Convert a Standalone Mongodb instance to a Replica Set on a single machine with multiple instances

This tutorial describes the process for converting a standalone instance into a replica set.

In this scenario, there is only a single machine, but we run multiple (for this example, two) mongod instances. One instance acts as PRIMARY and other instances act as SECONDARY.

Steps:- 
1. Shut down the standalone mongod instance.
2. Restart the instance. Use the --replSet option to specify the name of the new replica set.

For example, use the following command to create a new replica set 

mongod --dbpath c:\replica\rs1\data --port 27017 --dbpath c:\replica\rs1\log\mongodb.log --replSet myset

3. Connect a mongo shell to the mongod instance.

4. Use rs.initiate() to initiate the new replica set:

Start Secondary mongod instance.

We should use unique set of port and data path for each instance, otherwise we may get port binding error and data path issues. For this instance we shall use 27018 port and /replica/rs2/data data path.

mongod --dbpath c:\replica\rs2\data --port 27018 --dbpath c:\replica\rs2\log\mongodb.log --replSet myset

Add a MongoDB instance to the Replica Set.

To add the MongoDB instances, run the following command in the mongo shell

rs.add(<hostname:port>);

rs.add("localhost:27018")

Check the Status.

You may check the status of the Replica Set by running the following command

rs.status();

Now there are two members in the Replica Set, with localhost:27017 being PRIMARY and localhost:27018 being SECONDARY.

Check Replication.

Now we can check if the replication is happening correctly. Add document in primary instance, it should reflect in all replica set 

myset:PRIMARY> use Cars

switched to db Cars

myset:PRIMARY> db.Car.insertOne({ name: "Z4", brand: "BMW"})

myset:PRIMARY> 

Now check in SECONDARY instance, if this has replicated. To connect to MongoDB instance running at 27018, run the following command

mongo --port 27018

myset:SECONDARY> use Cars

switched to db Cars

myset:SECONDARY> db.Car.find();

error: { "$err" : "not master and slaveOk=false", "code" : 13435 }

myset:SECONDARY> 

Set "slave okay" mode to let the mongo shell know that We're allowing reads from a secondary. This is to protect applications from performing eventually consistent reads by accident. 

We can do this in the shell with:

rs.slaveOk()

Now find the document in secondary instance.

myset:SECONDARY> use Cars

switched to db fruits

myset:SECONDARY> db.Car.find();

{ "_id" : ObjectId("5a018ea7c89da78ba2076f25"), "name" : "Z4", "brand" : "BMW" }

myset:SECONDARY> 

Same we can do with another instance.