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:
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:
For details, see Configuring Displayed Metrics.