If an entity representation (JSON-LD, XML etc.) is received and deserialized to an entity object we should make sure that the fields on the entity are valid. This has to be done on write operations that create or update on entity ==> POST, PATCH and PUT in our case.
Example: A node title must not be longer than 256 characters. If a node is submitted with a title length of 257 characters the node should not be saved and a HTTP response with status 400 should be returned.
Before we can start to implement this we need a working entity validation system in place. Issues: