Replication Lag

Intermediate

How primary-replica replication lag forms, accumulates under write pressure, causes stale reads, and how applications should handle it.

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Steady State: Replica in Sync

Under steady load, a PostgreSQL streaming replica receives WAL records from the primary nearly instantaneously. Replication lag is typically <5ms on a well-provisioned replica in the same datacenter. The replica's LSN (Log Sequence Number) closely tracks the primary's.

Primary LSN

1,000

500 writes/s

Replica LSN

998

500 apply/s

Primary1000

Replica998

Lag: 2ms: in sync

Events

LSN 990

writeP:✓ R:✓

LSN 995

writeP:✓ R:✓

LSN 1,000

writeP:✓ R:·

Steady state: 2ms lag, replica current

Key Takeaways

→Under normal load, streaming replication lag stays under 10ms
→LSN on primary and replica stay within a few records of each other

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