What Is the Time Delay for Primary/Standby Replication?

When standby instances cannot keep up with the updates on the primary, this generates replication delay. If the standby SQL and I/O thread are running, the replication delay is a positive value measured in seconds. If the standby SQL thread is not running, or if the SQL thread has consumed all of the relay log and the standby I/O thread is running, then it is NULL (undefined or unknown)

The delay for primary/standby replication cannot be calculated using a formula as the delay is affected by the following factors:

• Network communication status
• Transaction workload on the primary DB instance in transactions per second (TPS)
• The size of the transaction executed by the primary DB instance (this affects the duration of transaction executions)
• Load balancing of the standby DB instance and read replicas

If the primary DB instance has a heavy load for a certain period of time and executes a large number of transactions per second, replication to the standby DB instance will be delayed. This delay is generally a few seconds.

• RDS for MySQL: Click the DB instance name on the Instances page to go to the Basic Information page. The replication source is the primary DB instance. When the replication is normal, view Real-Time Replication Delay to obtain the primary/standby replication delay.
• RDS for PostgreSQL: To check data consistency between the primary and standby DB instances, view Replication Lag on the Cloud Eye console to obtain the value of the primary/standby replication delay.
• RDS for SQL Server: View Replication Delay on the Cloud Eye console to obtain the primary/standby replication delay.