Why we run MongoDB as a replica set everywhere

Publishing an article writes three things: the article, a revision and an audit event. They commit together or not at all — and multi-document transactions require a replica set.

A single-node replica set

services:
  mongo:
    image: mongo:8
    command: ["--replSet", "rs0", "--bind_ip_all"]
    ports: ["27017:27017"]

One container, one command to initiate it, and local development behaves exactly like production. CI does the same; a missing database fails the build rather than skipping tests.

Optimistic concurrency

Every update matches on { _id, version } and increments the version. Zero matched documents means someone else won — we surface that, we never overwrite.