MongoDB Replica Set with Docker-compose

picture credit: MongoDb

This post walks you through how to set up a Mongo db replica set with docker-compose. This assumes some familiarity with Docker and MongoDB. It’s worth noting that for production setups, it’s advisable to distribute replica set nodes across multiple servers. Before diving in, here is a brief intro to replica set.

What is a Replica Set?

A MongoDB replica set is a group of interconnected separate instances of MongoDB that work together as one to provide high availability and fault tolerance. In a replica set, one node is elected as the primary to handle write operations while the rest serve as secondaries and replicate its data. In case of a primary node failure, a secondary node steps up to take over as the new primary, guaranteeing high availability.

To create this, the following files will be created in your project folder

setup.sh

openssl rand -base64 756 > ${PWD}/rs_keyfile
chmod 0400 ${PWD}/rs_keyfile
chown 999:999 ${PWD}/rs_keyfile

Next, create a docker-compose.yml file with the following configuration:

version: "3.8"

services:
  mongo1:
    container_name: mongo1
    image: mongo:7.0
    command: ["--replSet", "rs0", "--bind_ip_all", "--port", "27017", "--keyFile", "/etc/mongodb/pki/keyfile"]
    restart: always
    ports:
      - 27017:27017
    networks:
      mongo-cluster:
        ipv4_address: 111.222.32.2
    volumes:
      - ${PWD}/rs_keyfile:/etc/mongodb/pki/keyfile
      - "mongo1_data:/data/db"
      - "mongo1_config:/data/configdb"
    environment:
      MONGO_INITDB_ROOT_USERNAME: root
      MONGO_INITDB_ROOT_PASSWORD: example



  mongo2:
    image: mongo:7.0
    container_name: mongo2
    command: ["--replSet", "rs0" ,"--bind_ip_all", "--port", "27018","--keyFile", "/etc/mongodb/pki/keyfile"]
    restart: always
    ports:
      - 27018:27018
    networks:
      mongo-cluster:
        ipv4_address: 111.222.32.3
    volumes:
      - ${PWD}/rs_keyfile:/etc/mongodb/pki/keyfile
      - "mongo2_data:/data/db"
      - "mongo2_config:/data/configdb"
    environment:
      MONGO_INITDB_ROOT_USERNAME: root
      MONGO_INITDB_ROOT_PASSWORD: example



  mongo3:
    image: mongo:7.0
    container_name: mongo3
    command: ["--replSet", "rs0" ,"--bind_ip_all", "--port", "27019","--keyFile", "/etc/mongodb/pki/keyfile"]
    restart: always
    ports:
      - 27019:27019
    networks:
      mongo-cluster:
        ipv4_address: 111.222.32.4
    volumes:
      - ${PWD}/rs_keyfile:/etc/mongodb/pki/keyfile
      - "mongo3_data:/data/db"
      - "mongo3_config:/data/configdb"
    environment:
      MONGO_INITDB_ROOT_USERNAME: root
      MONGO_INITDB_ROOT_PASSWORD: example



volumes:
  mongo1_data:
  mongo2_data:
  mongo3_data:
  mongo1_config:
  mongo2_config:
  mongo3_config:

networks:
  mongo-cluster:
    ipam:
      config:
        - subnet: 111.222.32.0/24

Initializing the Replica Set:

1. Generate authentication key file bash setup.sh
2. Start the containers: docker-compose up -d
3. Bash into one container: docker exec -it mongo1 bash
4. Run mongosh -u root -p example
5. Initiate the replica set using:

 rs.initiate(
  {
    _id: "rs0",
    version: 1,
    members: [
      { _id: 0, host: "mongo1:27017" },
      { _id: 1, host: "mongo2:27018" },
      { _id: 2, host: "mongo3:27019" }
    ]
  }
)

you can confirm replica status using rs.status()

Note:

1. you might need to wait a few seconds for the primary to be elected
2. if you have trouble connecting to it via compass, you'll need to add the container ip addresses to your etc/hosts in this format ip_adress container_name
3. connection string: mongodb://root:example@localhost:27017,localhost:27018,localhost:27019/?replicaSet=rs0&authSource=admin

Congratulations! You’ve successfully set up a MongoDB replica set with Docker Compose.