CAP-THEOREM-EDI

CAP theorem (EDI) is Brewer's 2000 theorem stating that a distributed system can only guarantee simultaneously 2 of the 3 properties: Consistency, Availability, Partition tolerance.

Definition

CAP forces a choice: on network partition (P is inevitable), should you favour Consistency (reject writes) or Availability (accept with divergence)? CP: MongoDB, HBase. AP: Cassandra, DynamoDB. CA does not really exist (no partition tolerance).

Origin

Conjecture published by Eric Brewer (UC Berkeley) at the PODC 2000 keynote. Formally proven by Lynch & Gilbert (MIT) in 2002.

Use

A multi-region EDI gateway must choose: CP mode — a primary-region outage freezes everything (safe, but degraded SLA), AP mode — each region accepts its local messages (high availability, but divergence to reconcile later). PEPPOL leans AP with asynchronous reconciliation.

Related terms

• ACID vs BASE — consistency axis.
• Active-active — AP topology.
• Active-passive — CP topology.
• Event sourcing — solves convergence.