Why MVC doesn't fit the web

A common set of questions that come up on IRC around node web services revolve around how to do MVC “right” using tools like express.

The short answer: Don’t.

A little history. “MVC” is an abbreviation for “Model, View, Controller”. It’s a particular way to break up the responsibilities of parts of a graphical user interface application. One of the prototypical examples is a CAD application: models are the objects being drawn, in the abstract: models of mechanical parts, architectural elevations, whatever the subject of the particular application and use is. The “Views” are windows, rendering a particular view of that object. There might be several views of a three-dimensional part from different angles while the user is working. What’s left is the controller, which is a central place to collect actions the user is performing: key input, the mouse clicks, commands entered.

The responsibility goes something like “controller updates model, model signals that it’s been updated, view re-renders”.

This leaves the model relatively unencumbered by the design of whatever system it’s being displayed on, and lets the part of the software revolving around the concepts the model involves stay relatively pure in that domain. Measurements of parts in millimeters, not pixels; cylinders and cogs, rather than lines and z-buffers for display.

The View stays unidirectional: it gets the signal to update, it reads the state from the model and displays the updated view.

The controller even is pretty disciplined and takes input and makes it into definite commands and updates to the models.

Now if you’re wondering how this fits into a web server, you’re probably wondering the same thing I wondered for a long time. The pattern doesn’t fit.

On the web, we end up with a pipeline something like “Browser sends request to server, server picks a handler, handler reads request and does actions, result of those actions is presented to a template or presentation layer, which transforms it into something that can be sent, which goes out as a response to the browser.”

request -> handler -> presentation -> response

It still makes sense to separate out the meat of the application from the specifics of how it’s being displayed and interfaced to the world, often, especially if the application manipulates objects that are distinctly separate. A example might be that an accounts ledger makes no sense to bind the web portions to the data model particularly tightly. That same ledger might be used to generate emails, to generate print-outs, and later to generate reports in a completely different system. The concept of a “model” or a “business domain logic” layer to an application makes sense:

request -> handler -> presentation -> response
business logic

But some time in the mid-2000s, someone thought to try to shoehorn the MVC concept into this pipeline, and did so by renaming these components:

request -> controller -> model -> view -> response

And this is why we end up with relatively well-defined models, since that makes sense, and ‘views’ are a less-descriptive name for templating and presentation logic. What’s left ends up being called a ‘controller’ and we start a lot of arguments about whether a given bit of logic belongs there or in the model.

So in express, let’s refer to models and domain logic, to handlers and to templates. We’ll have an easier time of it.

Handlers accept web-shaped data: query strings and post data, and shape them into something the business logic can deal with. When the business logic emits something we should display, that same handler can pass it off to templates, or in the case of data being rendered in the browser by a client there, serialized directly as json and sent off as the response. Let the business logic know little about the web, unless its concern is the web as in a content management sytem. Let our handlers adapt the HTTP interface to the business logic, and the responses out to our presentation layer, even if that’s as simple as filling in values in a template.

We’ll all be a lot happier if MVC keeps its meaning as a paradigm for breaking up responsibility within a GUI.

Updated: read part 2, Why MVC doesn’t fit the browser.