# Monitoring replication lag for MySQL read replicas
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For MySQL read replicas, you can monitor replication lag in Amazon CloudWatch by viewing the Amazon RDS `ReplicaLag` metric. The `ReplicaLag` metric reports the value of the `Seconds_Behind_Master` field of the `SHOW REPLICA STATUS` command. 

Common causes for replication lag for MySQL are the following: 
+ A network outage.
+ Writing to tables that have different indexes on a read replica. If the `read_only` parameter is set to `0` on the read replica, replication can break if the read replica becomes incompatible with the source DB instance. After you've performed maintenance tasks on the read replica, we recommend that you set the `read_only` parameter back to `1`.
+ Using a nontransactional storage engine such as MyISAM. Replication is only supported for the InnoDB storage engine on MySQL.

When the `ReplicaLag` metric reaches 0, the replica has caught up to the source DB instance. If the `ReplicaLag` metric returns -1, then replication is currently not active. `ReplicaLag` = -1 is equivalent to `Seconds_Behind_Master` = `NULL`.