Distributed Systems

Multi-node systems fail partially and non-deterministically. Every design decision spans a space of consistency, availability, and latency trade-offs — name them explicitly.

CAP and PACELC

CAP: under a network Partition, choose Consistency or Availability — you cannot guarantee both.

PACELC extends this to normal operation: even without a partition, every request trades Latency for Consistency. Always name both axes: "This is AP/EL — available under partition, latency-favoured otherwise."

Common placements: Cassandra (AP/EL), HBase (CP/EC), Spanner (CP/EC via TrueTime), DynamoDB (AP/EL by default, tunable).

Consistency Models

Ordered strongest → weakest: