Running a Mongo Replicaset Locally using Docker and Docker Compose

#mongodb #testing #nosql

TLDR

You can use Docker and Docker Compose to start a Mongo replica set locally
GitHub repo: docker-compose-mongo-replica-set
Use docker-compose up -d and off you go

Docker Compose File

version: "3.8"

services:
  mongo1:
    image: mongo:4.2
    container_name: mongo1
    command: ["--replSet", "my-replica-set", "--bind_ip_all", "--port", "30001"]
    volumes:
      - ./data/mongo-1:/data/db
    ports:
      - 30001:30001
    healthcheck:
      test: test $$(echo "rs.initiate({_id:'my-replica-set',members:[{_id:0,host:\"mongo1:30001\"},{_id:1,host:\"mongo2:30002\"},{_id:2,host:\"mongo3:30003\"}]}).ok || rs.status().ok" | mongo --port 30001 --quiet) -eq 1
      interval: 10s
      start_period: 30s

  mongo2:
    image: mongo:4.2
    container_name: mongo2
    command: ["--replSet", "my-replica-set", "--bind_ip_all", "--port", "30002"]
    volumes:
      - ./data/mongo-2:/data/db
    ports:
      - 30002:30002

  mongo3:
    image: mongo:4.2
    container_name: mongo3
    command: ["--replSet", "my-replica-set", "--bind_ip_all", "--port", "30003"]
    volumes:
      - ./data/mongo-3:/data/db
    ports:
      - 30003:30003

What is it?

A way to run 3 instances of Mongo in a replica set on your machine
This docker-compose setup starts a local mongo replica set with 3 instances running on:
- mongo1:30001
- mongo2:30002
- mongo3:30003

Are there any prerequisites?

• Docker
• Docker Compose
• The following in your /etc/hosts file:

127.0.0.1       mongo1
127.0.0.1       mongo2
127.0.0.1       mongo3

How do I run the Replica Set?

Simples:

docker-compose up -d

How do I access the Mongo Shells for each Instance?

docker exec -it mongo1 sh -c "mongo --port 30001"
docker exec -it mongo2 sh -c "mongo --port 30002"
docker exec -it mongo3 sh -c "mongo --port 30003"

How does it work?

Starts three instances of Mongo
On the first instance it runs the following Mongo Shell command:

rs.initiate(
  {
    s_id : 'my-replica-set',
    members: [
      { _id : 0, host : "mongo1:30001" },
      { _id : 1, host : "mongo2:30002" },
      { _id : 2, host : "mongo3:30003" }
    ]
  }
)

This causes all 3 instances to join the replica set named my-replica-set and start talking to each other
One is elected to become the PRIMARY and the other two become SECONDARY instances
The Docker healthcheck config is used to cause the initialisation of the replica set. More info in the further reading links.