Monday, January 28, 2013

Setup a MongoDB Replica Set

Database replication is a service for shipping changes to your database, off to a copy housed on another server, potentially even in another data center.It ensures redundancy, backup, and automatic failover.
Replication occurs through groups of servers known as replica sets, each of which stores the same dataset as the other members of the replica set.

Replication in MongoDB

In MongoDB, replication is implemented in two forms:
  1. Replica Sets: Group of replication nodes containing one primary node and multiple secondary nodes. Primary accepts write operations, and secondaries are the replicating members. MongoDB’s replica sets provide automated failover. If a primary fails, the remaining members will automatically try to elect a new primary.
  2. Master-Slave Architecture: When the number of the replica nodes need to be more than 12, then Master-Slave architecture comes in. It is similar to Replica Sets except that it does not provide automated failover.
The figure below showcases the concept of Replica Sets in MongoDB, where one node is primary and rest all are secondaries. Also all of them are storing the same documents i.e. Doc 1 and 2

 Replication in MongoDB

Deploying a Replica Set

With trivial knowledge gathered about Replica Sets in MongoDB, try deploying one using the steps below:

Step 1: Follow Step 1,2 and 3 for setting up MongoDB on all the nodes of the Replica Set as given in my previous post.

Step 2: Create a conf file (for example "mongod.conf") at a location owned and accessible by the user with the following content. You can configure "port" and name your replica set by setting the field "replSet" to that name which would be same on all the nodes of the replica set.

nojournal = true
port = 27017
bind_ip = machine_ip (for eg.
fork = true
replSet = rsName                                                                              

Step 3: Start the MongoD daemon on each node of the Replica Set using:

/bin/mongod --config /pathToFile/mongod.conf                                

Step 4: Open a mongo terminal on the node which you want to be the primary node using: 

bin/mongo --host ip_of_the_machine                                                
To make this node the primary run the following command in the opened mongo terminal :


Step 5: To add the other nodes, run the add commands in the opened mongo terminal on the primary node:
and so on.                                                                                          
Check the status of the replica set using:


Add secondaries to the Replica Set using a JavaScript

After initializing the primary using "rs.initiate()" add the secondaries using :

mongo primary_ip:mongod_port /path_to_js_file                             

where the .js file contains 


To check, you can open the mongo terminal on the primary  node and run the "rs.conf()" command to see the status post running the JavaScript file.

Hope that the above steps helped you create a Replica Set successfully. My next post would be on "Creating a MongoDB Sharded Cluster".


  1. I am confused little bit.
    a) Can we have multinode on a single machine? How do we go for it?
    b) rs.add("ip_of_secondary_node_1"), Is this ip of a different machine? Can we have it on same machine with different port?
    c) Do we need to execute Step 2 on all nodes with same port and rsname? What if we want to start multiple nodes on a single machine?

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