Transactional Outbox for Domain Events

Design a transactional outbox so we do not write to the database and then fail to publish the event, or publish the event and then roll back the write. CONTEXT Database: [POSTGRES] Publisher: [QUEUE NAME] Events: [LIST THE 3 EVENTS] Consumer requirement: at-least-once, handlers must be idempotent. DELIVER 1) The outbox table: id, aggregate type, aggregate id, type, payload jsonb, created_at, published_at. 2) The application transaction: business write and outbox insert commit together. Show the pseudocode. 3) The publisher loop: select unpublished for update skip locked, publish, mark published. What happens if publish succeeds and the mark fails (duplicate delivery) and why the consumer can stand it. 4) The consumer idempotency key (event id) and the table it uses. 5) Operational bits: lag metric, a poison event that fails forever, and how a human replays one event. 6) What this does not solve: ordering across aggregates. Say that. Do not dual-write to the queue inside the request without the outbox.