3.3.1 CAP Theorem
If two datacenters cannot talk, a CP system refuses some writes; an AP system accepts them and reconciles later.
Why it matters
It is the vocabulary for “why is this database like this?” It is not an excuse to skip backups.
How it is used
Postgres primary + failover is mostly CP. Dynamo-style stores lean AP with conflict rules. Choose per use case (ledger vs likes).