MongoDB - Setting up a Replica Set

cd \mongodbdir\

mkdir db1
mkdir db2
mkdir db3

Primary

mongod --dbpath ./db1 --port 30000 --replSet "demo"

Secondary

mongod --dbpath ./db2 --port 40000 --replSet "demo"

Arbiter

mongod --dbpath ./db3 --port 50000 --replSet "demo"

Configuration

You need to connect to one of the database sets and setup the configuration. The mongo client is a javascript repl so you can do all the usual javascript commands.

var demoConfig = { 
    _id: "demo", 
    members: [
        { _id: 0, 
          host: 'localhost:30000', 
          priority: 10
        },
        { _id: 1, 
          host: 'localhost:40000'
        }, 
        { _id: 2, 
          host: 'localhost:50000', 
          arbiterOnly: true
        }
    ] 
 };

Initialize the Replica Set

The rs stands for replica set.

rs.initiate(demoConfig)

Please note that this can take some time to setup the databases initially. When it is complete it will return back:

{
    "info" : "Config now saved locally.  Should come online in about a minute.",
    "ok" : 1
}

The prompt will change to demo:PRIMARY>.

Test

To test add some data to the primary:

db.foo.save({_id:1, value:'hello world'})

Check that the data is saved on the primary:

db.foo.find()

This will return:

{ "_id" : 1, "value" : "hello world" }

Then log onto the secondary. This will have a prompt of demo:SECONDARY>.

db.foo.find()

This will return:

{ "_id" : 1, "value" : "hello world" }

Error not master and slaveOk=false

You may get an initial error error: { "$err" : "not master and slaveOk=false", "code" : 13435 } when you are trying to run db.foo.find(). This is because the secondary (ie the Slave) is not setup to perform reads. This can be enabled using the following command:

db.setSlaveOk()

You should then be able to access the data.

Replica Set Status

To check on the status of a replica set just use:

rs.status()

checkaayush/mongodb_setting_up_a_replica_set.md