The last major subsystem that I will include here is the caching layer. Drupal 8 has gone to great lengths to improve the performance of building pages and rendering data. To this end, the caching system has become an important part to consider whenever we either do complex or heavy calculations or render content.
From a module developer's perspective, there are two main pillars of the caching system. The first one provides developers with a cache backend to store the results of complex data calculations. This can be read in the next requests to avoid the need for reprocessing that task. This goes hand in hand with the cache invalidation that happens when something in the system changes that would require the calculations to be redone. The second pillar is the render cache, which allows developers to wrap their output with metadata that describes when the cache of that output needs to be invalidated.
We will see these in action in a later chapter dedicated to caching.