When To Go For MongoDB

It is an Open-Source Database runs on an collection of architecture of collections and documents. And a No SQL Product.

You should consider MongoDB if you have one of following challenges :

Expect a High Level Write Load: It prefers High Insert rate over transaction safety.  

In an Unreliable Environment Need High Availability: Setting Replica Sets [Set of servers Act as Master and slave] is easy and fast. Recovery and failover management is fast and automatic.

Need Shared Data: If Shared or Portioned database is required, MongoDB is best option to choose.

Need Location Based Data: It has built-in functions, so finding relevant data is accurate and fast.

Unstable Schema / Data Set Going To Be Big: Including new column in RDBMS can create a major load and degrade the performance in others or may lock entire database. Usually it happens when table size is larger than 1024 MB i.e 1GB. As MongoDB is Schema-less so adding new columns would not affect old documents and instant.