Running Mongodb Replication Using Docker

For a proper mongodb replication, we are going to start 3 containers for this exercise.

First, start the first container, we will call it mongorep1. We need to set it so that it has hostname, configured to listen to all interfaces, and set a replSet for it called myrepl

docker run -dit --name mongorep1 --hostname mongorep1 mongo:6 --bind_ip_all --replSet myrepl

Once running, we need to get the ip address of mongorep1

docker inspect mongorep1 | grep -w IPAddress

            "IPAddress": "172.17.0.2",

Then, we will start the second container. We need to feed the ip address of the first container to the second container as hosts so that mongo will not have issue setting up the replication

docker run -dit --name mongorep2 --hostname mongorep2 --add-host mongorep1:172.17.0.2 mongo:6 --bind_ip_all --replSet myrepl

Start the third and final container, with command almost similar to the second container.

docker run -dit --name mongorep3 --hostname mongorep3 --add-host mongorep1:172.17.0.2 mongo:6 --bind_ip_all --replSet myrepl

Once we have all the nodes running, access mongosh on the first container, and initiate replica set

docker exec -it mongorep1 mongosh

test> rs.initiate()

Add the other node into the replicaset

myrepl [direct: secondary] test> rs.add("172.17.0.3")

myrepl [direct: primary] test> rs.add("172.17.0.4")

Check the status of the replica set, make sure the first node is the primary node, and the other 2 are the secondary nodes

myrepl [direct: primary] test> rs.status()

...

  members: [

    {

      _id: 0,

      name: 'mongorep1:27017',

      health: 1,

      state: 1,

      stateStr: 'PRIMARY',

...

    {

      _id: 1,

      name: '172.17.0.3:27017',

      health: 1,

      state: 2,

      stateStr: 'SECONDARY',

...

    {

      _id: 2,

      name: '172.17.0.4:27017',

      health: 1,

      state: 2,

      stateStr: 'SECONDARY',

... 

Check if the other node is lagged in terms of replicating data

myrepl [direct: primary] test> db.printSecondaryReplicationInfo()

source: 172.17.0.3:27017

{

  syncedTo: 'Mon Dec 19 2022 15:48:01 GMT+0000 (Coordinated Universal Time)',

  replLag: '0 secs (0 hrs) behind the primary '

}

---

source: 172.17.0.4:27017

{

  syncedTo: 'Mon Dec 19 2022 15:48:01 GMT+0000 (Coordinated Universal Time)',

  replLag: '0 secs (0 hrs) behind the primary '

}

We can test the replication, by adding data into the first node, and check if the data is being replicated into the second and third node. 

docker exec -it mongorep1 mongosh

myrepl [direct: primary] test> use mynewdb

myrepl [direct: primary] mynewdb> db.people.insertOne( { name: "John Rambo", occupation: "Soldier" } ) 

exit

Now access mongosh in the second node and view the data, The data should be similar to the mongorep1

docker exec -it mongorep2 mongosh

myrepl [direct: secondary] test> show dbs

myrepl [direct: secondary] test> use mynewdb

myrepl [direct: secondary] test> db.people.find()

[

  {

    _id: ObjectId("63a08880e1c97fba6959ec15"),

    name: 'John Rambo',

    occupation: 'Soldier'

  }

]

If you encounter this error:

MongoServerError: not primary and secondaryOk=false - consider using db.getMongo().setReadPref() or readPreference in the connection string

Run below command to enable read on the secondary nodes

myrepl [direct: secondary] test> db.getMongo().setReadPref("secondary")

Do the same for the third node, the data should also be the same.

docker exec -it mongorep3 mongosh

myrepl [direct: secondary] test> use mynewdb

myrepl [direct: secondary] test> db.people.find() 

[

  {

    _id: ObjectId("63a08880e1c97fba6959ec15"),

    name: 'John Rambo',

    occupation: 'Soldier'

  }

]