Charmed MongoDB Tutorials > Deploy a replica set > 4. Scale your replicas
Scale your replicas
A replica set in MongoDB is a group of processes that copy stored data in order to make a database highly available. Replication provides redundancy, which means the application can provide self-healing capabilities in case one replica fails.
Disclaimer: This tutorial hosts all replicas on the same machine. This should never be done in a production environment.
To enable high availability in a production environment, replicas should be hosted on different servers to maintain isolation.
Summary
- Add replicas to your MongoDB cluster
- Remove replicas from your MongoDB cluster
Add replicas
You can add two replicas to your deployed MongoDB application with:
juju add-unit mongodb -n 2
It usually takes several minutes for the replicas to be added to the replica set. You’ll know that all three replicas are ready when juju status --watch 1s reports:
Model Controller Cloud/Region Version SLA Timestamp
tutorial overlord localhost/localhost 3.1.6 unsupported 14:42:04Z
App Version Status Scale Charm Channel Rev Exposed Message
mongodb active 3 mongodb 5/edge 96 no Replica set primary
Unit Workload Agent Machine Public address Ports Message
mongodb/0* active idle 0 10.23.62.156 27017/tcp Replica set primary
mongodb/1 active idle 1 10.23.62.55 27017/tcp Replica set secondary
mongodb/2 active idle 2 10.23.62.243 27017/tcp Replica set secondary
Machine State Address Inst id Series AZ Message
0 started 10.23.62.156 juju-d35d30-0 jammy Running
1 started 10.23.62.55 juju-d35d30-1 jammy Running
2 started 10.23.62.243 juju-d35d30-2 jammy Running
If you wanted to verify the replica set configuration, you could connect to MongoDB via charmed-mongodb.mongo. Since your replica set has 2 additional hosts, you will need to update the hosts in your URI. You can retrieve these host IPs with:
export HOST_IP_1=$(juju show-unit mongodb/1 | awk '/public-address:/{print $NF;exit}')
export HOST_IP_2=$(juju show-unit mongodb/2 | awk '/public-address:/{print $NF;exit}')
Then recreate the URI using your new hosts and reuse the username, password, database name, and replica set name that you previously used when you first connected to MongoDB:
export URI=mongodb://$DB_USERNAME:$DB_PASSWORD@$HOST_IP,$HOST_IP_1,$HOST_IP_2/$DB_NAME?replicaSet=$REPL_SET_NAME
Now view and save the output of the URI:
echo $URI
Like earlier, we access mongo by sshing into one of the Charmed MongoDB hosts:
juju ssh mongodb/0
While sshd into mongodb/0, we can access mongo with charmed-mongodb.mongo, using our new URI that we saved above.
charmed-mongodb.mongosh <saved URI>
Now, run rs.status() and you should see your replica set configuration. The first few lines should look something like this:
{
set: 'mongodb',
date: ISODate("2022-12-02T14:39:52.732Z"),
myState: 1,
term: Long("1"),
syncSourceHost: '',
syncSourceId: -1,
heartbeatIntervalMillis: Long("2000"),
majorityVoteCount: 2,
writeMajorityCount: 2,
votingMembersCount: 3,
writableVotingMembersCount: 3,
optimes: {
lastCommittedOpTime: { ts: Timestamp({ t: 1669991990, i: 1 }), t: Long("1") },
lastCommittedWallTime: ISODate("2022-12-02T14:39:50.020Z"),
readConcernMajorityOpTime: { ts: Timestamp({ t: 1669991990, i: 1 }), t: Long("1") },
appliedOpTime: { ts: Timestamp({ t: 1669991990, i: 1 }), t: Long("1") },
durableOpTime: { ts: Timestamp({ t: 1669991990, i: 1 }), t: Long("1") },
lastAppliedWallTime: ISODate("2022-12-02T14:39:50.020Z"),
lastDurableWallTime: ISODate("2022-12-02T14:39:50.020Z")
},
...
Return to original shell
Leave the MongoDB shell by typing exit.
You will be back in the host of Charmed MongoDB (mongodb/0). Exit this host by typing exit again.
You should now be at the original shell where you can interact with Juju and LXD.
Remove replicas
Removing a unit from the application scales the replicas down. Before we scale down the replicas, list all the units with juju status, here you will see three units mongodb/0, mongodb/1, and mongodb/2. Each of these units hosts a MongoDB replica.
To remove the replica hosted on the unit mongodb/2 enter:
juju remove-unit mongodb/2
You’ll know that the replica was successfully removed when juju status --watch 1s reports:
Model Controller Cloud/Region Version SLA Timestamp
tutorial overlord localhost/localhost 3.1.6 unsupported 14:44:25Z
App Version Status Scale Charm Channel Rev Exposed Message
mongodb active 2 mongodb 5/edge 96 no Replica set primary
Unit Workload Agent Machine Public address Ports Message
mongodb/0* active idle 0 10.23.62.156 27017/tcp Replica set primary
mongodb/1 active idle 1 10.23.62.55 27017/tcp Replica set secondary
Machine State Address Inst id Series AZ Message
0 started 10.23.62.156 juju-d35d30-0 jammy Running
1 started 10.23.62.55 juju-d35d30-1 jammy Running
You can also see that the replica was successfully removed by using the new URI (where the removed host has been excluded).
Next step: 5. Manage passwords
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