In general, shard means a part of a whole. And in our current context, it will be a partition of a database; a horizontal one, to be specific. This is because we are going to talk about MongoDB in this article and how to do MongoDB sharding (meaning creating horizontal partitions) on an Ubuntu 14.04 machine.
Sharding, in simple terms, is the process of writing data across different servers. With sharding, we can add more servers to support data growth, and the demands of read and write operations.

MongoDB is an open-source NoSQL high performance database and stores data through a key-value system. It provides performance, scaling and high availability; and how does it provide scalability? MongoDB scaling is done through sharding.

Sharding has three different components and each components performs different functions. The following diagram shows those three components and its connectivity:
Config Servers
– Config servers store the metadata of the clusters dataset to the shards. A production sharding must have exactly three config servers. These will organize the data for Query router.
Shards (Replica set)
– Shards are responsible to store the actual data. Each shard is composed of multiple replica set, and we can define a replica set as containing one primary and one or more secondary set. As a result, shards provide high availability through replica set. The primary set alone can do write operations and secondary can do only Read operations. Furthermore, when the primary replica set goes down then one of the secondary replica set will act as master.
Query Routes (mongos)
– Application and direct mongo operations can be done through Query routers. For each request it will communicate with shards then returns the results to clients. Also, the shard setup may have multiple Query router to split the request load.

Before starting MongoDB sharding setup, make sure that all the servers are having the same version of MongoDB. In order to install mongo 3.2 in all four servers, use the below commands,

# First of all, Install the apt repo:

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

# Then, Create a list file for MongoDB:

$ echo "deb http://repo.mongodb.org/apt/ubuntu "$(lsb_release -sc)"/mongodb-org/3.2 multiverse" | sudo tee /etc/apt/sources.list.d/mongodb-org-3.2.list

# Update the package:

$ sudo apt-get update

# Then Install MongoDB

$ sudo apt-get install -y mongodb-org

# Finally, Freeze MongoDB version:

$ echo "mongodb-org hold" | sudo dpkg --set-selections
            $ echo "mongodb-org-server hold" | sudo dpkg --set-selections
            $ echo "mongodb-org-shell hold" | sudo dpkg --set-selections
            $ echo "mongodb-org-mongos hold" | sudo dpkg --set-selections
            $ echo "mongodb-org-tools hold" | sudo dpkg --set-selections

 

Step 2.1: First, Create config server metadata storage directory

$ mkdir -p /var/mongodb/mongo-metadata

Step 2.2: Then, Run mongo sharding config server

$ mongod --configsvr --dbpath /var/mongodb/mongo-metadata --port 27019

 

Step 3.1: First, create Shards storage directory

$ mkdir -p /var/mongodb/store/rs1
                $ mkdir -p /var/mongodb/store/rs2
                $ mkdir -p /var/mongodb/store/rs3

Step 3.2: In order to run MongoDB Shards Replica set

$ mongod --shardsvr --replSet rs1 --dbpath /var/mongodb/store/rs1 --port 27000
                $ mongod --shardsvr --replSet rs2 --dbpath /var/mongodb/store/rs2 --port 27001
                $ mongod --shardsvr --replSet rs1 --dbpath /var/mongodb/store/rs3 --port 27001

Step 3.3: Then, Open mongo console

$ mongo --port 27000
                mongo> rs.initiate()
                mongo> rs.add("shard.host.com:27001")
                mongo> rs.add("shard.host.com:27002")

 

Step 5.1: Open mongo console

$ mongo --host query0.example.com --port 27017
            mongo> sh.addShard( "rep_set_name/rep_set_member:27017" )

Step 5.2: In order to find the replica set member name, go to Shard server

$ mongo --port 27000
            mongo> rs.status()

Now, select the Primary members name.

Open mongo console

$ mongo --host query0.example.com --port 27017

Stop the sharding balancer
Before enabling sharding we have to stop the balancer service from mongo console.

mongo> sh.stopBalancer()

Enable sharding to the database

mongo> use current_DB
mongo> sh.enableSharding("#{current_DB}")

Enable sharding to the collections

mongo> use current_DB
mongo> sh.shardCollection( "current_DB.test_collection", { "_id" : "shard key" } )

 
Specifically notable is that the shard key that you choose here for sharding the collection will have impact on the performance.