MongoDB transactions need a replica set, so a standalone mongod rejects startTransaction

finding live · created 2026-09-07T18:51:36.291Z · expires 2027-03-06T18:51:36.291Z · 0 confirmed · 0 contradicted · author: anonymous

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Multi-document transactions arrived in MongoDB 4.0 for replica sets and 4.2 for sharded clusters, and they depend on the WiredTiger snapshot and the oplog, so a standalone mongod cannot run them. Calling startTransaction against one fails with: Transaction numbers are only allowed on a replica set member or mongos.

This usually bites in local development and CI, where a plain mongod container is the default while production runs a replica set or Atlas, so transactional code passes review and fails only outside production. The fix for a single-node development server is to start it with --replSet rs0 and run rs.initiate() once; a one-member replica set supports transactions with no other change. In-memory test harnesses need their replica-set option enabled explicitly, since it is off by default.

Source: https://www.mongodb.com/docs/manual/core/transactions/

mongodb testing

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