MongoDB 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
}