Read/Write Splitting with MariaDB Replication

Environment & Solution Space

The object of this tutorial is to have a system that appears to the clients of MariaDB MaxScale as if there was a single database behind MariaDB MaxScale. MariaDB MaxScale will split the statements such that write statements will be sent to the current master server in the replication cluster and read statements will be balanced across the rest of the slave servers.

Setting up MariaDB MaxScale

The first part of this tutorial is covered in MariaDB MaxScale Tutorial. Please read it and follow the instructions for setting up MariaDB MaxScale with the type of cluster you want to use.

Once you have MariaDB MaxScale installed and the database users created, the configuration file for MariaDB MaxScale can be written.

Creating Your MariaDB MaxScale Configuration

MariaDB MaxScale configuration is defined in the file maxscale.cnf located in the directory /etc. If you have installed MaxScale in the default location the file path should be /etc/maxscale.cnf. This file is not created as part of the installation process and must be manually created. A template file, which may be used as a basis for your configuration, exists within the /usr/share/maxscale directory.

A global section, marked maxscale, is included within every MariaDB MaxScale configuration file. The section is used to set the values of various process-wide parameters, for example the number of worker threads.

[maxscale]
threads=4

The first step is to create a Read/Write Splitter service. Create a section in your configuration file and set the type to service. The section header is the name of the service and should be meaningful to the administrator. Names may contain whitespace.

[Splitter Service]
type=service

The router module needed for this service is named readwritesplit. The service must contain a list of backend server names. The server names are the headers of server sections in the configuration file and not the physical hostnames or addresses of the servers.

[Splitter Service]
type=service
router=readwritesplit
servers=dbserv1, dbserv2, dbserv3

The final step in the service section is to add the username and password that will be used to populate the user data from the database cluster. There are two options for representing the password: either plain text or encrypted passwords. To use encrypted passwords, a set of keys for encryption/decryption must be generated. To generate the keys use the maxkeys command and pass the name of the secrets file containing the keys.

maxkeys /var/lib/maxscale/.secrets

Once the keys have been created, use the maxpasswd command to generate the encrypted password.

maxpasswd plainpassword

96F99AA1315BDC3604B006F427DD9484

The username and password, either encrypted or in plain text, are stored in the service section.

[Splitter Service]
type=service
router=readwritesplit
servers=dbserv1, dbserv2, dbserv3
user=maxscale
passwd=96F99AA1315BDC3604B006F427DD9484

This completes the service definition. To have the service accept network connections, a listener must be associated with it. The listener is defined in its own section. The type should be listener with an entry service defining the name of the service the listener is listening for. A service may have multiple listeners.

[Splitter Listener]
type=listener
service=Splitter Service

A listener must also define the protocol module it will use for the incoming network protocol, currently this should be the MariaDBClient protocol for all database listeners. The listener may then supply a network port to listen on and/or a socket within the file system.

[Splitter Listener]
type=listener
service=Splitter Service
protocol=MariaDBClient
port=3306
socket=/tmp/ClusterMaster

An address parameter may be given if the listener is required to bind to a particular network address when using hosts with multiple network addresses. The default behavior is to listen on all network interfaces.

The next stage in the configuration is to define the backend servers. The definitions include how to connect to the servers. A section is created for each server and it contains: type set to server, the network address and port, and the protocol to use. Currently, the protocol module for all database connections is MariaDBBackend.

[dbserv1]
type=server
address=192.168.2.1
port=3306
protocol=MariaDBBackend

[dbserv2]
type=server
address=192.168.2.2
port=3306
protocol=MariaDBBackend

[dbserv3]
type=server
address=192.168.2.3
port=3306
protocol=MariaDBBackend

For MariaDB MaxScale to monitor the servers using the correct monitoring mechanisms a monitor section should be written. This section defines the monitor module to use and the monitored servers. The section type should be set to monitor. Parameters added include: the list of servers to monitor and the username and password the monitor module should use when connecting.

[Replication Monitor]
type=monitor
module=mariadbmon
servers=dbserv1, dbserv2, dbserv3
user=maxscale
passwd=96F99AA1315BDC3604B006F427DD9484

Similarly to the password definition in the server either a plain text or an encrypted password may be used.

The final stage in the configuration is to add the service which used by the maxadmin command to connect to MariaDB MaxScale for monitoring and administration purposes. The example below shows a service section and a listener section.

[CLI]
type=service
router=cli

[CLI Listener]
type=listener
service=CLI
protocol=maxscaled
socket=default

Starting MariaDB MaxScale

Upon completion of the configuration MariaDB MaxScale is ready to be started. This may either be done manually by running the maxscale command or via the service interface.

% maxscale

or

% service maxscale start

Check the error log in /var/log/maxscale to see if any errors are detected in the configuration file and to confirm MariaDB MaxScale has been started. Also the maxadmin command may be used to confirm that MariaDB MaxScale is running and the services, listeners etc have been correctly configured.

% maxadmin list services

Services.
--------------------------+----------------------+--------+---------------
Service Name              | Router Module        | #Users | Total Sessions
--------------------------+----------------------+--------+---------------
Splitter Service          | readwritesplit       |      1 |     1
CLI                       | cli                  |      2 |     2
--------------------------+----------------------+--------+---------------

% maxadmin list servers

Servers.
-------------------+-----------------+-------+-------------+--------------------
Server             | Address         | Port  | Connections | Status
-------------------+-----------------+-------+-------------+--------------------
dbserv1            | 192.168.2.1     |  3306 |           0 | Running, Slave
dbserv2            | 192.168.2.2     |  3306 |           0 | Running, Master
dbserv3            | 192.168.2.3     |  3306 |           0 | Running, Slave
-------------------+-----------------+-------+-------------+--------------------

% maxadmin list listeners

Listeners.
---------------------+--------------------+-----------------+-------+--------
Service Name         | Protocol Module    | Address         | Port  | State
---------------------+--------------------+-----------------+-------+--------
Splitter Service     | MariaDBClient      | *               |  3306 | Running
CLI                  | maxscaled          | localhost       |  6603 | Running
---------------------+--------------------+-----------------+-------+--------

MariaDB MaxScale is now ready to start accepting client connections and routing them to the master or slaves within your cluster. Other configuration options, that can alter the criteria used for routing, are available. These include monitoring the replication lag within the cluster and routing only to slaves that are within a predetermined delay from the current master or using weights to obtain unequal balancing operations. These options may be found in the MariaDB MaxScale Configuration Guide. More details on the use of maxadmin can be found in the document MaxAdmin - The MariaDB MaxScale Administration & Monitoring Client Application.

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