With the new Version 3.2 we have introduced a new feature called SatelliteCollections. This post explains what this is all about, how it can help you, and explains a concrete use case for which it is essential.
Background and Overview
Join operations are very useful but can be troublesome in a distributed database. This is because quite often, a join operation has to bring together different pieces of your data that reside on different machines. This leads to cluster internal communication and can easily ruin query performance. As in many contexts nowadays, data locality is very important to avoid such headaches. There is no silver bullet, because there will be many cases in which one cannot do much to improve data locality.
One particular case in which one can achieve something, is if you need a join operation between a very large collection (sharded across your cluster) and a small one, because then one can afford to replicate the small collection to every server, and all join operations can be executed without network communications.