25.7.6 Starting NDB Cluster Replication (Single Replication Channel)

This section outlines the procedure for starting NDB Cluster replication using a single replication channel.

Start the MySQL replication source server by issuing this command, where id is this server's unique ID (see Section 25.7.2, “General Requirements for NDB Cluster Replication”):
```
shellS> mysqld --ndbcluster --server-id=id \
        --log-bin --ndb-log-bin &
```
This starts the server's mysqld process with binary logging enabled using the proper logging format. It is also necessary to enable logging of updates to NDB tables explicitly, using the --ndb-log-bin option.
Note
You can also start the source with --binlog-format=MIXED, in which case row-based replication is used automatically when replicating between clusters. Statement-based binary logging is not supported for NDB Cluster Replication (see Section 25.7.2, “General Requirements for NDB Cluster Replication”).
Start the MySQL replica server as shown here:
```
shellR> mysqld --ndbcluster --server-id=id &
```
In the command just shown, id is the replica server's unique ID. It is not necessary to enable logging on the replica.
Note
Unless you want replication to begin immediately, delay the start of the replication threads until the appropriate START REPLICA statement has been issued, as explained in Step 4 below. You can do this by starting the replica with --skip-replica-start.
It is necessary to synchronize the replica server with the source server's replication binary log. If binary logging has not previously been running on the source, run the following statement on the replica:
```
mysqlR> CHANGE REPLICATION SOURCE TO
     -> SOURCE_LOG_FILE='',
     -> SOURCE_LOG_POS=4;
```
This instructs the replica to begin reading the source server's binary log from the log's starting point. Otherwise—that is, if you are loading data from the source using a backup—see Section 25.7.8, “Implementing Failover with NDB Cluster Replication”, for information on how to obtain the correct values to use for SOURCE_LOG_FILE and SOURCE_LOG_POS in such cases.
Finally, instruct the replica to begin applying replication by issuing this command from the mysql client on the replica:
```
mysqlR> START REPLICA;
```
This also initiates the transmission of data and changes from the source to the replica.

It is also possible to use two replication channels, in a manner similar to the procedure described in the next section; the differences between this and using a single replication channel are covered in Section 25.7.7, “Using Two Replication Channels for NDB Cluster Replication”.

It is also possible to improve cluster replication performance by enabling batched updates. This can be accomplished by setting the system variable replica_allow_batching on the replicas' mysqld processes. Normally, updates are applied as soon as they are received. However, the use of batching causes updates to be applied in batches of 32 KB each; this can result in higher throughput and less CPU usage, particularly where individual updates are relatively small.

Note

Batching works on a per-epoch basis; updates belonging to more than one transaction can be sent as part of the same batch.

All outstanding updates are applied when the end of an epoch is reached, even if the updates total less than 32 KB.

Batching can be turned on and off at runtime. To activate it at runtime, you can use either of these two statements:

SET GLOBAL replica_allow_batching = 1;
SET GLOBAL replica_allow_batching = ON;

If a particular batch causes problems (such as a statement whose effects do not appear to be replicated correctly), batching can be deactivated using either of the following statements:

SET GLOBAL replica_allow_batching = 0;
SET GLOBAL replica_allow_batching = OFF;

You can check whether batching is currently being used by means of an appropriate SHOW VARIABLES statement, like this one:

mysql> SHOW VARIABLES LIKE 'replica%';