Matthias Noback continues his highly interesting series of posts with programming guidelines. Part four is about messages.

Besides having a type and a particular value, messages can also be categorized:
  • A command message is a request for change. One object wants to make the other object do something.
  • A query message is a request for information. One object wants to get something from the other object.
  • A document message is the response from the other object, based on the query that was sent to it.