Storing and using data are a critical part of any (web) application. Without somehow persisting data, we wouldn't be able to build much of anything. However, different uses of data warrant different systems for storing and manipulating it. For the purposes of this chapter, I will use the word data to mean almost anything that has to be persisted somewhere, for any given period of time.
If you've done development in Drupal 7, you already know a few ways of storing data--we had entities (primarily, the Node entity type, but others could be defined as well); the variables table, which was a relatively simple Key/Value store; and an API to interact with the database and do whatever we fancied. This caused many ...
