What are messages for?

They are for exception handling. Each message provides the info Angus needs to properly respond to an exception raised by your ruby code.

When do I need a message?

Let's say you were trying to get a post by it's id. But the post wasn't found. So what you should do is respond with 404 - not found. The way to do this in Angus is to raise a custom exception let's name it PostNotFoundError, which you should define as a class inheriting from StandardError.

class PostNotFoundError < StandardError
  def message
    "Post not found."
  end
end

Now we need to add an entry in messages.yml so angus knows how to handle this exception:

PostNotFoundError:
  status_code: 404
  level: error
  description: No Post found for the given id.

This will tell angus to return a 404 and the message given by the error's message method. So every time you throw an exception in your code remember to add the message definition in the messages.yml.