Distributed systems are at the heart of every modern tech company. When your interviewer asks "How would you scale this system?" or "What happens if a server fails?", they are really asking about distributed systems concepts.

Yet many candidates only scratch the surface: "We add more servers and a load balancer." That is not enough. Interviewers want to understand your knowledge of CAP theorem, consistency models, failure detection, consensus algorithms, and the fundamental trade-offs that shape every distributed system.

This chapter provides a deep understanding of distributed systems for system design interviews. We will work through CAP theorem, consistency models, consensus algorithms, failure detection, and the patterns that make real systems work at scale.

1. Why Distributed Systems