MongoDB Single Node Replica Set

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The Problem

If you’re like me and you’re building non-trivial software and you use MongoDB as a datastore you’d know that it can be a pain in the ass to work around the fact that MongoDB doesn’t support transactions when running it locally, so you might opt to use a shared instance on MongoDB Altas or look for a docker compose file that spins up a replica set, but that’s not ideal because it’s an extra thing you have to setup.

Setup

You can run MongoDB as a single-node replica-set on your local machine. Depending on how you installed MongoDB this setup might be different. I’m currently on Ubuntu 20.04.4 LTS, and I’m using the apt package manager. Installing mongodb using apt will create a daemon that runs mongodb in the background.

The Solution

According to MongoDB’s documentation, a mongod standalone instance can be converted to a replica set by passing a cli flag with the replica set name.

  1. Stop the currently running MongoDB Daemon
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systemctl stop mongod.service
  1. Edit the service unit file
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vi /lib/systemd/system/mongod.service
  1. Update the ExecStart command
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...
ExecStart=/usr/bin/mongod --config /etc/mongod.conf --replSet [my-replica-set-name]
...
  1. Reload systemd
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sudo systemctl reload mongod
  1. The permission settings on /var/lib/mongodb and /tmp/mongodb-27017.lock are wrong, so you will have to change the owner to monogdb. You can read more about it here and here.
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chown -R mongodb:mongodb /var/lib/mongodb
chown mongodb:mongodb /tmp/mongodb-27017.sock
  1. Restart the MongoDB Daemon
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sudo systemctl start mongod.service
  1. Connect to the MongoDB instance with this connection URI
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mongodb://localhost:27017/test?retryWrites=true&w=majority&replicaSet=[my-replica-set-name]

This would get you up and running in a few minutes but once the package is updated you’d have to redo the process again.

A Shorter Solution

I built a cli tool that makes the process trivial, it’s called rs and you can snag the binary from here.

It’s really easy to setup a single node replica set, you can run there commands to get started

  1. This downloads and unpacks MongoDB (the default is v4.2.21) into a folder in your home directory
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rs download
  1. Run the replica set
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rs run
  1. Connect to the MongoDB instance with this connection URI
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mongodb://localhost:27017/test?retryWrites=true&w=majority&replicaSet=localhost

Notes

  1. There’s a known issue with the rs cli tool where mongod would fail to start, the fix is delete the data folder for that distribution. Haha, I know it’s not elegant but the tool has served it’s purpose.
  2. if you want you can use rs to download and run different mongodb distributions if that’s something you want to do.