A read replica is a PostgreSQL standby that receives replicated data and serves reads while the primary accepts writes. It can scale read traffic, but replication lag makes its view of the database temporarily stale. section 5.6
The stale-balance mistake
Suppose alpha is the primary and beta is a read replica configured with a 60-second apply delay. Beta reports an account balance of 100; alpha then changes it to 90. A later read from beta still returns 100, so application logic applies a +50 bonus and writes 150 to alpha. The result is an incorrect business decision, not a PostgreSQL ACID violation: the application attempted atomic work across two database nodes. section 5.6
