From 2012 to mid-2014, MongoDB Inc. sponsored the 8.x-1.x branch, the goal of which was to create a complete NoSQL version of Drupal 8 running entirely on MongoDB, without any SQL database.

The result of that effort is an installable pre-8.0 Drupal NoSQL distribution, including the base (driver) extension, to use as a Drupal standard SQL database driver, Cache, Flood, Key-Value, Logger, Queue, Route, and partial Entity-Field storage.

Many more subsystems had patches waiting for review or extra work, to increase the features coverage of the distribution, when the project stopped receiving sponsoring, and did not recover. Yet ?

To try this version, you will need:

• Drupal core at commit 9b4dd5, which is the highest version known to have worked during Drupal Dev Days Szeged. This is a version between 8.0.0-beta9 and -beta10.
• PHP 5.4.5 to 5.6.*, with the legacy mongo extension. For extra geek credit, try PHP 7.x with the Alcaeus adapter and the mongodb extension
• The closed patches for 8.x-1.x contain lots of answers to the questions you will have

Outside this project, @daffie has been working hard on a Drumongous NoSQL distribution based on the 8.x-1.x branch, but evolved separately.

You can use Drupal867 a ready-to-run DDEV environment running the NoSQL distribution, for an easier start.