", user presses a button) A controller contains the input part of the user interface. The controller accesses the model, possibly updating it in a way appropriate to the user's action (e . g. , controller updates user's shopping cart) A view uses the model to generate an appropriate user interface (e. g. , view produces a screen listing the shopping cart contents). The view gets its own data from the model (or direct from the data access layer - sometimes there is no domain logic to do). Model Portfolio Hosting The model should have no direct knowledge of the view. However, the observer pattern can be used to provide some indirection between model and view, allowing the model to notify interested parties of a change. "

" If the deviation from model to original cannot be tolerated then the original has to be changed. Design models are in a prescriptive reference mode to their originals, i. e. , the implementation. (3) idealizing, i. e. , the model is supposed to deviate from the original and as long as there is a good reason for that one doesn't worry. Model Portfolio Atlanta Software process models are in an idealizing reference mode to the actually realized software process. As the software process model describes how software shall be developed under ideal conditions there is no reason to be worried if one deviates from the model because the real conditions are not ideal. "