MySQL Replication Example

WARNING:

This is only a Proof-Of-Concept example and it is not meant to be used in any production. Use at your own risk.

What is MySQL replication?

Replication enables data from one MySQL database server (the master) to be replicated to one or more MySQL database servers (the slaves). Replication is asynchronous - slaves do not need not be connected permanently to receive updates from the master. This means that updates can occur over long-distance connections and even over temporary or intermittent connections such as a dial-up service. Depending on the configuration, you can replicate all databases, selected databases, or even selected tables within a database.

See: https://dev.mysql.com/doc/refman/en/replication.html

How does this example work?

The provided JSON file (mysql_replica.json) contains a Template resource that groups the Kubernetes and OpenShift resources which are meant to be created. This template will start with one MySQL master server and one slave server.

Persistent storage

In order to provide persistent storage for MySQL, this example requires two persistent volumes of at least 512 MiB each.

The OpenShift cluster administrator needs to create persistent volumes to be claimed when the template is instantiated. Refer to the OpenShift documentation to learn how to create persistent volumes.

Service 'mysql-master'

This resource provides 'headless' Service for the MySQL server(s) which acts as the 'master'. The headless means that the Service does not use IP addresses but it uses the DNS sub-system. This behavior is configured by setting the clusterIP attribute to None.

In this case, you can query the DNS (eg. dig mysql-master A +search +short) to obtain the list of the Service endpoints (the MySQL servers that subscribe to this service).

Service 'mysql-slave'

This resource provides the 'headless' Service for the MySQL servers that the MySQL master uses as 'slaves' which are used to replicate the data from the MySQL master.

You can use the same DNS lookup as mentioned above to obtain the list of the Service endpoints.

ReplicationController 'mysql-master'

This resource defines the PodTemplate of the MySQL server that acts as the 'master'. The Pod uses the openshift/mysql-55-centos7 image, but it sets the special 'entrypoint' named mysqld-master. This will tell the MySQL image to configure the MySQL server as the 'master'.

To configure the 'master', you have to provide the credentials for the user that will act as the 'master' admin. This user has special privileges to add or remove 'slaves'. The other thing you have to provide is the regular MySQL username that you can use to connect to the MySQL server. This user has lower privileges and it is safe to use it in your application.

Optionally you can define MYSQL_DATABASE and MYSQL_ROOT_PASSWORD. The first one sets the name of the initial database that will be created and the MYSQL_USER will be granted access to it. This parameter is optional and if you don't specify it, the database name will default to the value of MYSQL_USER.

If you want to perform administration tasks, you can also set the MYSQL_ROOT_PASSWORD. In that case you will be able to connect to the MySQL server as the 'root' user and create more users or more databases.

Once the MySQL master server is started, it has no slaves preconfigured as the slaves registers automatically.

Note that currently the multiple-master configuration is not supported (even though the mysql-master is defined as ReplicationController. If you increase the number of replicas, then a new MySQL master server is started, but it will not receive any slaves. This will be solved in future.

To check that the master MySQL server is working, you can issue the following command on the master container:

mysql> SHOW MASTER STATUS;
+------------------+----------+--------------+------------------+
| File             | Position | Binlog_Do_DB | Binlog_Ignore_DB |
+------------------+----------+--------------+------------------+
| mysql-bin.000002 |      107 | foo          |                  |
+------------------+----------+--------------+------------------+
1 row in set (0.00 sec)

ReplicationController 'mysql-slave'

This resource defines the PodTemplate of the MySQL servers that act as the slaves to the master server. In the provided JSON example, this Replication Controller starts with 3 slaves. Each slave server first waits for the master server to become available (getting the master server IP using the DNS lookup). Once the master is available, the MySQL 'slave' server is started and connected to the master. The unique server-id configuration property is generated from the unique IP address of the container (and hashed to a number). Each slave must have unique server-id.

Once the slave is running, it will fetch the database and users from the master server, so you don't have to configure the user accounts for this resources.

To check the 'slave' status, you can issue the following command on the slave container:

mysql> SHOW SLAVE STATUS\G
*************************** 1. row ***************************
               Slave_IO_State: Waiting for master to send event
                  Master_Host: 172.17.0.17
                  Master_User: master
                  Master_Port: 3306
                Connect_Retry: 60

This output means that the 'slave' is successfully connected to the 'master' MySQL server running on '172.17.0.17'.

To see the 'slave' hosts from the 'master', you can issue the following command on the 'master' container:

mysql> SHOW SLAVE HOSTS;
+------------+-------------+------+------------+
| Server_id  | Host        | Port | Master_id  |
+------------+-------------+------+------------+
| 3314680171 | 172.17.0.20 | 3306 | 1301393349 |
| 3532875540 | 172.17.0.18 | 3306 | 1301393349 |
+------------+-------------+------+------------+
2 rows in set (0.01 sec)

You can add more slaves if you want, using the following oc command.

$ oc scale rc mysql-slave --replicas=4