Multi-region architecture
Run in several parts of the world for lower latency and to survive a whole region going down.
Why more than one region
- Latency: a round trip between Mumbai and Virginia takes about 200 ms over real networks, before any work happens. Serving users from a nearby region removes that.
- Disaster recovery: whole cloud regions do go down. A second region keeps you running.
- Data residency: some laws require users' data to stay in their country or region.
Patterns
| Pattern | How it works | Trade-off |
|---|---|---|
| Active-passive | One region serves traffic; a standby region gets replicated data and takes over if the first fails | Simple, but no latency benefit and some data loss on failover (async replication) |
| Active-active, read local | Every region serves reads from local replicas; writes go to one home region | Fast reads everywhere; writes from far away are slow |
| Active-active, write local | Every region accepts writes and replicates to the others | Fastest, but concurrent writes to the same data conflict and must be resolved |
| Partition by geography | Each user's data lives in their home region (EU users in the EU) | Local and compliant; cross-region interactions need care |
The pieces
- Geo-DNS or anycast sends each user to the nearest healthy region.
- A CDN already makes static content global; multi-region is about the dynamic part.
- Cross-region replication is almost always asynchronous (synchronous would add a cross-continent round trip to every write), so expect lag. See CAP.
- Conflict resolution for write-local setups: last-write-wins, merging (CRDTs), or routing each record's writes to one owner region.
- Practise failover. A standby region nobody has tested usually doesn't work when it's needed.
In the interview
- Only go multi-region if the requirements need it: global users with latency targets, strict uptime, or data laws.
- Say which pattern and why, and be explicit that cross-region replication is asynchronous.
- Mention how users are routed (geo-DNS) and how you'd fail over.