17.6.8. Implementing Failover with MySQL Cluster Replication

In the event that the primary Cluster replication process fails, it is possible to switch over to the secondary replication channel. The following procedure describes the steps required to accomplish this.

Obtain the time of the most recent global checkpoint (GCP). That is, you need to determine the most recent epoch from the ndb_apply_status table on the slave cluster, which can be found using the following query:
```
mysqlS'> SELECT @latest:=MAX(epoch)      ->        FROM mysql.ndb_apply_status;
```
Using the information obtained from the query shown in Step 1, obtain the corresponding records from the ndb_binlog_index table on the master cluster.

In MySQL Cluster NDB 7.3, you can use the following query to obtain the needed records from the master's ndb_binlog_index table:
```
mysqlM'> SELECT      ->     @file:=SUBSTRING_INDEX(next_file,
                    '/', -1),      ->     @pos:=next_position      -> FROM mysql.ndb_binlog_index      -> WHERE epoch = @latest      -> ORDER BY epoch ASC LIMIT 1;
```
These are the records saved on the master since the failure of the primary replication channel. We have employed a user variable @latest here to represent the value obtained in Step 1. Of course, it is not possible for one mysqld instance to access user variables set on another server instance directly. These values must be "plugged in" to the second query manually or in application code.

Important

You must ensure that the slave mysqld is started with --slave-skip-errors=ddl_exist_errors before executing START SLAVE. Otherwise, replication may stop with duplicate DDL errors.
Now it is possible to synchronize the secondary channel by running the following query on the secondary slave server:
```
mysqlS'> CHANGE MASTER TO      ->     MASTER_LOG_FILE='@file',      ->     MASTER_LOG_POS=@pos;
```
Again we have employed user variables (in this case @file and @pos) to represent the values obtained in Step 2 and applied in Step 3; in practice these values must be inserted manually or using application code that can access both of the servers involved.

Note

@file is a string value such as '/var/log/mysql/replication-master-bin.00001', and so must be quoted when used in SQL or application code. However, the value represented by @pos must not be quoted. Although MySQL normally attempts to convert strings to numbers, this case is an exception.
You can now initiate replication on the secondary channel by issuing the appropriate command on the secondary slave mysqld:
```
mysqlS'> START SLAVE;
```

Once the secondary replication channel is active, you can investigate the failure of the primary and effect repairs. The precise actions required to do this will depend upon the reasons for which the primary channel failed.

Warning

The secondary replication channel is to be started only if and when the primary replication channel has failed. Running multiple replication channels simultaneously can result in unwanted duplicate records being created on the replication slaves.

If the failure is limited to a single server, it should (in theory) be possible to replicate from M to S', or from M' to S; however, this has not yet been tested.