The AEM MicroKernel (extremely high altitude view)

The microkernel is the layer on which all other things are built. It determines the type of storage that AEM is going to use. To put it simply it is the persistence layer for AEM.

The Options

There are two choices available. Each of these has specific pro's and cons that must be considered when setting up an AEM environment.

TarMK

The TarMK option is the one that is deployed by default on all new AEM author instances, and publish instances will most likely always be deployed as TarMK. It is designed specifically for performance, but is not as strong in terms of scalability.

MongoMK

The primary reason for choosing the MongoMK persistence backend over TarMK is to scale the instances horizontally. 

This means having two or more active author instances running at all times and using MongoDB as the persistence storage system. 

The need to run more than one author instance results generally from the fact that the CPU and memory capacity of a single server, supporting all concurrent authoring activities, is no longer sustainable.

It is not recommended to deploy MongoMK for publish instances.

Moving from one to the other

It is possible to migrate from TarMK to MongoMK. 

Adobe recommends first deploying to TarMK and then moving to MongoMK, unless you foresee conditions that demand MongoMK within the first 18months after a deployment.

How to decide which MicroKernel fits your scenario best (for Author instances)

You will first need to know the following :

• Number of named users connected in a day: in the thousands or more.
• Number of concurrent users: in the hundreds or more.
• Volume of asset ingestions per day: in hundreds of thousands or more.
• Volume of page edits per day: in hundreds of thousands or more (including automated updates via Multi Site Manager or news feed ingestions for example).
• Volume of searches per day: in tens of thousands or more.

A tool called Tough Day can be used to evaluate an existing deployment, in terms of the hardware configuration.

Once you have the above details, the decision process is as follows:

Sources: 

https://docs.adobe.com/docs/en/aem/6-0/deploy/recommended-deploys.html
https://docs.adobe.com/docs/en/aem/6-0/deploy/upgrade/data-store-config.html