Jul 11, 2015

MongoDB- Replication

  • Replication is the process of synchronizing data across multiple servers.
  • Provides redundancy and increases data availability with multiple copies of data of different database servers.
  • It is protects a database from the loss of a single server and recover from hardware failure and service interruptions.
  • Replication also allows you to recover from hardware failure and service interruptions. With additional copies of the data, you can dedicate one to disaster recovery, reporting, or backup.
Why we need to use Replication:
1-It is a process to keep your data safe ,
2-high availability of data,
3-disaster recovery ,
3-no down time for maintenance ,
4-read scaling and it is  a transparent to the application.

How replication works in MongoDB

A replica set is a group of MongoDB instances that host the same data set. There is two node first is primary node that receives all write operations and secondary, apply operations from the primary so that they have the same data set. Replica set can have only one primary node.
1. Replica set is the group of nodes generally minimum three nodes are required, one node is primary node and remaining nodes are secondary.
2. At the time of automatic failover or maintenance, election establishes for primary and a new primary node is elected.
3 .After the recovery of failed node, it again join the replica set and works as a secondary node.

A typical diagram of mongodb replication is shown in which client application always interact with primary node and primary node then replicate the data to the secondary nodes.

Replica set features

A cluster of N nodes
Anyone node can be primary
All write operations goes to primary
Automatic failover
Automatic Recovery
Consensus election of primary

Set up a replica set 

In this codefari we will convert standalone mongodb instance to a replica set. To convert to replica set follow the below given steps:
·         Shutdown already running mongodb server.
Now start the mongodb server by specifying --replSet option. Basic syntax of --replSet is given below:

mongod --port "PORT" --dbpath "YOUR_DB_DATA_PATH" --replSet "REPLICA_SET_INSTANCE_NAME"


mongod --port 27017 --dbpath "D:\set up\mongodb\data" --replSet rs0

It will start a mongod instance with the name rs0, on port 27017. Now start the command prompt and connect to this mongod instance. In mongo client issue the command rs.initiate() to initiate a new replica set. To check the replica set configuration issue the command rs.conf(). To check the status of replica sete issue the command rs.status().

Add members to replica set

To add members to replica set, start mongod instances on multiple machines. Now start a mongo client and issue a command rs.add().


Basic syntax of rs.add() command is as follows:



Suppose your mongod instance name is mongod1.net and it is running on port 27017. To add this instance to replica set issue the command rs.add() in mongo client.


You can add mongod instance to replica set only when you are connected to primary node. To check whether you are connected to primary or not issue the command db.isMaster() in mongo client.