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'
}
]
