"

Note that this is only supported on legacy SysV systems.

Checking The Status Of The MariaDB MaxScale Services

It is possible to use the maxctrl command to obtain statistics about the services that are running within MaxScale. The maxctrl command list services will give very basic information regarding services. This command may be either run in interactive mode or passed on the maxctrl command line.

$ maxctrl list services
┌────────────────────────┬────────────────┬─────────────┬───────────────────┬────────────────────────────────────┐
│ Service                │ Router         │ Connections │ Total Connections │ Servers                            │
├────────────────────────┼────────────────┼─────────────┼───────────────────┼────────────────────────────────────┤
│ CLI                    │ cli            │ 1           │ 1                 │                                    │
├────────────────────────┼────────────────┼─────────────┼───────────────────┼────────────────────────────────────┤
│ RW-Split-Router        │ readwritesplit │ 1           │ 1                 │ server1, server2, server3, server4 │
├────────────────────────┼────────────────┼─────────────┼───────────────────┼────────────────────────────────────┤
│ RW-Split-Hint-Router   │ readwritesplit │ 1           │ 1                 │ server1, server2, server3, server4 │
├────────────────────────┼────────────────┼─────────────┼───────────────────┼────────────────────────────────────┤
│ SchemaRouter-Router    │ schemarouter   │ 1           │ 1                 │ server1, server2, server3, server4 │
├────────────────────────┼────────────────┼─────────────┼───────────────────┼────────────────────────────────────┤
│ Read-Connection-Router │ readconnroute  │ 1           │ 1                 │ server1                            │
└────────────────────────┴────────────────┴─────────────┴───────────────────┴────────────────────────────────────┘

Network listeners count as a user of the service, therefore there will always be one user per network port in which the service listens. More details can be obtained by using the "show service" command.

Persistent Connections

When clients who are accessing a database system through MariaDB MaxScale make frequent short connections, there may be a benefit in using persistent connections. This feature is controlled by two configuration values that are specified per server in the relevant server section of the configuration file. The configuration options are persistpoolmax and persistmaxtime.

Normally, when a client connection is terminated, all the related back end database connections are also terminated. If the persistpoolmax options is set to a non-zero integer, then up to that number of connections will be kept in a pool for that server. When a new connection is requested by the system to handle a client session, then a connection from the pool will be used if possible.

The connection will only be taken from the pool if it has been there for no more than persistmaxtime seconds. The connection will also be discarded if it has been disconnected by the back end server. Connections will be selected so that they match the user name and protocol for the new request.

Starting with MaxScale 2.1, when a MySQL protocol connection is taken from the pool, the backend protocol module resets the session state. This allows persistent connections to be used with no functional limitations.

The session state is reset when the first outgoing network transmission is done. This lazy initialization of the persistent connections allows MaxScale to take multiple new connections into use but only initialize the ones that it actually needs.

Please note that in versions before 2.1 the persistent connections may give a different environment when compared to a fresh connection. For example, if the previous use of the connection issued a "USE mydatabase;" statement then this setting will be carried over into the reuse of the same connection. For many applications this will not be noticeable, since each request will assume that nothing has been set and will issue fresh requests such as "USE" to establish the desired configuration. In exceptional cases this feature could be a problem.

It is possible to have pools for as many servers as you wish, with configuration values in each server section.

What Clients Are Connected To MariaDB MaxScale

To determine what client are currently connected to MariaDB MaxScale, you can use the list sessions command within maxctrl. This will give you IP address and the ID of the session for that connection. As with any maxctrl command this can be passed on the command line or typed interactively in maxctrl.

$ maxctrl list sessions
┌────┬─────────┬──────────────────┬──────────────────────────┬──────┬─────────────────┐
│ Id │ User    │ Host             │ Connected                │ Idle │ Service         │
├────┼─────────┼──────────────────┼──────────────────────────┼──────┼─────────────────┤
│ 6  │ maxuser │ ::ffff:127.0.0.1 │ Thu Aug 27 10:39:16 2020 │ 4    │ RW-Split-Router │
└────┴─────────┴──────────────────┴──────────────────────────┴──────┴─────────────────┘

Rotating the Log File

MariaDB MaxScale logs messages of different priority into a single log file. With the exception if error messages that are always logged, whether messages of a particular priority should be logged or not can be enabled via the maxctrl interface or in the configuration file. By default, MaxScale keeps on writing to the same log file. To prevent the file from growing indefinitely, the administrator must take action.

The name of the log file is maxscale.log. When the log is rotated, MaxScale closes the current log file and opens a new one using the same name.

Log file rotation is achieved by use of the rotate logs command in maxctrl.

maxctrl rotate logs

As there currently is only the maxscale log, that is the only one that will be rotated.

This may be integrated into the Linux logrotate mechanism by adding a configuration file to the /etc/logrotate.d directory. If we assume we want to rotate the log files once per month and wish to keep 5 log files worth of history, the configuration file would look as follows.

/var/log/maxscale/maxscale.log {
monthly
rotate 5
missingok
nocompress
sharedscripts
postrotate
\# run if maxscale is running
if test -n "`ps acx|grep maxscale`"; then
/usr/bin/maxctrl rotate logs
fi
endscript
}

MariaDB MaxScale will also rotate all of its log files if it receives the USR1 signal. Using this the logrotate configuration script can be rewritten as

/var/log/maxscale/maxscale.log {
monthly
rotate 5
missingok
nocompress
sharedscripts
postrotate
kill -USR1 `cat /var/run/maxscale/maxscale.pid`
endscript
}

In older versions MaxScale renamed the log file, behavior which is not fully compliant with the assumptions of logrotate and may lead to issues, depending on the used logrotate configuration file. From version 2.1 onward, MaxScale will not itself rename the log file, but when the log is rotated, MaxScale will simply close and reopen (and truncate) the same log file. That will make the behavior fully compliant with logrotate.

Taking A Database Server Out Of Use

MariaDB MaxScale supports the concept of maintenance mode for servers within a cluster, this allows for planned, temporary removal of a database from the cluster within the need to change the MariaDB MaxScale configuration.

To achieve the removal of a database server you can use the set server command in maxctrl to set the maintenance mode flag for the server. This may be done interactively within maxctrl or by passing the command on the command line.

maxctrl set server dbserver3 maintenance

This will cause MariaDB MaxScale to stop routing any new requests to the server, however if there are currently requests executing on the server these will not be interrupted.

To bring the server back into service use the "clear server" command to clear the maintenance mode bit for that server.

sudo maxctrl clear server dbserver3 maintenance

If multiple MariaDB MaxScale instances are configured to use the node them maintenance mode must be set within each MariaDB MaxScale instance.