MongoDB is a NoSQL database that has Drupal integration for various scenarios.
One of these scenarios is using MongoDB as the caching layer for Drupal.
This article describes what is needed to get MongoDB working as a caching layer for your Drupal site. We assume that you have an Ubuntu Server LTS 14.04 or similar Debian derived distro.
Download The Drupal Module
First, download the MongoDB Drupal module. You do not need to enable any MongoDB modules.