Using Two Replication Channels for NDB Cluster Replication 4. Finally, instruct the replica to begin applying replication by issuing this command from the  mysql  client on the replica: mysql R >  START SLAVE; In MySQL 8.0.22 and later, you can also use the following statement: mysql R >  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 23.6.7, “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  slave_allow_batching  system variable 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. 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. Note Batching can be turned on and off at runtime. To activate it at runtime, you can use either of these two statements: SET GLOBAL slave_allow_batching = 1; SET GLOBAL slave_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 slave_allow_batching = 0; SET GLOBAL slave_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 ’slave%’; +---------------------------+-------+ | Variable_name | Value | +---------------------------+-------+ | slave_allow_batching | ON | | slave_compressed_protocol | OFF | | slave_load_tmpdir | /tmp | | slave_net_timeout | 3600 | | slave_skip_errors | OFF | | slave_transaction_retries | 10 | +---------------------------+-------+ 6 rows in set (0.00 sec) 23.6.7 Using Two Replication Channels for NDB Cluster Replication In a more complete example scenario, we envision two replication channels to provide redundancy and thereby guard against possible failure of a single replication channel. This requires a total of four replication 4627