Integration Patterns

Priority: P1 (HIGH)

Each pattern buys one guarantee and charges operational complexity. Adopt on evidence, not on architecture fashion.

Constraint observed	Pattern	Cost accepted
A write must update the database and publish an event atomically	Transactional outbox	A relay (poller or CDC) to run and monitor
Read shape and write shape have diverged and one model serves neither	CQRS	Two models, sync lag, doubled tests
The sequence of changes is itself the product (audit, replay, temporal queries)	Event sourcing	Projections, event versioning, no easy delete
A business transaction spans services and cannot hold a lock	Saga with compensations	Compensating action per step; no isolation
Exactly one instance may run a job or own a resource	Leader election (lease-based)	Fencing tokens; split-brain handling
Cross-cutting concerns repeated in every service	Sidecar or ambassador	Another process per pod to operate
A legacy or third-party model would infect the domain	Anti-corruption layer	A translation layer to maintain
Each client surface needs a different aggregate shape	Backends-for-frontends	One backend per surface, owned by that team
A monolith must be replaced without a rewrite freeze	Strangler fig	Dual routing and a live seam until the last slice moves