MongoDB ReplicaSet Setup Explained

For rstarting up a 3 node cluster with 1 master and 2 slaves follow this https://medium.com/swlh/mongodb-creating-a-3-node-replica-set-cluster-7ca94849b139

First we need a keyfile and appropriate permissions.

If the mongod processes were actually running on different machines, then each machine will have a copy of this keyfile.

Use OpenSSL to create keyfile -

sudo mkdir -p /usr/local/var/mongodb/pki/
openssl rand -base64 741 > /usr/local/var/mongodb/pki/replicaset-keyfile
chmod 400 /usr/local/var/mongodb/pki/replicaset-keyfile

Create Mongod config files to start mongod processes. Here we will fork the processes to run multiple mongo.

Create and save these files node1.conf

storage:
  dbPath: /usr/local/var/mongodb/node1
net:
  bindIp: localhost
  port: 27011
security:
  authorization: enabled
  keyFile: /usr/local/var/mongodb/pki/replicaset-keyfile
systemLog:
  destination: file
  path: /usr/local/var/log/mongodb/node1/mongod.log
  logAppend: true
processManagement:
  fork: true
replication:
  replSetName: replicaset-example

Create dbpath and log path for same

mkdir -p /usr/local/var/mongodb/node1
mkdir -p /usr/local/var/log/mongodb/node1

node2.conf

storage:
  dbPath: /usr/local/var/mongodb/node2
net:
  bindIp: localhost
  port: 27012
security:
  authorization: enabled
  keyFile: /usr/local/var/mongodb/pki/replicaset-keyfile
systemLog:
  destination: file
  path: /usr/local/var/log/mongodb/node2/mongod.log
  logAppend: true
processManagement:
  fork: true
replication:
  replSetName: replicaset-example

mkdir -p /usr/local/var/mongodb/node2
mkdir -p /usr/local/var/log/mongodb/node2

node3.conf

storage:
  dbPath: /usr/local/var/mongodb/node3
net:
  bindIp: localhost
  port: 27013
security:
  authorization: enabled
  keyFile: /usr/local/var/mongodb/pki/replicaset-keyfile
systemLog:
  destination: file
  path: /usr/local/var/log/mongodb/node3/mongod.log
  logAppend: true
processManagement:
  fork: true
replication:
  replSetName: replicaset-example

mkdir -p /usr/local/var/mongodb/node3
mkdir -p /usr/local/var/log/mongodb/node3

Now launch the 3 mongod processes by following commands -

mongod -f node1.conf
mongod -f node2.conf
mongod -f node3.conf

Do netstat -tulnp to check them if they are running.

Connect and Add replicas in Replica Set

mongosh --port 27011

Inside it, run command,

rs.initiate(); // to initiate the replica set
use admin;     //switch to admin database for creating root user
db.createUser({
 user: "test-user",
 pwd: "test-pass",
 roles: [{ role: "root", db: "admin"}]
});

Here we password protect the database and assign role as well.

Now disconnect by ctrl+c and login to replica by credentials u entered as - mongo --host "replicaset-example/localhost:27011" -u "test-user" -p "test-pass" --authenticationDatabase "admin"

The master in replica is the only that recieves read/write and is called the Primary. (Learn about running is master mode and other things as well in Documentation)

Check using command rs.isMaster() to check if current is master node and other hosts connected as well.

To add other nodes write the command -

rs.add("localhost:27012") //to add second node running on port 27012
rs.add("localhost:27013") //to add third node running on port 27013

Run rs.isMaster() to check output again and info.

(items returned after rs.isMaster())

You can change master by making a secondary as master using rs.stepDown(). This does internal election and elect new primary and this is called Failover.

Connect and Add replicas in Replica Set#

Connect and Add replicas in Replica Set