Design an active-active multi-region architecture on AWS. What are the hard parts?
What they are really testing: Principal-level: do you know the concrete AWS building blocks AND that the real difficulty is data consistency and write conflicts, not stateless compute. They want honesty about the trade-offs.
A real interview question
Design an active-active multi-region architecture on AWS. What are the hard parts?
What most people say
drag me
“I would deploy the app in two regions and use Route 53 to send traffic to both, with database replication between them.”
It names the topology but waves away the actual hard problem: how writes are kept consistent across regions and what happens on conflict. "Database replication between them" hides the multi-master conflict question that defines the design.
The follow-ups they ask next
What is the single hardest part of active-active, and why?
Write conflicts and data consistency across regions. Stateless compute replicates trivially; concurrent writes in two regions to the same record force an explicit conflict-resolution strategy (LWW, home-region partitioning, or merge), each with real trade-offs.
When would you steer a team away from active-active toward active-passive?
When the driver is disaster recovery rather than latency or residency. Active-passive (warm standby with fast failover, for example Aurora Global) meets most RTO/RPO needs at far lower cost and complexity, without the multi-master data problem.
What the interviewer is listening for
- Challenges the requirement first
- Names Route 53, DynamoDB Global Tables, Aurora Global, S3 CRR correctly
- Centers data consistency / conflict resolution
- States the CAP choice and tests failover
What sinks the answer
- Jumps to topology with no requirement check
- Hand-waves "database replication"
- No conflict-resolution or consistency reasoning
If you genuinely do not know
Say this instead of freezing. Reasoning out loud from what you do know beats silence every single time, and a good interviewer is listening for exactly that.
“First [challenge it: active-active is costly, is the driver latency, resilience, or residency? often active-passive DR is better]. Routing: [Route 53 latency/geo + health checks]. [Stateless compute in each region is the easy part]. Data is the hard part, per class: [DynamoDB Global Tables = multi-master, last-writer-wins, eventual; Aurora Global = fast replicas but one write region; S3 CRR]. Own [the conflict-resolution + CAP choice, keep ops idempotent, and test failover].”
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