Some commands needed to starting up a replica set in MongoDB

mongodb-repl-replica-set-config-up.txt

MongoDB Setting up a replica set

The configuration file for the first node (/data/node1.conf):
	storage:
	  dbPath: /var/mongodb/db/node1
	net:
	  bindIp: 192.168.103.100,localhost
	  port: 27011
	security:
	  authorization: enabled
	  keyFile: /var/mongodb/pki/m103-keyfile
	systemLog:
	  destination: file
	  path: /var/mongodb/db/node1/mongod.log
	  logAppend: true
	processManagement:
	  fork: true
	replication:
	  replSetName: m103-example


Creating the keyfile and setting permissions on it:
	sudo mkdir -p /var/mongodb/pki/
	sudo chown vagrant:vagrant /var/mongodb/pki/
	openssl rand -base64 741 > /var/mongodb/pki/m103-keyfile
	chmod 400 /var/mongodb/pki/m103-keyfile


Creating the dbpath for node1:
	mkdir -p /var/mongodb/db/node1


Starting a mongod with node1.conf:
	mongod -f node1.conf


Copying node1.conf to node2.conf and node3.conf:
	cp node1.conf node2.conf
	cp node2.conf node3.conf


node2.conf, after changing the dbpath, port, and logpath:
	storage:
	  dbPath: /var/mongodb/db/node2
	net:
	  bindIp: 192.168.103.100,localhost
	  port: 27012
	security:
	  keyFile: /var/mongodb/pki/m103-keyfile
	systemLog:
	  destination: file
	  path: /var/mongodb/db/node2/mongod.log
	  logAppend: true
	processManagement:
	  fork: true
	replication:
	  replSetName: m103-example

node3.conf, after changing the dbpath, port, and logpath:
	storage:
	  dbPath: /var/mongodb/db/node3
	net:
	  bindIp: 192.168.103.100,localhost
	  port: 27013
	security:
	  keyFile: /var/mongodb/pki/m103-keyfile
	systemLog:
	  destination: file
	  path: /var/mongodb/db/node3/mongod.log
	  logAppend: true
	processManagement:
	  fork: true
	replication:
	  replSetName: m103-example


Creating the data directories for node2 and node3:
	mkdir /var/mongodb/db/{node2,node3}


Starting mongod processes with node2.conf and node3.conf:
	mongod -f node2.conf
	mongod -f node3.conf


Connecting to node1:
	mongo --port 27011


Initiating the replica set:

This command rs.initiate will initiate the replica set. And we actually need to run it on one of the nodes. Because we ran it here, we just have to add the other two nodes from this node.
	rs.initiate()


Creating a user:
	use admin
	db.createUser({
	  user: "m103-admin",
	  pwd: "m103-pass",
	  roles: [
		{role: "root", db: "admin"}
	  ]
	})


Exiting out of the Mongo shell and connecting to the entire replica set:

All right, so this command created our m103 super user, called m103-admin, that has root access and authenticates against the admin database.
Now I'm just going to exit out of this mongod and then log back in as that user.
So this is the command that we're going to use to connect to the replica set.
And in addition to authenticating here with a username password, we have to specify the name of the replica set in the host name.
This will tell the mongo shell to connect directly to the replica set, instead of just this one node that we specify.

	exit
	mongo --host "m103-example/192.168.103.100:27011" -u "m103-admin" -p "m103-pass" --authenticationDatabase "admin"


Getting replica set status:
	rs.status()


Adding other members to replica set:
	rs.add("m103.mongodb.university:27012")
	rs.add("m103.mongodb.university:27013")
	

Getting an overview of the replica set topology:
	rs.isMaster()


Stepping down the current primary:
	rs.stepDown()


Checking replica set overview after election:
	rs.isMaster()


===========================================================================================================================
					Some information about replica set
===========================================================================================================================
After initiating our node and adding the node to the replica set, the oplog.rs collection is created.
By default we take 5% of the available disk.
But this value can also be set by configuring it through the oplog size in megabytes under the replication section of our configuration file.
As operations get logged into the oplog, like inserts or deletes kind of operations, the oplog.rs collection starts to accumulate the operations and statements, until it reaches the oplog size limit.
Once that happens, the first operations in our oplog start to be overwritten with newer operations.
The time it takes to fill in fully our oplog and start rewriting the early statements determines the replication window.
Every node in our replica set has its own oplog.
Now if for some reason one of the nodes gets disconnected the replica set keeps on accumulating new writes.
Basically what will happen is that a recovering node will check for its last oplog entry and try to find it in one of the available nodes.
The size of our oplog.rs collection is an important aspect to keep in mind.
The replication window measured in hours will be proportional to the load of your system.