Cross-Region Replication: When and Why
Cross-region replication sounds like resilience but is sometimes complexity for its own sake. Pick when the data, not the buzzword, demands it.
Three good reasons
Disaster recovery beyond what multi-AZ provides. Some failures are regional, not just zonal.
Latency. Replicate read replicas to the regions where users live.
Compliance. Specific data must reside in specific geographies; replication enforces it.
Two reasons it is theatre
- “Resilience” without RTO/RPO targets. Without the targets, the replica is paid-for backup nobody tests.
- For workloads multi-AZ already covers. Multi-AZ within one region handles 99% of zone failures at fraction of the cost.
The consistency tradeoff
Synchronous cross-region replication kills write latency. The Atlantic is 80ms one way; your write doubles.
Asynchronous replication accepts a window of data loss on failover (RPO). Most teams pick this and accept seconds-to-minutes of loss.
Read-only replicas plus cross-region failover with manual promotion is the realistic posture for most workloads.
When to combine with multi-AZ
Multi-AZ first; cross-region for the workloads that genuinely need it. Most apps need multi-AZ; few need cross-region.
When you do cross-region, document the RPO/RTO and rehearse. Otherwise it is decoration.
Antipatterns
- Cross-region without rehearsing failover. Replication does not equal recovery.
- Synchronous replication for non-trivial workloads. Latency tax kills the user experience.
- Confusing replication with backup. Replicas inherit deletes; backups do not.
What to do this week
Three moves. (1) Pick the most exposed instance of the pattern in your environment. (2) Apply the lightest fix and measure for one week. (3) Schedule a quarterly review so the discipline does not rot.