Configure a replica set for MongoDB
Rocket.Chat uses the MongoDB replica set to improve performance via Meteor Oplog tailing.
Note please check release notes to ensure you are using a compatible version of Mongo
To configure the replica set add this section into mongod.conf:
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:
> rs.initiate()
The output of the command should look like this:
"info2" : "no configuration specified. Using a default configuration for the set",
"me" : "",
"ok" : 1,
"operationTime" : Timestamp(1538772048, 1),
"$clusterTime" : {
"clusterTime" : Timestamp(1538772048, 1),
"signature" : {
"keyId" : NumberLong(0)
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
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 }

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:
Reload systemd state and restart Rocket.Chat:
sudo systemctl daemon-reload && sudo systemctl restart rocketchat