Replication

Basic configuration with three nodes

The replica set is a group of mongod instances that maintain the same data set.

This example shows how to configure a replica set with three instances on the same server.

Creating data folders

mkdir /srv/mongodb/data/rs0-0
mkdir /srv/mongodb/data/rs0-1
mkdir /srv/mongodb/data/rs0-2

Starting mongod instances

mongod --port 27017 --dbpath /srv/mongodb/data/rs0-0 --replSet rs0
mongod --port 27018 --dbpath /srv/mongodb/data/rs0-1 --replSet rs0
mongod --port 27019 --dbpath /srv/mongodb/data/rs0-2 --replSet rs0

Configuring replica set

mongo --port 27017    // connection to the instance 27017

rs.initiate();                // initilization of replica set on the 1st node
rs.add("<hostname>:27018")    // adding a 2nd node
rs.add("<hostname>:27019")    // adding a 3rd node

Testing your setup

For checking the configuration type rs.status(), the result should be like:

{
        "set" : "rs0",
        "date" : ISODate("2016-09-01T12:34:24.968Z"),
        "myState" : 1,
        "term" : NumberLong(4),
        "heartbeatIntervalMillis" : NumberLong(2000),
        "members" : [
                {
                        "_id" : 0,
                        "name" : "<hostname>:27017",
                        "health" : 1,
                        "state" : 1,
                        "stateStr" : "PRIMARY",
                        ...........................
                },
                {
                        "_id" : 1,
                        "name" : "<hostname>:27018",
                        "health" : 1,
                        "state" : 2,
                        "stateStr" : "SECONDARY",
                        ...........................
                },
                {
                        "_id" : 2,
                        "name" : "<hostname>:27019",
                        "health" : 1,
                        "state" : 2,
                        "stateStr" : "SECONDARY",
                        ...........................
                }
        ],
        "ok" : 1
}