System Design for Reliability

How would you design a highly available web application? What components and patterns would you use?

Architectureadvanced

// interview question

How would you design a highly available web application? What components and patterns would you use?

Sample answer

For high availability: 1) Multi-AZ deployment with load balancers distributing traffic across zones. 2) Stateless application servers that can scale horizontally. 3) Database replication with automatic failover (primary-replica or multi-master). 4) Caching layer (Redis/Memcached) to reduce database load. 5) CDN for static assets and geographic distribution. 6) Circuit breakers and retries for external dependencies. 7) Health checks at every layer. 8) Async processing with message queues for non-critical operations. Design for failure: assume every component can fail and plan accordingly.

Why this matters

System design for reliability is a senior-level skill that combines architectural knowledge, cloud expertise, and operational experience. It requires understanding trade-offs between consistency, availability, cost, and complexity.

Code examples

High Availability Architecture

text

                    [CDN]
                      |
              [Load Balancer]
              /      |      \
         [App-1] [App-2] [App-3]  <- Stateless, multi-AZ
              \      |      /
              [Cache Layer]
                    |
           [Database Primary]
                    |
           [Database Replica]
                    |
              [Backup/DR]

Common mistakes to avoid

Single points of failure in the architecture
Not testing failover scenarios before production
Over-engineering for availability that isn't required

Likely follow-ups

How would you handle a database failover scenario?
What is the CAP theorem and how does it affect your design decisions?
How do you test system resilience?

Answer out loud first, then check yourself against the model answer.

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#system-design#high-availability#architecture#reliability#senior

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