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MongoDB has become the best-known NoSQL database on the market. MongoDB is document-oriented, and its schemaless design makes it popular in a variety of WEB applications. One of my favorite features is its Replica Set, which places multiple copies of the same data on a set of mongod nodes, resulting in data redundancy and high availability.

This tutorial will show you how to configure a MongoDB replica set.

The most common configuration of replica sets requires a primary node and multiple secondary nodes. The replication behavior that is initiated after that will go from this primary node to other secondary nodes. Replica sets not only protect the database from unexpected hardware failures and downtime events, but also improve the throughput of database client data reads because it provides more nodes.

Configure the environment

In this tutorial, we will configure a replica set that includes one primary node and two secondary nodes.

To achieve this, we used three virtual machines running on VirtualBox. I will install Ubuntu 14.04 on these virtual machines and install the MongoDB official package.

I will configure the required environment on one virtual machine instance and then clone it to another virtual machine instance. Therefore, select a virtual machine named master and perform the following installation procedure.

First, we need to add a MongoDB key to apt:

$sudo apt-key adv--keyserver hkp://keyserver.ubuntu.com:80-recv 7F0CEB10

Then, add the official MongoDB repository to source.list:

$sudo su

# echo "deb http://repo.mongodb.org/apt/ubuntu" $(lsb_release-sc) "/ mongodb-org/3.0 multiverse" | sudo tee/etc/apt/sources.list.d/mongodb-org-3.0.list

Next, update the apt repository and install MongoDB.

$sudo apt-get update

$sudo apt-get install-ymongodb-org

Now make some changes to / etc/mongodb.conf

Auth = true

Dbpath=/var/lib/mongodb

Logpath=/var/log/mongodb/mongod.log

Logappend=true

KeyFile=/var/lib/mongodb/keyFile

ReplSet=myReplica

The purpose of the first line is to indicate that our database needs to be verified before it can be used. Keyfile configures the key file used for replication between MongoDB nodes. ReplSet sets a name for the replica set.

Next we create a key file for all instances.

$echo-n "MyRandomStringForReplicaSet" | md5sum > keyFile

This will create a key file that contains MD5 strings, but because it contains some noise, we need to clean them up before we can officially use them in MongoDB.

$echo-n "MyReplicaSetKey" | md5sum | grep-o "[0-9a-z]\ +" > keyFile

The function of the grep command is to print out the MD5 string after filtering out spaces and other unwanted content.

Now let's do something with the key file to make it really available.

$sudo cp keyFile/var/lib/mongodb

$sudo chown mongodb:nogroupkeyFile

$sudo chmod 400 keyFile

Next, shut down the virtual machine. Clone its Ubuntu system to another virtual machine.

This is the cloned secondary node 1 and secondary node 2. Make sure you have reinitialized their MAC addresses and cloned the entire hard drive.

Note that the three virtual machine examples need to be on the same network in order to communicate with each other. Therefore, we need them to get on the Internet.

It is recommended that you set a static IP address for each virtual machine instead of using DHCP. So they don't lose connectivity when DHCP assigns them an IP address.

Edit the / etc/networks/interfaces file for each virtual machine as follows.

On the primary node:

Auto eth2

Iface eth2 inet static

Address 192.168.50.2

Netmask 255.255.255.0

On secondary node 1:

Auto eth2

Iface eth2 inet static

Address 192.168.50.3

Netmask 255.255.255.0

On secondary node 2:

Auto eth2

Iface eth2 inet static

Address 192.168.50.4

Netmask 255.255.255.0

Since we don't have a DNS service, we need to set up the / etc/hosts file and manually put the host name in this file.

On the primary node:

127.0.0.1 localhost primary

192.168.50.2 primary

192.168.50.3 secondary1

192.168.50.4 secondary2

On secondary node 1:

127.0.0.1 localhostsecondary1

192.168.50.2 primary

192.168.50.3 secondary1

192.168.50.4 secondary2

On secondary node 2:

127.0.0.1 localhostsecondary2

192.168.50.2 primary

192.168.50.3 secondary1

192.168.50.4 secondary2

Use the ping command to check the connectivity between the nodes.

$ping primary

$ping secondary1

$ping secondary2

Configure copy set

After verifying that the nodes are connected properly, we can create a new administrator user for subsequent replica set operations.

On the main node, open the / etc/mongodb.conf file and comment out the auth and replSet.

Dbpath=/var/lib/mongodb

Logpath=/var/log/mongodb/mongod.log

Logappend=true

# auth = true

KeyFile=/var/lib/mongodb/keyFile

# replSet=myReplica

Before configuring any users or replica sets on a newly installed MongoDB, you need to comment out the auth lines. By default, MongoDB does not create any users. And if you enable auth before you create a user, you can't do anything. You can enable auth again after creating a user.

After modifying / etc/mongodb.conf, restart the mongod process.

$sudo service mongod restart

Now connect to MongoDB master:

$mongo:27017

After connecting to MongoDB, create a new administrator user.

> use admin

> db.createUser ({

User: "admin"

Pwd: "

})

Restart MongoDB:

$sudo service mongod restart

Connect to MongoDB again and add the secondary node 1 and secondary node 2 nodes to our replica set with the following command.

> use admin

> db.auth ("admin", "myreallyhardpassword")

> rs.initiate ()

> rs.add ("secondary1:27017")

> rs.add ("secondary2:27017")

Now that we have the copy set, we can start our project. Refer to the official driver documentation to learn how to connect to a copy set. If you want to use Shell to request data, you need to connect to the primary node to insert or request data, not the secondary node. If you insist on trying to operate with a copy set, the following error message pops up to greet you.

MyReplica:SECONDARY >

MyReplica:SECONDARY > showdatabases

2015-05-10T03:09:24.131+0000E QUERY Error: listDatabases failed: {"note": "from execCommand", "ok": 0, "errmsg": "not master"}

At Error ()

At Mongo.getDBs (src/mongo/shell/mongo.js:47:15)

At shellHelper.show (src/mongo/shell/utils.js:630:33)

At shellHelper (src/mongo/shell/utils.js:524:36)

At (shellhelp2): 1:1 at src/mongo/shell/mongo.js:47

If you want to connect to the entire replica set from shell, you can install the following command. Failed switching in the replica set is automatic.

$mongoprimary,secondary1,secondary2:27017/?replicaSet=myReplica

If you use other driver languages (for example, JavaScript, Ruby, etc.), the format may be different.

I hope this tutorial will be helpful to you. You can use Vagrant to automate your local environment configuration and speed up your code.

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