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.

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