shlomiassaf / mongo-replica-set

A replica set of MongoDB in Docker Container

MongoDB Replica Set for test

A MongoDB Replica Set running in a single docker container.

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Not For Production

The key motivation for this image is to have a ready-made replica set of MongoDB running inside docker container for CI tests.

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This repo is based on CandisIO/mongo-replica-set.

Adding the following enhenchments:

• Initial startup wait for all nodes to initialize before configuring the replica set.
• Logs from all nodes print to stdout
• $PUBLIC_HOST environment variable to expose the hostname instead of ipaddress

Docker CLI

To run the container, execute the following command:

docker run -d -p 27017:27017 -p 27018:27018 -p 27019:27019 shlomiassaf/mongo-replica-set

Once ready, the replica-set can be accessed using the following connection string:

mongodb://localhost:27017,localhost:27018,localhost:27019/?replicaSet=rs0 #... your additional config

Note that localhost (or 127.0.0.1) is accessible from the host machine and will not work inside other containers.

For example, running unit tests on you maching (the host) against the replica-set container at 127.0.0.1

Access the ReplicaSet from other containers

To access the replica-set from other containers we need to use the IP Address of the replica-set container or an alias (hostname) the resolve to that ip address.

Docker provide this (assuming we're using a brige network) for us, we just need to define the --hostname:

docker run --hostname mongodb -d -p 27017:27017 -p 27018:27018 -p 27019:27019 shlomiassaf/mongo-replica-set

And inside service container use the following connection string:

mongodb://mongodb:27017,mongodb:27018,mongodb:27019/?replicaSet=rs0 #... your additional config

This will be able to connect to the replica set but will fail when trying to connect to each replica set node.

The reason is that the replica set configuration, by default, is using 127.0.0.1 as the default replica-set node address:

rs.initiate({
    _id : 'rs0',
    members: [
        { _id : 0, host : "127.0.0.1:27017" },
        { _id : 1, host : "127.0.0.1:27018" },
        { _id : 2, host : "127.0.0.1:27019" } 
    ]
});

So when the client in a container try to connect it will first resolve connectoin string, successfully connect to the replica-set (via mongdb hostname) just to get an updated replica-set server list resolving to 127.0.0.1 for each server thus failing to connect.

To solve this, we can use the $PUBLIC_HOST environment variable which will ensure the replica set is configured properly:

docker run --env PUBLIC_HOST=mongodb --hostname mongodb -d -p 27017:27017 -p 27018:27018 -p 27019:27019 shlomiassaf/mongo-replica-set

And now the replica set is configured propertly:

rs.initiate({
    _id : 'rs0',
    members: [
        { _id : 0, host : "mongodb:27017" },
        { _id : 1, host : "mongodb:27018" },
        { _id : 2, host : "mongodb:27019" } 
    ]
});

Note that with this setup you will not be able to reach the replica-set servers from the host on OSX.

mongodb://localhost:27017,localhost:27018,localhost:27019/?replicaSet=rs0 #... your additional config

The above will get you to the replica-set but, as seen above, the replica-set will update the servers so each node is accessible using the hostname mongodb which is unreachable from the docker host machine.

To solve this issue you need to add dns mapping to the file /etc/hosts

127.0.0.1       mongodb

To avoid adding multiple hosts to your machine, syncing it with the team, you can use a single host host.docker.internal which is also what you put in PUBLIC_HOST and now accessing the replica set is working from host and containers.

Docker Compose

version: "3.9"
services:
  web:
    build: .
    ports:
        - "8000:80"
    depends_on:
        - db
    depends_on:
      - mongodb
  mongodb:
    profiles:
      - mongo
    image : shlomiassaf/mongo-replica-set:5.0.10
    hostname: mongodb
    volumes:
      - "/docker-mount/mongodb/mongo1:/var/lib/mongo1"
      - "/docker-mount/mongodb/mongo2:/var/lib/mongo2"
      - "/docker-mount/mongodb/mongo3:/var/lib/mongo3"
    environment:
      - PUBLIC_HOST=mongodb
    ports:
      - 27017:27017
      - 27018:27018
      - 27019:27019