The Ultimate Guide to High Availability with MariaDB
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MariaDB MaxScale Administration Tutorial

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 MariaDB MaxScale

There are several ways to start MariaDB MaxScale, the most convenient mechanism is probably using the Linux service interface. When a MariaDB MaxScale package is installed, the package manager will also install a script in /etc/init.d which may be used to start and stop MariaDB MaxScale either directly or via the service interface.

    $ service maxscale start

or

    $ /etc/init.d/maxscale start

It is also possible to start MariaDB MaxScale by executing the maxscale command itself. Running the executable /usr/bin/maxscale will result in MariaDB MaxScale running as a daemon process, unattached to the terminal in which it was started and using configuration files that it finds in the /etc directory.

Options may be passed to the MariaDB MaxScale binary that alter this default behavior. For a full list of all parameters, refer to the MariaDB MaxScale help output by executing maxscale --help.

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"

Stopping MariaDB MaxScale

There are numerous ways in which MariaDB MaxScale can be stopped; using the service interface, killing the process or by using the maxadmin utility.

Stopping MariaDB MaxScale with the service interface is simply a case of using the service stop command or calling the init.d script with the stop argument.

    $ service maxscale stop

or

    $ /etc/init.d/maxscale stop

MariaDB MaxScale will also stop gracefully if it received a terminate signal, to find the process id of the MariaDB MaxScale server use the ps command or read the contents of the maxscale.pid file located in the /var/run/maxscale directory.

    $ kill `cat /var/run/maxscale/maxscale.pid`

In order to shutdown MariaDB MaxScale using the maxadmin command you may either connect with maxadmin in interactive mode or pass the "shutdown maxscale" command you wish to execute as an argument to maxadmin.

    $ maxadmin shutdown maxscale

Checking The Status Of The MariaDB MaxScale Services

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

    $ maxadmin
    MaxScale> list services

    Services.

    --------------------------+----------------------+--------+---------------

    Service Name              | Router Module        | #Users | Total Sessions

    --------------------------+----------------------+--------+---------------

    RWSplitter                | readwritesplit       |      2 |     4

    Cassandra                 | readconncouter       |      1 |     1

    CLI                       | cli                  |      2 |     2

    --------------------------+----------------------+--------+---------------

    MaxScale>

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 disconnectedby 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 clients" command within maxadmin. This will give you IP address and the ID’s of the DCB and session for that connection. As with any maxadmin command this can be passed on the command line or typed interactively in maxadmin.

    $ maxadmin list clients

    Client Connections

    -----------------+------------------+----------------------+------------

     Client          | DCB              | Service              | Session

    -----------------+------------------+----------------------+------------

     127.0.0.1       |   0x7fe694013410 | CLI                  | 0x7fe69401ac10

    -----------------+------------------+----------------------+------------

    $

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 maxadmin 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 "flush log" or “flush logs” command in maxadmin.

    $ maxadmin flush logs

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

The maxscale log can also be flushed explicitly.

    $ maxadmin
    MaxScale> flush log maxscale
    MaxScale>

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/maxadmin flush logs
fi
endscript
}

Note:

If 'root' user is no longer available for maxadmin connection and for example 'user1' is one of the allowed users, the maxadmin command should be run as:

su - user1 -c '/usr/bin/maxadmin flush logs'

If listening socket is not the default one, /tmp/maxadmin.sock, use -S option.

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
}

Since MaxScale currently renames the log file, the behaviour 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 onwards, 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 behaviour 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 the maxadmin utility to set the maintenance mode flag for the server. This may be done interactively within maxadmin or by passing the command on the command line.

    MaxScale> set server dbserver3 maintenance
    MaxScale>

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.

    MaxScale> clear server dbserver3 maintenance
    MaxScale>

Note that maintenance mode is not persistent, if MariaDB MaxScale restarts when a node is in maintenance mode a new instance of MariaDB MaxScale will not honor this mode. If multiple MariaDB MaxScale instances are configured to use the node them maintenance mode must be set within each MariaDB MaxScale instance. However if multiple services within one MariaDB MaxScale instance are using the server then you only need set the maintenance mode once on the server for all services to take note of the mode change.

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