Consistency & Coordination

Decide what a reader is guaranteed to see when data lives on more than one node, and how independent nodes agree on a single answer. Get this wrong and the system either serves stale or conflicting data silently, or stalls the moment a network link drops — the most common and most punishing distributed-systems failure.

When to reach for this

Any time state is replicated, sharded, or coordinated across nodes: choosing a replication or quorum scheme, picking a consistency level, electing a leader, spreading keys across an elastic fleet (consistent hashing), or committing a change that spans services. Reach here the instant someone asks "what does a read see right after a write?" or "what happens during a partition?"