Filling the Gap CAP Leaves
CAP only describes behavior during a partition. But partitions are rare while latency choices happen on every request. PACELC extends the picture: if there is a Partition then choose Availability or Consistency, Else in normal operation choose Latency or Consistency.
The Two Halves
- The PAC half is just CAP, the partition time trade between availability and consistency.
- The ELC half is the everyday trade. To confirm a write on every replica you wait for acknowledgements, which adds latency. To answer faster you reply before all replicas agree, which weakens consistency.
Reading Real Systems
- A store tuned for speed is often described as PA EL, available under partition and low latency otherwise.
- A store tuned for correctness is often PC EC, consistent in both modes at the cost of speed.
PACELC matters because most of a system life is spent with a healthy network, so the else branch shapes the user experience far more than the rare partition branch.
Key idea
PACELC adds the else branch CAP forgot, the daily trade of latency against consistency when the network is healthy.