mongo-replicas.md

Configure a replica set for MongoDB

Rocket.Chat uses the MongoDB replica set to improve performance via Meteor Oplog tailing.

{% hint style="info" %} Note please check release notes to ensure you are using a compatible version of Mongo {% endhint %}

To configure the replica set add this section into mongod.conf:

replication:
  replSetName: "rs01"

for example by running this command:

echo -e "replication:\n  replSetName: \"rs01\"" | sudo tee -a /etc/mongod.conf

Restart MongoDB and initiate the replica set

Restart MongoDB service:

sudo systemctl restart mongod

Start the MongoDB shell and initiate the replica set:

mongo
> rs.initiate()

The output of the command should look like this:

{
    "info2" : "no configuration specified. Using a default configuration for the set",
    "me" : "127.0.0.1:27017",
    "ok" : 1,
    "operationTime" : Timestamp(1538772048, 1),
    "$clusterTime" : {
        "clusterTime" : Timestamp(1538772048, 1),
        "signature" : {
            "hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
            "keyId" : NumberLong(0)
        }
    }
}
rs01:SECONDARY>

Note the "ok" value should be 1. Any other value, i.e. 93, means something is wrong.

Hit enter, you should see your prompt turn into rs01:PRIMARY>, this indicates the replica set is being used. Type exit to get back to your regular shell:

rs01:PRIMARY> exit
bye

Depending on your network settings, including /etc/hosts and DNS, if you have configured Rocket.Chat to use localhost:27017 you may see an error similar to this:

[conn1] created this configuration for initiation : { _id: "rs0", version: 1, members: [ { _id: 0, host: "test:27017" } ] }

rs.initiate() may be defaulting to the host name e.g. 'test'

You can force a different hostname as follows:

rs.initiate({ _id: 'rs01', members: [ { _id: 0, host: 'localhost:27017' } ]})

You should get a response similar to:

> rs.initiate({ _id: 'rs01', members: [ { _id: 0, host: 'localhost:27017' } ]})
{ "ok" : 1 }
rs0:OTHER>

Reconfigure and restart Rocket.Chat service

After you configured the replica set, you MUST add the MONGO_OPLOG_URL environment variable to the service definition and restart Rocket.Chat server.

In CentOS open file /usr/lib/systemd/system/rocketchat.service

In Ubuntu or Debian open file /lib/systemd/system/rocketchat.service

and add this to the Environment line:

MONGO_OPLOG_URL=mongodb://localhost:27017/local?replSet=rs01

Reload systemd state and restart Rocket.Chat:

sudo systemctl daemon-reload && sudo systemctl restart rocketchat