MVC is What Exactly?

MVC (Model, View, Controller) is a design pattern or application architecture, depending on how you want to look at it, used to separate the storage of data from the presentation of data from the processing that takes place when the user interacts with the software.

It's also a huge buzzword in software development that everyone has heard about and thinks they understand but that very few people have ever actually researched or read about.

The concept of MVC was first introduced in the Smalltalk language. The Model stores data, the View presents a user interface, and the Controller handles user interaction.

It was recognized that MVC typically took one of two general forms, Passive or Active. Each form differs based on how the controller and the model behave.

Active Form:

1. Controller handles user iteration events and updates model
2. State changes in the model cause the view to update (observer pattern keeps model independent of view)

Note: This means the model can be changed by things other than the controller and the view will still updatePassive Form:

1. Controller handles user interaction events and updates the model and causes the view to update
2. Model is completely independent of view and controller

Clearly the Model must carry a lot of responsibility in the Active Form by maintaining state or whatever else is needed to keep the UI up to date. In the Passive Form the Model isn't required to do anything more than house the data. The Controller can manage the state changes.

MVC also appears in a slightly different form called the Document-View pattern used by the MFC (Microsoft Foundation Classes) framework which combines the controller and the model into one object, the Document.

MVC as seen in Ruby on Rails is basically the passive model. The controller is invoked by a user interaction. The Controller builds the model (or tells it to build itself) and/or modifies it as needed, the controller then picks what view to render and passes it any data it may need.

ASP.NET's new MVC framework works almost exactly the same way as Ruby on Rails in terms of MVC (surprise surprise!).

Passive MVC makes tons of sense on the web because the Request/Response nature of HTTP requires a very structured data passing model. Ultimately, that's all MVC is, a standard for how to pass data around:

Data is housed in the Model, accessed and modified by the controller, passed to the view to be displayed, passed back to the controller from the view w/ changes made by the user, etc...

So, MVC is a great thing on the web. It simplifies the code, makes it obvious where to go for what, standardizes the flow of data and rendering of web pages, and exactly matches the behavior of HTTP.

That being said, there are still some variations:
Does the Model match the database schema and just house data (as in .NET Typed Datasets or the new LINQ to SQL objects)? Or does the Model present data in an intelligent (problem domain specific) way and contain high level logic for modifying the Model? I'm not sure it actually matters, honestly. Its just a question of where you want to put your logic, all in the controller or spread between the controller and the model?

So, now we've surveyed MVC, talked about some frameworks that use it, and decided that it works really well for the web. In the next post I'll look at if MVC makes sense in a Windows Application.

Update 01/25/08: Added chicken scratch images