Replica sets in MongoDB

This chapter follows Geospatial queries in MongoDB. Production Atlas clusters run as replica sets by default. Self-hosted deployments should too unless you are doing a single-node local experiment.

Why replica sets exist

A replica set is a group of mongod processes that share the same data set. One member is primary and receives all writes; secondaries replicate the primary’s oplog and can serve reads depending on read preference. If the primary fails, members elect a new primary automatically.

Elections and roles

Each member has a priority (default 1). Higher priority members are preferred in elections. Check status in mongosh on the primary:

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rs.status()
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rs.isMaster().primary

To influence future elections (connect to the current primary):

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const cfg = rs.conf()
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cfg.members[0].priority = 3
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cfg.members[1].priority = 2
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cfg.members[2].priority = 1
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rs.reconfig(cfg)

Local three-node set with Docker Compose

A minimal compose file for learning (not production hardening):

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services:
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  mongo1:
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    image: mongodb/mongodb-community-server:8.0-ubuntu2204
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    command: ["--replSet", "rs0", "--bind_ip_all"]
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    ports: ["27017:27017"]
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  mongo2:
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    image: mongodb/mongodb-community-server:8.0-ubuntu2204
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    command: ["--replSet", "rs0", "--bind_ip_all"]
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    ports: ["27018:27017"]
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  mongo3:
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    image: mongodb/mongodb-community-server:8.0-ubuntu2204
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    command: ["--replSet", "rs0", "--bind_ip_all"]
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    ports: ["27019:27017"]

Initiate once:

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rs.initiate({
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  _id: 'rs0',
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  members: [
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    { _id: 0, host: 'localhost:27017' },
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    { _id: 1, host: 'localhost:27018' },
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    { _id: 2, host: 'localhost:27019' },
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  ],
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})

Read preference and write concern

From the driver:

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const col = db.collection('events', {
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  readPreference: 'secondaryPreferred',
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  writeConcern: { w: 'majority', j: true },
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});

Stale reads are possible on secondaries; use primary for read-your-writes consistency.

Connection strings

Include the replica set name and multiple hosts:

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mongodb://localhost:27017,localhost:27018,localhost:27019/myapp?replicaSet=rs0

Atlas mongodb+srv URIs include replica set parameters automatically.

Next: Transactions in MongoDB.