# ADR-0004: Postgres transactional outbox first, Kafka later

**Status**: accepted · 2026-09

**Context**: Event bus carries business objects (P4) and is the audit backbone
(P5). Phase-1 volume is bid-frequency, not telemetry-frequency.

**Decision**: Phase 1 uses a Postgres transactional outbox (business write +
event in one transaction) with polling/LISTEN consumers, behind a thin bus
interface. Phase 2 relays the outbox into Kafka when telemetry-scale events
arrive; consumer interface unchanged.

**Consequences**: Exactly-once-ish semantics via same-DB transactionality in
phase 1; events reconcile trivially against business data. Bus abstraction must
not leak Postgres specifics.