Controller Micro-Architecture

A Controller supplies one or more web-service API's.  Those API's are most often closely related to a single Model.  For example, one has a Widget model and a WidgetsController.  That close relationship can cause the lines of responsibility to become blurred.

Controllers also coordinate with the presentation of the response.  Again, the lines of responsibility can be a bit a fuzzy - does the logic go in the controller or in the .erb file?

Here is my sense for controllers:

• Functional Authorization
• Does the user have to be signed-in to access the controller's method?
• Is the user allowed to access the method?  For example, is it superuser-only?
• Check for basic argument correctness
• Required parameters
• Extraneous parameters (e.g. white-list)
• Data Authorization
• Is the user allowed to see/modify/delete that particular data?
• Invocation of domain logic
• Error handling
• Assigning domain logic results to instance variables
• For use by result rendering (HTML, XML, JSON)
• Selection and invocation of the result rendering code
• Often done implicitly by Rails!
• Correct presentation of errors

Next:  Best (for some relative value of best) Practices for each area of responsibility.