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PACELC

The PACELC theorem is indeed a direct extension of the CAP theorem.

If Partition exists choose between Availability or Consistency Else Latency or Consistency

What If Partition Exists (P) means

• A network partition has occurred
• Some nodes cannot communicate with others
• Messages are dropped, not just delayed

When CAP exists why PACELC?

CAP focuses exclusively on what happens during a network failure (a “partition”), PACELC addresses a major critique: it accounts for how a system behaves during normal, healthy operation.

• Most systems run without network partitions most of the time
• Datacenters are engineered to avoid partitions
• Partitions are rare but catastrophic
• So when everything works, you still trade consistency vs latency.

                Distributed System
                        |
                        v
             Is there a network partition?
                        |
            +-----------+-----------+
            |                       |
          YES (P)                NO (ELSE)
            |                       |
            v                       v
   Availability (A)         Low Latency (L)
            |                       |
   - Keep serving            - Read nearest replica
   - May return              - Async replication
     inconsistent data       - Possible staleness
            |
            |
            v
     Consistency (C)         Consistency (C)
            |                       |
   - Block / error           - Quorum / consensus
   - Wait for quorum         - Higher latency
   - Data always correct     - Strong guarantees