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System Design Fundamentals Interview Questions — What Senior Engineers Need to Know

System design interviews test whether you can reason about trade-offs at scale. Interviewers aren’t looking for a single correct answer — they want to see how you think through constraints, identify bottlenecks, and justify architectural decisions with concrete reasoning.

These 20 questions cover the fundamental building blocks that appear in nearly every system design discussion: consistency models, caching layers, partitioning strategies, load balancing, and failure handling. Master these concepts and you’ll have the vocabulary to tackle any open-ended design problem.

All 20 Questions

Redis Caching PatternsWhat are the common Redis caching patterns?Vertical vs Horizontal ScalingWhat is the difference between vertical and horizontal scaling?API VersioningWhat are the common approaches to API versioning?SLOs vs SLAsWhat is the difference between SLOs and SLAs?Availability — Five 9sWhat does 'five 9s' availability mean and how do you achieve it?Latency vs ThroughputWhat is the difference between latency and throughput?Cache Eviction — LRU, LFU, FIFOWhat are LRU, LFU, and FIFO cache eviction strategies?Stateless vs Stateful Web TierWhat is the difference between a stateless and stateful web tier?Multi-data CenterWhat are the challenges of a multi-data center architecture?Back-of-envelope: QPSHow do you estimate QPS in a back-of-envelope calculation?Back-of-envelope: StorageHow do you estimate storage needs in a back-of-envelope calculation?Back-of-envelope: BandwidthHow do you estimate bandwidth in a back-of-envelope calculation?Power of 2 TableWhat are the key powers of 2 every engineer should know?Latency NumbersWhat are the key latency numbers every engineer should know?Framework Wrap-upHow do you wrap up a system design answer effectively?Cache InvalidationWhat are the common cache invalidation strategies?CRUD → HTTPHow do CRUD operations map to HTTP methods?API Design Best PracticesWhat are the best practices for API design?Write-around CacheWhat is a write-around cache strategy?Reliability, Fault Tolerance, RedundancyWhat is the relationship between reliability, fault tolerance, and redundancy?

How to Prepare

Focus on understanding concepts deeply enough to explain them in your own words. For each topic, practice articulating the trade-offs and real-world applications — interviewers care about practical judgment, not textbook definitions.

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