Running a Local Mongo Replica Set

Contents

• What is it?
• Versions
• Are there any prerequisites?
• How do I run the Replica Set?
• How do I access the Mongo Shells for each Instance?
• How does it work?
• Robo 3T
• Thanks / Further Reading

Disclaimer

:warning: This setup is purely for local development purposes.

This setup should not be used for production applications as it was not built with that in mind.

What is it?

This docker-compose setup starts a local mongo replica set with 3 instances running on:

• mongo1:30001
• mongo2:30002
• mongo3:30003

Versions

• Different versions are available to cater for the various major versions of Mongo
• You can find the most suitable one by looking at the tags or releases

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(
  {
    _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.

Robo 3T

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

Connecting with URI

mongodb://mongo1:30001,mongo2:30002,mongo3:30003/?replicaSet=my-replica-set

Thanks / Further Reading

• How to turn standalone MongoDB server into a replica set with Docker-Compose
• Creating a MongoDB replica set using Docker 🍃
• asoorm/docker-compose-mongo-replicaset.yml