Read Replicas Can Cause Silent Data Bugs When Replication Lag Is Ignored

Database read replicas are a widely used scaling technique that routes read traffic away from the primary database, but they come with a key drawback: replication lag. Because data written to the primary takes time to propagate to replicas, applications that immediately read after a write may retrieve stale data. This creates a confusing bug where users appear to lose their submitted changes, even though the write actually succeeded. The problem rarely surfaces during testing or low-traffic conditions, making it difficult to catch before production. Mitigation strategies exist — such as routing post-write reads to the primary — but each introduces its own trade-offs in complexity or reduced scaling benefit.