Distributed systems show up whenever one machine is no longer enough for the workload, availability target, or latency target. In interviews, that usually appears as questions like "How would you scale this?", "What happens if a node fails?", or "Can this system tolerate a region outage?"

A common answer stops at "add more servers and a load balancer." Stronger answers explain the trade-offs that come with that choice: partial failures, replication lag, coordination, consistency, recovery, and operational complexity.

This chapter covers the distributed-systems concepts interviewers commonly probe: CAP and PACELC, failure modes, clocks and ordering, failure detection, consensus, consistency models, and practical patterns for building systems that keep working under stress.