1. Create a Docker network.

2. Start three instances of MongoDB.

3. Initiate the Replica Set.

Create a Docker Network

First, create a Docker network by running the following command:

$ docker network create mongodbCluster

if the network has been created, you can get its details by running the following command:

$ docker network inspect mongodbCluster

The output of the above command looks like this

[
    {
        "Name": "mongodbCluster",
        "Id": "ebde001d9f02012f803a5914a3b450f6e6092a972ccfd513afb61399cbdca2a4",
        "Created": "2023-02-23T16:02:13.629671156Z",
        "Scope": "local",
        "Driver": "bridge",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": {},
            "Config": [
                {
                    "Subnet": "172.19.0.0/16",
                    "Gateway": "172.19.0.1"
                }
            ]
        },
        "Internal": false,
        "Attachable": false,
        "Ingress": false,
        "ConfigFrom": {
            "Network": ""
        },
        "ConfigOnly": false,
        "Containers": {},
        "Options": {},
        "Labels": {}
    }
]

Start MongoDB Instances

Create your primary node and add it to the network you created before:

$ docker run -d -p 27017:27017 --name mongo1 --network mongodbCluster mongo:6 mongod --replSet myReplicaSet --bind_ip localhost,mongo1

The parameters in the above command are:

• -d. Running the container in detached mode

• -p. The port in the host machine that will receive and redirect all the requests to the port in the container

• --name. Name of the container

• --network. Name of the network to use

• mongo:6. Name of the base image used for creating the container

The following command will be run in the initialized container:

mongod --replSet myReplicaSet --bind_ip localhost,mongo1

Where:

• The --replSet option is used to set a name (myReplicaSet) for your replica set

• The --bind_ip option is used to ensure that MongoDB listens for connections from applications on configured addresses. In this case, the mongod instance binds to both the localhost and the hostname (mongo1)

Once the primary node is created, you must create the secondary nodes of your replica set:

$ docker run -d -p 27018:27017 --name mongo2 --network mongodbCluster mongo:6 mongod --replSet myReplicaSet --bind_ip localhost,mongo2

$ docker run -d -p 27019:27017 --name mongo3 --network mongodbCluster mongo:6 mongod --replSet myReplicaSet --bind_ip localhost,mongo3

Initiate the Replica Set

After your cluster is created, you must initiate the replica set. Run the rs.initiate() method in the primary node of your cluster:

$ docker exec -it mongo1 mongosh 

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

If the replica set was initiated, you will get the following output:

{ ok: 1 }

Check the status of your replica set by running:

docker exec -it mongo1 mongosh --eval "rs.status()"

Authentication

For authenticating on your MongoDB cluster, create a user and assign userAdminAnyDatabase and readWriteAnyDatabase roles, following the instructions here.

Log into your MongoDB server:

$ docker exec -it mongo1 mongosh

Switch to the adming database

$ use admin

And add a new user:

db.createUser(
 {
   user: "myUserAdmin",
   pwd: passwordPrompt(), 
   roles: [
     { role: "userAdminAnyDatabase", db: "admin" },
     { role: "readWriteAnyDatabase", db: "admin" }
   ]
 }
)

Replace myUserAdmin with your desired username. With passwordPrompt() you will be asked to write the password you want to assign, or you can write the value there instead of calling that method.

Once the user is created, you can log in from the command line:

$ mongosh --username myUserAdmin --password --host mongo1

It will ask you to type in your password. If you’re accessing from outside the cluster, you must replace mongo1 with the IP address of the primary node of your cluster.

Or you can pass a connection string:

$ mongosh “mongodb://myUserAdmin:password@localhost/mydb”

Replacing password, localhost, and mydb with your authentication details.

Conclusion

Through this blog post, I have explained a set up of MongoDB cluster with replication enabled using Docker.

Thanks !!!