The MVC Architecture

Back in 1979, Trygve Reenskaug came up with a new architecture for developing interactive applications. In his design, applications were broken into three types of components: models, views, and controllers.

The model is responsible for maintaining the state of the application. Sometimes this state is transient, lasting for just a couple of interactions with the user.

Sometimes the state is permanent and is stored outside the application,often in a database. A model is more than data; it enforces all the business rules that apply to that data. For example, if a discount shouldn’t be applied to orders of less than $20, the model enforces the constraint. This makes sense; by putting the implementation of these business rules in the model, we make sure that nothing else in the application can make our data invalid. The model acts as both a gatekeeper and a data store.

The view is responsible for generating a user interface, normally based on data in the model. For example, an online store has a list of products to be displayed on a catalog screen. This list is accessible via the model, but it’s a view that formats the list for the end user. Although the view might present the user with various ways of inputting data, the view itself never handles incoming data. The view’s work is done once the data is displayed. There may well be many views that access the same model data, often for different purposes. The online store has a view that displays product information on a catalog page, and another set of views used by administrators to add and edit products.

Controllers orchestrate the application. Controllers receive events from the outside world (normally, user input), interact with the model, and display an appropriate view to the user.

This triumvirate — the model, view, and controller — together form an architecture known as MVC.

This style of architecture was originally intended for conventional GUI applications, where developers found that the separation of concerns led to far less coupling, which in turn made the code easier to write and maintain. Each concept or action was expressed in a single, well-known place. Using MVC was like constructing a skyscraper with the girders already in place — it was a lot easier to hang the rest of the pieces with a structure already there.

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