Comparison
Read replicavsReplication lag
Read replica
you point the reporting queries at a copy of the database so they stop competing with checkout traffic.
A follower serving read-only queries to offload the primary. It is the first and cheapest scaling move for a read-heavy workload. Everything read from it is stale by the replication lag, and a heavy analytical query on a replica can itself increase that lag for everyone else using it.
Full entry →Replication lag
the replica is forty seconds behind the primary and the report you generated from it is missing this morning's orders.
How far behind a follower is from the leader, measured in time or log position. It grows under write bursts, long transactions, and single-threaded replication apply. It is the number to alert on before you send any read traffic to replicas, and the number that decides whether a failover loses data.
Full entry →