How to design a multi-region, highly available AWS system?

Long Answer

Designing a multi-region, highly available system on AWS requires trade-offs between latency, cost, and fault tolerance. The key is to mix edge services, replication, routing policies, and cost-aware operations into a layered architecture.

1. Edge, entry, and routing
Start with Amazon CloudFront to cache assets and protect origins. Combine with Route 53 latency-based routing or AWS Global Accelerator to direct users to the nearest healthy region. Weighted or geo-based policies let you shift traffic for failover or cost optimization.

2. Active-active vs active-passive
Keep stateless services (web/API) active-active across regions with ECS/Fargate or EKS, fronted by Application Load Balancers. For databases, choose patterns based on workload:

• Aurora Global Database offers low-lag replicas and fast failover.
  
• DynamoDB Global Tables enable multi-writer setups with eventual consistency.
  
• Cost-sensitive apps may run single-writer active-passive, promoting a standby region only during failures.
  

3. Data replication and consistency

• S3 Cross-Region Replication (CRR): durable object storage synced globally.
  
• Aurora Global Database: replicate with sub-second lag; use readers in secondary regions.
  
• DynamoDB global tables: support concurrent writes across regions, requiring conflict-free design.
  
• ElastiCache/OpenSearch: run region-local clusters to reduce latency.
  

4. Failure isolation
Treat each region as its own failure domain. Duplicate VPCs, subnets, and gateways per region. Keep synchronous dependencies local; rely on asynchronous replication (queues, S3, EventBridge) for cross-region tasks. Use timeouts and circuit breakers to isolate stalls.

5. Messaging and backpressure
Cross-region fanout can use SQS, SNS, or EventBridge, with DLQs and idempotency keys for safe retries. Batch low-priority workloads and replicate asynchronously to control peak costs.

6. Security and compliance
Replicate Secrets Manager and KMS keys across regions. For multi-tenant systems, partition data by tenant/region and enforce IAM boundaries. Use geo routing to pin data to compliant regions.

7. Observability and SLOs
Centralize monitoring with CloudWatch metrics/logs, X-Ray traces, and OpenTelemetry. Track RED metrics (Rate, Errors, Duration) per region. Define SLOs for p95 latency and availability. Run synthetic canaries from multiple geographies to validate DNS/routing health.

8. Deployment and change safety
Use IaC (CloudFormation, CDK, or Terraform) to stamp identical stacks per region. Release with blue/green or canary deployments and progressively shift traffic via Route 53 weights. Keep configs in AppConfig with fail-safe defaults.

9. Cost controls
Use Graviton and Spot Instances for stateless compute, safeguarded with capacity buffers. Cache aggressively at CloudFront and APIs. Prefer read-local patterns to reduce inter-region data transfer. Right-size Aurora replicas and global tables, and consider hibernating pure DR regions off-hours.

10. Failover readiness
Automate regional evacuation with Route 53 health checks or Global Accelerator weights. Promote Aurora or switch DynamoDB write regions with rehearsed runbooks. Regularly chaos-test (disable AZs, block traffic) to confirm RTO/RPO goals are met.

Result: Users are routed to the closest healthy region, data is consistent with known RPO, and the system survives region-level failures while keeping spend under control. It’s AWS multi-region done pragmatically—fast where needed, frugal where possible, and always resilient.

‍

Table

‍

Common Mistakes

Typical errors include over-engineering every tier as active-active, driving up costs without matching business needs. Many teams replicate data synchronously across regions, hurting latency and introducing cross-region stalls. Others forget Route 53 health checks, leaving failover manual. Using DynamoDB global tables without conflict-free keys creates data drift. Skipping canary tests means failovers are unproven. Cost control is often ignored—running idle clusters 24/7 in DR regions. Finally, poor IAM scoping across replicated resources risks compliance breaches. The right approach balances business RTO/RPO goals against budget, not maximal redundancy everywhere.

‍

Sample Answers (Junior / Mid / Senior)

Junior:
“I’d use CloudFront with Route 53 to send users to the nearest region. Stateless APIs run on ECS in both regions. For data, I’d replicate S3 buckets and use Aurora Global Database for quick failover. Monitoring is with CloudWatch.”

Mid:
“I’d architect active-active for APIs on EKS across two regions, fronted by ALBs. Aurora Global DB handles reads locally, while a single writer ensures consistency. Route 53 latency-based routing + health checks automate failover. For costs, I’d use Spot/Graviton for stateless compute and CloudFront caching.”

Senior:
“I’d define RTO/RPO per workload, then mix patterns: stateless active-active on ECS/EKS, DynamoDB global tables for multi-writer, Aurora Global DB for fast failover. Route 53/Global Accelerator balances latency and shifts traffic safely. Security via multi-region Secrets Manager/KMS. Monitoring with RED metrics + synthetic canaries. Cost controls: aggressive caching, right-sized replicas, and hibernated DR regions. Regular chaos drills validate the design.”

‍

Evaluation Criteria

Interviewers look for a balanced architecture, not just “make everything multi-region.” Strong answers define trade-offs: when to run active-active vs active-passive, how to replicate data with Aurora or DynamoDB, and how to keep cross-region traffic asynchronous. They’ll check if you know to use CloudFront + Route 53/Global Accelerator for low-latency routing and health-checked failover. Cost control is important: Spot/Graviton for compute, caching for transfer, DR hibernation. Observability matters: CloudWatch, X-Ray, synthetic canaries, SLOs. Weak answers: “replicate everything everywhere.” Strong answers show nuance: per-workload RTO/RPO, compliance pinning, chaos tests, and automation runbooks.

‍

Preparation Tips

Hands-on prep: Build a demo with two AWS regions. Deploy a stateless API to ECS/Fargate in both, fronted by ALBs. Add Route 53 latency-based routing with health checks. Replicate an S3 bucket with CRR. Configure Aurora Global Database with a writer in Region A and a reader in Region B. Test failover by simulating an outage in Region A. Add CloudFront in front to serve static content. Instrument with CloudWatch and X-Ray to capture p95 latency and RED metrics. Practice chaos drills: kill a region, block cross-region traffic, and measure recovery. Document RTO/RPO goals. Optimize cost by using Spot/Graviton for compute and caching at CloudFront. Be ready to explain trade-offs: why you didn’t make every tier active-active, how you balanced latency vs spend, and how regulatory compliance shapes routing.

‍

Real-world Context

A global SaaS firm moved from single-region to dual-region AWS. By adding CloudFront + Route 53 latency-based routing, they cut EU latency 35%. Aurora Global Database replicated from us-east-1 to eu-west-1, giving sub-second lag and 1-minute failover. They ran stateless APIs on ECS active-active, but left background jobs active-passive for cost. During a chaos drill simulating us-east-1 loss, Route 53 health checks shifted traffic in ~90 seconds, and Aurora promoted the replica in under 2 minutes. Costs rose 18%, but controlled with Spot/Graviton and hibernating a third DR site. Another enterprise mis-used DynamoDB global tables without conflict-free design, causing overwrites. Lesson: success comes from per-workload trade-offs, not one-size-fits-all HA.

‍

Key Takeaways

• Use CloudFront + Route 53/Global Accelerator for geo-routing.
  
• Run stateless tiers active-active; stateful based on RTO/RPO vs cost.
  
• Aurora Global DB/DynamoDB global tables replicate with low lag.
  
• Keep cross-region dependencies async; test failover regularly.
  
• Control cost: caching, Spot/Graviton, DR hibernation.

‍

Practice Exercise

Scenario: You must design a customer-facing SaaS that needs <200 ms p95 latency in NA/EU, survive full region loss, and stay within budget.

Tasks:

1. Place CloudFront in front with edge caching. Configure Route 53 latency routing + health checks to direct to nearest healthy region.
   
2. Deploy APIs on ECS/EKS active-active in us-east-1 and eu-west-1. Use ALBs for each.
   
3. For storage:

• S3 buckets with CRR.
  
• Aurora Global DB (writer in us-east-1, reader in eu-west-1).
  
• DynamoDB global tables for multi-writer workloads.
  

4. Add Secrets Manager/KMS multi-region replication for security.
   
5. Instrument with CloudWatch/X-Ray and define SLOs (availability 99.95%, p95 latency 200 ms). Add synthetic canaries.
   
6. Run a chaos drill: simulate us-east-1 outage. Confirm Route 53 reroutes, Aurora promotes replica, APIs still serve. Measure RTO/RPO.
   
7. Add cost controls: Graviton/Spot for compute, caching, read-local queries, and off-hours DR hibernation.
   

Deliverable: Write a 2-minute explanation of design choices. Be explicit about trade-offs (e.g., Aurora vs DynamoDB, active-active vs passive) and cost management.