ADR-0004 — Postgres outbox instead of a Redis queue

Status: accepted · Date: 2026-09-14

Context

The deferred push path needs durable hand-off from the request to a worker. The reflexive choice is Redis with BullMQ.

Decision

A Postgres outbox table polled with SELECT ... FOR UPDATE SKIP LOCKED.

Consequences

• No second datastore to run, back up, secure, or explain in the README.
• The enqueue shares a transaction with the conversation write, so a message can never be recorded as sent without being queued.
• SKIP LOCKED gives safe concurrent workers without external coordination.
• Polling adds latency (bounded by the poll interval) and load that a push-based queue avoids. At this volume — tens of messages an hour — this is irrelevant.
• If throughput ever reaches thousands of messages per minute, revisit. The worker interface is narrow enough to swap the transport behind it.