Difference between Sharding And Replication on MongoDB

I am just confuse about the Sharding and Replication that how they works..According to Definition

Replication: A replica set in MongoDB is a group of mongod processes that maintain the same data set.

Sharding: Sharding is a method for storing data across multiple machines.

As per my understanding if there is data of 75 GB then by replication (3 servers), it will store 75GB data on each servers means 75GB on Server-1, 75GB on server-2 and 75GB on server-3..(correct me if i am wrong)..and by sharding it will be stored as 25GB data on server-1, 25Gb data on server-2 and 25GB data on server-3.(Right?)...but then i encountered this line in the tutorial

Shards store the data. To provide high availability and data consistency, in a production sharded cluster, each shard is a replica set

As replica set is of 75GB but shard is of 25GB then how they can be equivalent...this makes me confuse a lot...I think i am missing something great in this. Please help me in this.

Solution

Lets try with this analogy. You are running the library.

As any person who has is running a library you have books in the library. You store all the books you have on the shelf. This is good, but your library became so good that your rival wants to burn it. So you decide to make many additional shelves in other places. There is one the most important shelf and whenever you add some new books you quickly add the same books to other shelves. Now if the rival destroys a shelf - this is not a problem, you just open another one and copy it with the books.

This is replication (just substitute library with application, shelf with a server, book with a document in the collection and your rival is just failed HDD on the server). It just makes additional copies of the data and if something goes wrong it automatically selects another primary.

This concept may help if you

want to scale reads (but they might lag behind the primary).
do some offline reads which do not touch main server
serve some part of the data for a specific region from a server from that specific region
But the main reason behind replication is data availability. So here you are right: if you have 75Gb of data and replicate it with 2 secondaries - you will get 75*3 Gb of data.

Look at another scenario. There is no rival so you do not want to make copy of your shelves. But right now you have another problem. You became so good that one shelf is not enough. You decide to distribute your books between many shelves. You decide to distribute them between shelves based on the author name (this is not be a good idea and read how to select sharding key here). So everything that starts with name less then K goes to one shelf everything that is K and more goes to another. This is sharding.

This concept may help you:

distribute a workload
be able to save data which much more then can fit on a single server
do map-reduce things
store more data in ram for faster queries

Here you are partially correct. If you have 75Gb, then in sum on all the servers there will be still 75 Gb, but it does not necessarily be divided equally.

But here is a problem with only sharding. Right now your rival appeared and he just came to one of your shelves and burned it. All the data on that shelf is lost. So you want to replicate every shard as well. Basically the notion that

each shard is a replica set

is not true. But if you are doing sharding you have to create a replication for every shard. Because the more shards you have, the bigger is the probability that at least one will die.