Need help understanding MVC

Question

To my understanding, MVC is a way to implement the separation of presentation tier from business and data tier. Am I understanding this correctly? If so, MVC should separate the business logic completely from presentation, right?

So to me it seems like javascript (or jquery) is somehow violating the MVC design since it takes over some of the logic on the client side, isn't it? Is model = data tier, controller = business tier, view = presentation tier? I think I have misunderstood the whole concept.

Answer 1

You seem to have a decent understanding of MVC. The trouble is that you are looking at two different potential MVC structures as one and the same. On the server, you can have data models, controllers, and views. On the client side, you can ALSO have data models, controllers, and views. If you want to look at your client side JavaScript as MVC, then jQuery is simply a utility that the view controllers can use to manipulate the view (the DOM).

Simply put, the client side doesn't always have to be only the view. If you use a web application client-side framework like Backbone, for example, then you can have models, views, and controllers all on the client side, which communicate with another, SEPARATE MVC structure on your server.

Answer 2

What you describe does actually pose a challenge for a lot of implementations. Frameworks such as the ASP.NET MVC Framework have been making attempts to auto-render JavaScript to the UI based on business logic in the middle tier (validation rules for form fields, primarily). But they're a long way off from having a truly compelling JavaScript user experience which doesn't repeat logic.

Personally, I like to think of the JavaScript as purely a UI concern. The application internally handles all of the logic. The JavaScript, as part of the UI, may duplicate some of that logic... but only for strictly UI purposes. Remember that the application should regress gracefully into a still-working state if the user has JavaScript disabled. That is, it should still use server-side (middle-tier) code to get the job done. All the JavaScript did was add a richer user experience to the UI layer.

JavaScript isn't the only culprit for this, either. Suppose you have a lot of validation logic in your middle tier defining what's valid or invalid for your objects. When you persist those objects to a database (which is on the periphery of the application just like the UI is), doesn't that database also contain duplicate validation logic? Non-nullable fields and such.

Answer 3

_{Congratulations! Your understanding of MVC is completely wrong. It has nothing to do with n-tier architecture (which is what you seem to be confusing it with).}

The core idea of MVC is separation of concerns. This is used by dividing the application it two major layers:

• model layer: contains all of the domain business logic and rules.
• presentation layer: deals it user interface

The presentation then is further split into controllers (for handling the user input) and views (for dealing with response).

When applied to web applications, you either have MVC (or MVC-like) structure only on server-side, or, for larger and more complicated applications, you have separate MVC triads for both frontend and backend.

Also, when working with applications, the user of MVC is not human being, but the browser.

In latter case the backend acts like one data source for frontend application. An the whole frontend part of MVC is written in javascript.

_{P.S. In case if you are able to read PHP code, you can find a quite simple explanation of model layer in this answer. And, yes. It is the "simple version" because MVC is a pattern for enforcing a structure in large application, not for making a guesbook.}

Answer 4

You can go to http://www.asp.net/mvc site and refer tutorials / samples to learn about MVC using Microsoft technologies.