Then, it updates the Replication State field in the Amazon RDS console to Error if replication stops for any reason. Amazon RDS monitors the replication status of your read replica.
This metric shows how far a read replica has fallen behind its source DB instance. You can monitor replication lag in Amazon CloudWatch by viewing the Amazon RDS ReplicaLag metric. For more information, see How streaming replication works for different RDS for PostgreSQL versions. When the read replica can't find the WAL on the primary instance, the read replica is recovered from the archived WAL data in Amazon Simple Storage Service (Amazon S3). Therefore, the read replica checks the WAL logs to replicate the changes done on the primary instance.
The replica DB is created by a special connection that transmits the write ahead log (WAL) data from the source DB instance to the read replica. This means that sometimes the replica can't keep up with the primary DB instance. This read replica DB instance is an asynchronously created physical replica of the source DB instance. RDS for PostgreSQL uses PostgreSQL native streaming replication to create a read-only copy of a source DB instance. You can scale reads for your Amazon RDS for PostgreSQL DB instance by adding read replicas to the instance.
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