How to create MongoDB replica set on Windows

In this tutorial, we will discuss about how to create a mongodb replica set on windows-

1 First start 3 mongod processes individually-
mongod --port 30001  --dbpath C:\app\mongo\mongo_replica\mongoag1 --logpath C:\app\mongo\mongo_replica\mongoag1\log1  --replSet rs1 --logappend --bind_ip 127.0.0.1

mongod --port 30002  --dbpath C:\app\mongo\mongo_replica\mongoag2 --logpath C:\app\mongo\mongo_replica\mongoag2\log2  --replSet rs1 --logappend --bind_ip 127.0.0.1

mongod --port 30003  --dbpath C:\app\mongo\mongo_replica\mongoag3 --logpath C:\app\mongo\mongo_replica\mongoag3\log3  --replSet rs1 --logappend --bind_ip 127.0.0.1

2 Now login to each member  individually to check that mongo is up and running for all 3 members

mongo --port 30001
mongo --port 30002
mongo --port 30003

3 You may disconnect from all 3 and connect to one member which you want to make primary
mongo --port 30001

Issue the following commands to define the configuration of replica set and initiate it. Here replica set name is "rs1".

cfg= { _id : "rs1",   members: [
      { _id: 0, host: "127.0.0.1:30001" },
      { _id: 1, host: "127.0.0.1:30002" },
      { _id: 2, host: "127.0.0.1:30003" }
   ] }

rs.initiate (cfg)

The prompt will automatically change  to "OTHER", then to "SECONDARY" and finally to "PRIMARY"

4 Now issue the following command to validate the configuration of replica set -

rs1:PRIMARY> rs.status()
{
        "set" : "rs1",
        "date" : ISODate("2019-02-24T12:03:20.849Z"),
        "myState" : 1,
        "term" : NumberLong(1),
        "heartbeatIntervalMillis" : NumberLong(2000),
        "optimes" : {
                "lastCommittedOpTime" : {
                        "ts" : Timestamp(1551009787, 1),
                        "t" : NumberLong(1)
                },
                "readConcernMajorityOpTime" : {
                        "ts" : Timestamp(1551009787, 1),
                        "t" : NumberLong(1)
                },
                "appliedOpTime" : {
                        "ts" : Timestamp(1551009787, 1),
                        "t" : NumberLong(1)
                },
                "durableOpTime" : {
                        "ts" : Timestamp(1551009787, 1),
                        "t" : NumberLong(1)
                }
        },
        "members" : [
                {
                        "_id" : 0,
                        "name" : "127.0.0.1:30001",
                        "health" : 1,
                        "state" : 1,
                        "stateStr" : "PRIMARY",
                        "uptime" : 85,
                        "optime" : {
                                "ts" : Timestamp(1551009787, 1),
                                "t" : NumberLong(1)
                        },
                        "optimeDate" : ISODate("2019-02-24T12:03:07Z"),
                        "infoMessage" : "could not find member to sync from",
                        "electionTime" : Timestamp(1551009785, 1),
                        "electionDate" : ISODate("2019-02-24T12:03:05Z"),
                        "configVersion" : 1,
                        "self" : true
                },
                {
                        "_id" : 1,
                        "name" : "127.0.0.1:30002",
                        "health" : 1,
                        "state" : 2,
                        "stateStr" : "SECONDARY",
                        "uptime" : 26,
                        "optime" : {
                                "ts" : Timestamp(1551009787, 1),
                                "t" : NumberLong(1)
                        },
                        "optimeDurable" : {
                                "ts" : Timestamp(1551009787, 1),
                                "t" : NumberLong(1)
                        },
                        "optimeDate" : ISODate("2019-02-24T12:03:07Z"),
                        "optimeDurableDate" : ISODate("2019-02-24T12:03:07Z"),
                        "lastHeartbeat" : ISODate("2019-02-24T12:03:19.912Z"),
                        "lastHeartbeatRecv" : ISODate("2019-02-24T12:03:19.153Z"),
                        "pingMs" : NumberLong(1),
                        "syncingTo" : "127.0.0.1:30001",
                        "configVersion" : 1
                },
                {
                        "_id" : 2,
                        "name" : "127.0.0.1:30003",
                        "health" : 1,
                        "state" : 2,
                        "stateStr" : "SECONDARY",
                        "uptime" : 26,
                        "optime" : {
                                "ts" : Timestamp(1551009787, 1),
                                "t" : NumberLong(1)
                        },
                        "optimeDurable" : {
                                "ts" : Timestamp(1551009787, 1),
                                "t" : NumberLong(1)
                        },
                        "optimeDate" : ISODate("2019-02-24T12:03:07Z"),
                        "optimeDurableDate" : ISODate("2019-02-24T12:03:07Z"),
                        "lastHeartbeat" : ISODate("2019-02-24T12:03:19.915Z"),
                        "lastHeartbeatRecv" : ISODate("2019-02-24T12:03:19.635Z"),
                        "pingMs" : NumberLong(0),
                        "syncingTo" : "127.0.0.1:30001",
                        "configVersion" : 1
                }
        ],
        "ok" : 1,
        "operationTime" : Timestamp(1551009787, 1),
        "$clusterTime" : {
                "clusterTime" : Timestamp(1551009787, 1),
                "signature" : {
                        "hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
                        "keyId" : NumberLong(0)
                }
        }
}
rs1:PRIMARY>

NOTE: To connect to secondary nodes, we have to issue rs.slaveOk(). Like in this case, to connect to mongo on secondary nodes-
mongo  --port 30001 (or port 30002)
rs.slaveOk()

If you want to set up mongo replication on unix, then plz go through following link for creating mongo replica set-
http://www.dbatraininghub.com/2019/02/how-to-create-replica-set-in-unix.html

Replica Set in MongoDB

• Replica Set  in MongoDB ensure high availability and redundancy.
• Replica Set is  basically set of different  mongod processes which can run on different machines or on same machine. Typically in a prod environment, these should run on different machines/nodes.
• All members of replica set contains same data.
• There can be only 1  read-write member which is called primary member.
• All others members are read only members called secondary members.
• Data is replicated from read-write member to read only members asynchronously.
• Optionally, there can be arbiter which does not require dedicated hardware. Arbiter is not data bearing node. It just participates is elections to obtain majority.
• It is recommended to have add number of nodes in Mongo replica set. We typically add a arbiter to make odd number of nodes in replica set.
• When primary node fails, or does not communicate with other nodes in replica set( for by default 10 sec), an election happens. The purpose of elections is to choose a primary in mongo replica set, so that normal operations can be resumed (write can also happen now). Election can also happen in certain other circumstances.