Scale your Charmed MongoDB
This is part of the Charmed MongoDB Tutorial. Please refer to this page for more information and the overview of the content.
Adding and Removing units
Replication is a popular feature of MongoDB; replicas copy data making a database highly available. This means the application can provide self-healing capabilities in case one MongoDB replica fails.
! Disclaimer: this tutorial hosts replicas all on the same machine, this should not be done in a production environment. To enable high availability in a production environment, replicas should be hosted on different servers to maintain isolation.
Add replicas
You can add two replicas to your deployed MongoDB application with:
juju add-unit mongodb -n 2
You can now watch the replica set add these replicas with: juju status --watch 1s. 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 6/beta 140 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
You can trust that Charmed MongoDB added these replicas correctly. But if you wanted to verify the replicas got added correctly you could connect to MongoDB via charmed-mongodb.mongosh. 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.mongosh, using our new URI that we saved above.
charmed-mongodb.mongosh <saved URI>
Now type rs.status() and you should see your replica set configuration. It 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")
},
lastStableRecoveryTimestamp: Timestamp({ t: 1669991950, i: 1 }),
electionCandidateMetrics: {
lastElectionReason: 'electionTimeout',
lastElectionDate: ISODate("2022-12-02T11:24:09.587Z"),
electionTerm: Long("1"),
lastCommittedOpTimeAtElection: { ts: Timestamp({ t: 1669980249, i: 1 }), t: Long("-1") },
lastSeenOpTimeAtElection: { ts: Timestamp({ t: 1669980249, i: 1 }), t: Long("-1") },
numVotesNeeded: 1,
priorityAtElection: 1,
electionTimeoutMillis: Long("10000"),
newTermStartDate: ISODate("2022-12-02T11:24:09.630Z"),
wMajorityWriteAvailabilityDate: ISODate("2022-12-02T11:24:09.651Z")
},
members: [
{
_id: 0,
name: '10.23.62.156:27017',
health: 1,
state: 1,
stateStr: 'PRIMARY',
uptime: 11747,
optime: { ts: Timestamp({ t: 1669991990, i: 1 }), t: Long("1") },
optimeDate: ISODate("2022-12-02T14:39:50.000Z"),
lastAppliedWallTime: ISODate("2022-12-02T14:39:50.020Z"),
lastDurableWallTime: ISODate("2022-12-02T14:39:50.020Z"),
syncSourceHost: '',
syncSourceId: -1,
infoMessage: '',
electionTime: Timestamp({ t: 1669980249, i: 2 }),
electionDate: ISODate("2022-12-02T11:24:09.000Z"),
configVersion: 5,
configTerm: 1,
self: true,
lastHeartbeatMessage: ''
},
{
_id: 1,
name: '10.23.62.55:27017',
health: 1,
state: 2,
stateStr: 'SECONDARY',
uptime: 305,
optime: { ts: Timestamp({ t: 1669991990, i: 1 }), t: Long("1") },
optimeDurable: { ts: Timestamp({ t: 1669991990, i: 1 }), t: Long("1") },
optimeDate: ISODate("2022-12-02T14:39:50.000Z"),
optimeDurableDate: ISODate("2022-12-02T14:39:50.000Z"),
lastAppliedWallTime: ISODate("2022-12-02T14:39:50.020Z"),
lastDurableWallTime: ISODate("2022-12-02T14:39:50.020Z"),
lastHeartbeat: ISODate("2022-12-02T14:39:51.868Z"),
lastHeartbeatRecv: ISODate("2022-12-02T14:39:51.882Z"),
pingMs: Long("0"),
lastHeartbeatMessage: '',
syncSourceHost: '10.23.62.156:27017',
syncSourceId: 0,
infoMessage: '',
configVersion: 5,
configTerm: 1
},
{
_id: 2,
name: '10.23.62.243:27017',
health: 1,
state: 2,
stateStr: 'SECONDARY',
uptime: 300,
optime: { ts: Timestamp({ t: 1669991990, i: 1 }), t: Long("1") },
optimeDurable: { ts: Timestamp({ t: 1669991990, i: 1 }), t: Long("1") },
optimeDate: ISODate("2022-12-02T14:39:50.000Z"),
optimeDurableDate: ISODate("2022-12-02T14:39:50.000Z"),
lastAppliedWallTime: ISODate("2022-12-02T14:39:50.020Z"),
lastDurableWallTime: ISODate("2022-12-02T14:39:50.020Z"),
lastHeartbeat: ISODate("2022-12-02T14:39:51.861Z"),
lastHeartbeatRecv: ISODate("2022-12-02T14:39:52.372Z"),
pingMs: Long("0"),
lastHeartbeatMessage: '',
syncSourceHost: '10.23.62.55:27017',
syncSourceId: 1,
infoMessage: '',
configVersion: 5,
configTerm: 1
}
],
ok: 1,
'$clusterTime': {
clusterTime: Timestamp({ t: 1669991990, i: 1 }),
signature: {
hash: Binary(Buffer.from("dbe96e73cf659617bb88b6ad11152551c0dd9c8d", "hex"), 0),
keyId: Long("7172510554420936709")
}
},
operationTime: Timestamp({ t: 1669991990, i: 1 })
}
Now exit the MongoDB shell by typing:
exit
Now you should be back in the host of Charmed MongoDB (mongodb/0). To exit this host type:
exit
You should now be shell you started in 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 6/beta 140 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
As previously mentioned you can trust that Charmed MongoDB removed this replica correctly. This can be checked by verifying that the new URI (where the removed host has been excluded) works properly.