Rocket.Chat tails the Mongo oplog for realtime delivery, and a standalone
mongod has no oplog — so the database has to be a replica set even with one
member.
Uses the official mongo image rather than bitnami/mongodb (which upstream's own
compose uses) because Bitnami moved its catalog behind a paid registry and the
free tags are no longer dependable for a long-lived install. The cost is that
rs.initiate() is not automatic, so the post-start hook runs it once — guarded by
rs.status() so a reinstall over restored data doesn't re-initiate a live set,
and followed by a wait for the member to report itself primary.
Mongo runs without auth: enabling it on a replica set also requires a shared
keyfile for member-to-member auth, which is a lot of moving parts for a database
that is never published outside the docker network.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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