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Muhammed Ahmed
Muhammed Ahmed

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Running a Mongo Replicaset Locally using Docker and Docker Compose

#mongodb #testing #nosql

TLDR

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
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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
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How do I run the Replica Set?

Simples:

docker-compose up -d
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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"
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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" }
    ]
  }
)
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  • 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.

Robo 3T

I used Robo 3T to test it locally and used the following config for the connection:

Robo 3T Config

Acknowledgements / Further Reading

Cross posted from my blog here

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