Data Retention & History
Rule: keeping the data is not the same as keeping the history. is_deleted = true preserves a
Temporal Data
Rule: a system does not act on objective truth — it acts on what it believed at the time.
Event Sourcing, Snapshots & CQRS
Rule: storing state answers "what is it now"; storing events answers "how did it get there".
Message Delivery Semantics
Rule: exactly-once delivery does not exist. You get at-least-once, and you make the effect