Model – view – controller architecture

25 Jan

The model- view – controller architecture (MVC as it is generally termed), is the basis of many applications and development frameworks. We can say its a software architecture. The idea is simple. Divide any logic into three different sets or domains.

Model –

The model manages the behavior and data of the application domain, responds to requests for information about its state (usually from the view), and responds to instructions to change state (usually from the controller). In event-driven systems, the model notifies observers (usually views) when the information changes so that they can react.

View –

The view renders the model into a form suitable for interaction, typically a user interface element. Multiple views can exist for a single model for different purposes. A viewport typically has a one to one correspondence with a display surface and knows how to render to it.

controller –

The controller receives input and initiates a response by making calls on model objects. A controller accepts input from the user and instructs the model and viewport to perform actions based on that input.


As a simple analogy (derived from http://nuts-and-bolts-of-cakephp.com/2009/01/06/another-way-to-think-about-mvc/)

So, let’s imagine a bank.

The safe is the Database this is where all the most important goodies are stored, and are nicely protected from the outside world.
Then we have the bankers or in programmatic terms the Models. The bankers are the only ones who have access to the safe (the DB). They are generally fat, old and lazy, which follows quite nicely with one of the rules of MVC: *fat models, skinny controllers*. We’ll see why and how this analogy applies a little later.

Now we’ve got our average bank workers, the gophers, the runners, the Controllers. Controllers or gophers do all the running around, that’s why they have to be fit and skinny. They take the loot or information from the bankers (the Models) and bring it to the bank customers the Views.

The bankers (Models) have been at the job for a while, therefore they make all the important decisions. Which brings us to another rule: *keep as much business logic in the model as possible*. The controllers, our average workers, should not be making such decisions, they ask the banker for details, get the info, and pass it on to the customer (the View). Hence, we continue to follow the rule of *fat models, skinny controllers*. The gophers do not make important decisions, but they cannot be plain dumb (thus a little business logic in the controller is OK). However, as soon as the gopher begins to think too much the banker gets upset and your bank (or you app) goes out of business. So again, always remember to offload as much business logic (or decision making) to the model.

Now, the bankers sure as hell aren’t going to talk to the customers (the View) directly, they are way too important in their cushy chairs for that. Thus another rule is followed: *Models should not talk to Views*. This communication between the banker and the customer (the Model and the View) is always handled by the gopher (the Controller).
(Yes, there are some exception to this rule for super VIP customers, but let’s stick to basics for the time being).

It also happens that a single worker (Controller) has to get information from more than one banker, and that’s perfectly acceptable. However, if the bankers are related (otherwise how else would they land such nice jobs?)… the bankers (Models) will communicate with each other first, and then pass cumulative information to their gopher, who will happily deliver it to the customer (View). So here’s another rule: *Related models provide information to the controller via their association (relation)*.

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One Response to “Model – view – controller architecture”

  1. ruining code January 28, 2011 at 12:23 pm #

    an example of MVC is cake PHP

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