CAP & PACELC — Simple

Problem statement (interviewer prompt)

Explain CAP and PACELC. Pick three real systems — one CP, one AP, one with tunable consistency — and contrast their behaviour during a network partition and during normal-mode latency-vs-consistency trade-offs.

CAP: under a network partition, a distributed system can be either consistent or available — pick one.

PACELC extends it: even when no partition exists, you trade latency vs consistency.

flowchart LR
  N[Network OK?]
  N -->|Partition| P[Choose: C or A]
  N -->|No partition| E[Choose: L or C]
  P --> CP[CP system<br/>HBase, Spanner, etcd]
  P --> AP[AP system<br/>Dynamo, Cassandra, Riak]
  E --> EL[EL low latency<br/>Dynamo, Cassandra]
  E --> EC[EC strong consistency<br/>Spanner, MongoDB majority]

    classDef client fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
    classDef edge fill:#cffafe,stroke:#0e7490,stroke-width:1px,color:#0f172a;
    classDef service fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
    classDef datastore fill:#fee2e2,stroke:#991b1b,stroke-width:1px,color:#0f172a;
    classDef cache fill:#fed7aa,stroke:#9a3412,stroke-width:1px,color:#0f172a;
    classDef queue fill:#ede9fe,stroke:#5b21b6,stroke-width:1px,color:#0f172a;
    classDef compute fill:#d1fae5,stroke:#065f46,stroke-width:1px,color:#0f172a;
    classDef storage fill:#e5e7eb,stroke:#374151,stroke-width:1px,color:#0f172a;
    classDef external fill:#fce7f3,stroke:#9d174d,stroke-width:1px,color:#0f172a;
    classDef obs fill:#f3e8ff,stroke:#6b21a8,stroke-width:1px,color:#0f172a;
    class N,P,E service;
    class CP,AP,EL,EC datastore;