Mechanism and Policy

There is no one-best-way to design interfaces. Instead there is mechanism and there is policy

November 25, 2003

What can we do about the tradeoff between flexibility and convenience in interface design? Do users want us to provide just the means to get the job done (the mechanism), or do they want to be told how to do that job (the policy)?

I’ve been reading Eric Raymond’s “The Art of Unix Programming” (a good book that could have been great had he managed to find a more balanced voice). In the section on user interfaces, he reminds his readers of the decision of the designers of the X windowing system not to impose look-and-feel constraints on X applications. The designers say that X supports “mechanism, not policy.”

The X windowing system provides the underlying graphical user interface for most Unix systems (the Mac is a notable exception, as we’ll see). Perhaps surprisingly, X itself offers almost no user-level features. Instead, it concentrates on providing a set of low-level primitives for drawing windows and filling those windows with graphics and text.

In order to make X usable, you need to supply an application program called a “window manager.” This hooks in to X and handles events: for example, X may create a window, but the window manager can decide where to place it on the screen. To fill windows with widgets (standard interface components) you need another layer of software, the various X toolkits.

The designers of X felt that building a lot of behavior and standard interaction models into X would limit the user of X. Instead, they provided a (fairly low-level) API, and allowed their users to build any style of interface they wanted. They provide the mechanism, but enforce no policies on how that mechanism is used.

By contrast, the windowing systems from Microsoft and Apple (as well as those from Be and NeXT) were rich in policy. The windowing systems imposed a number of look-and-feel constraints and behavioral similarities between applications. There were even documents for application designers dictating just how their applications should look and react.

So what are the tradeoffs? Raymond says:

The difference in approach [between X and Mac/Windows] ensured that X would have a long-run evolutionary advantage by remaining adaptable as new discoveries were made about human factors in interface design—but it also ensured that the X world would be divided by multiple toolkits, a profusion of window managers, and many experiments in look and feel.

Ignoring the interesting spin on “evolutionary advantage” (I don’t often see X applications edging out Windows and Mac ones on my client’s desktops), the point is a good one. By keeping the underlying framework free of particular implementation decisions, you make it more flexible. This flexibility is a two-edged sword. One the one hand, it allows multiple competing ideas to duke it out: the winners will be selected by their users, and not just by developers (perhaps this is what he meant by evolution). But on the other hand, it also leads to the fragmentation he describes.

But he’s also being disingenuous here: the reality is that it isn’t the X windowing system itself that’s adapting at all. Instead, it’s the efforts of hundreds of people writing the stuff on top of X that has provided the ongoing evolving interfaces he describes. Underneath the covers, X is basically the same old X. In some way, you could say that all their efforts were expended making up for stuff that X didn’t provide itself.

So, by providing policy, the designers of Windows and Mac interfaces have provided their end-users with a consistent look and feel, and a base set of application behaviors. By instead focusing on mechanism and ignoring policy, the designers of X allowed developers to experiment, but gave the users of X applications a very inconsistent interface experience. Arguing one approach is better than the other is pretty pointless: they’re just different.

What can we learn here when it comes to applications and designs?

When I first started thinking about this, I was reminded of the audience discussions that sometimes erupt when I talk about Naked Objects. A Naked Objects application exposes the core business objects of an application directly to the end user. They can manipulate these in any way the objects allow: there is no overall application GUI imposing a certain way of doing things. A Naked Objects system provides mechanism, but little in the way of policy. When I describe this, some folks have a strong reaction against the idea. “Without high-level policy imposed by the GUI (scripting, or series of modal dialogs that have to be filled in in a proscribed order) how can we ensure our users do everything that needs to be done?” they ask. And its a good question. Experienced users, folks who understand the domain, love Naked Object systems because they get the control and flexibility they need to get the job done. But inexperienced users can be confounded by that same flexibility—”what should I do now?”. (In a way, this also relates back to the Dreyfus model of skills acquisition: beginners need to be guided, while experts need to be left alone to get on with their jobs.)

In the Naked Objects world, it turns out that there’s a compromise. Because the Naked Objects are themselves just Java business objects, there’s nothing stopping you putting a more conventional view and controller on top of them, converting your Naked Objects application into a conventional GUI or Struts-style app. And, because the objects are the same beneath the covers, you could probably arrange to run both the Naked Objects and conventional application at the same time. The conventional application would have less flexibility and functionality, but would be easier for casual users. The Naked Objects system would have full flexibility for more experienced users.

In a way, this seems like the OS X way of doing things. Apple have taking a Unix operating system and wrapped it with a fantastic user interface. Not only does this interface work at the application level, but it also gives you the ability to do most of the administration of a box without dropping to the command line or editing files. I love this: I’ve spent all too many years administering Unix boxes the hard way. But what I love just as much is that when I need to, I can still get down and dirty. It’s the best of both worlds: regular users get a great, easy-to-use interface, but power users get to strip away the facade and work down at the lower levels.

Increasingly, I think the “one-size-fits-all” mentality is going to break down. We need to think about delivering our application functionality using multiple modalities, each targeted at specific user communities. Mechanism versus policy is one axis we need to consider, and one that’s relatively easily addressed in a well-designed application. We don’t need to decide up front whether to deliver one or the other; instead we need to work out how to provide both.