PACELC extends CAP theorem:
If Partition: Choose Availability or Consistency (CAP)
Else (normal operation): Choose Latency or Consistency
System classifications:
- PA/EL: Dynamo, Cassandra (availability and low latency)
- PC/EC: Traditional RDBMS (consistency always)
- PA/EC: MongoDB default (available in partition, consistent normally)
Why it matters: CAP only describes partition behavior. PACELC describes everyday operation. Most of the time there's no partition. Your latency/consistency choice affects every request.
In interviews, PACELC shows deeper understanding than just citing CAP.