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  6. What Is the Time Delay for Primary/Standby Synchronization?

What Is the Time Delay for Primary/Standby Synchronization?

A replication delay occurs when the slaves cannot keep up with the updates occurring on the master. The replication delay is a positive value in seconds if the slave SQL and I/O thread are running. It is NULL (undefined or unknown) if the slave SQL thread is not running, or if the SQL thread has consumed all of the relay log and the slave I/O thread is running.

The delay for primary/standby synchronization cannot be calculated using a formula. 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

Generally, synchronization delay is in seconds. However, if the primary DB instance is under a heavy load within a set period and executes a large number of transactions per second, the synchronization delay may increase.

The method of checking data consistency between primary and standby DB instances varies depending on the DB engine being used:

  • MySQL: Locate the target primary/standby DB instances on the Instance Management page. The Replication Source is the primary DB instance name. When Replication Status is Normal, view Replication Delay (s) to obtain the value of the primary/standby synchronization delay.
  • PostgreSQL: View Replication Lag on the Cloud Eye console to obtain the value of the primary/standby synchronization delay.

    For details, see Configuring Displayed Metrics.

  • Microsoft SQL Server: No method is currently available for checking data consistency between DB instances.