Essays

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Serving Client-Side Applications

December 5, 2004 in Essays, Software | Comments

[Update 2006: “Model N” never caught on. A few months after I wrote this, Jesse James Garrett coined the term AJAX. This is what the architecture described in this post is known as today.]

In a traditional server-side web application, the server renders a series of views which are downloaded, as HTML, to the client. A client-side web application is an application that is deployed from a server and displays data from a server, but can render a series of views on the client.

I’ve been watching server-side developers try to figure out how to serve client-side web applications for a few years now. Different developers, that is — it doesn’t take years for any individual developer to figure it out. There’s often an initial stumble, which is caused by a mismatch between the obvious way to deploy a client-side web application, and the right way. The right way is simpler, but elusive.

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The Apple Boutique

November 27, 2004 in Essays, Technology

One — but only one — reason for Apple’s appeal is that Apple products are luxury goods. (I’ll get to the second reason in a moment.) Apple products compete on design, not price. The Apple stores, with their hardwood floors and wide open spaces, are modeled after a luxury car showroom, and don’t share the convenience store layout and shelving of other computer stores.

Focusing on the high end of the market is a reasonable strategy1. (Sony is trying it with the VAIO.) In an area that depends on third parties to create programs and peripherals, this strategy has a benefit beyond high profit margins. Hardware and software makers are disproportionately interested in Apple’s customers, because these customers have shown themselves to be disproportionately willing to spend money on computer products. This is why Apple can have 1% of the desktop market but — unlike Linux in 2004 — command compatability from web sites and large software vendors. Selling to Apple customers is like opening a store in Beverly Hills; if the products are good enough to sell there, it’s worth the real estate cost.

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The IDE Divide

November 21, 2004 in Essays, Software Development | Comments

The developer world is divided into two camps. Language mavens wax rhapsodic about the power of higher-level programming — first-class functions, staged programming, AOP, MOPs, and reflection. Tool mavens are skilled at the use of integrated build and debug tools, integrated documentation, code completion, refactoring, and code comprehension. Language mavens tend to use a text editor such as emacs or vim — these editors are more likely to work for new languages. Tool mavens tend to use IDEs such as Visual Studio, Eclipse, or IntelliJ, that integrate a variety of development tools1.

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Responsive Interfaces and Effective People

September 11, 2004 in Essays, Systems Thinking | Comments

Patrick Roberts has written a fascinating post on Responsive User Interfaces. It’s easy to make a specific application responsive, through careful coding and by limiting what the user can do. An architecture for doing this, with arbitrary functionality, is one of the holy grails of GUI frameworks. Patrick’s post is a step towards this.

Patrick defines responsive as “the UI never locks up and provides at least partial results ASAP, not that every operation is completed instantaneously”. His design uses a UI thread that runs performs fast operations, and pushes slow operations onto a queue. A background thread runs these operations in priority order.

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Web MVC

August 27, 2004 in Essays, Software | Comments

The Model-View-Controller (MVC) architecture is a standard architecture for interactive applications. In client-server programming, the MVC components are distributed across at least two nodes of a network. This leads to a set of choices about where to deploy each component of the architecture. One solution is the traditional server-based MVC model. Another is the Rich Internet Application (RIA) model. In a real-world application with client-side validation, these are more similar than they might seem.

Desktop MVC

In an interactive application, there is typically a domain model, code to present the model to the user, and code to act upon the model in response to the user manipulation of input devices such as the keyboard and mouse. For example, in a word processor the domain model is the document, which contains entities such as paragraphs, spans, and styles. The system presents the document to the user as (for a sighted user) glyphs rendered as pixel patterns, and interprets keystrokes and mouse actions as edit, formatting, and control commands.

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XML Schema Versioning with RELAX NG

August 20, 2004 in Essays, XML | Comments

XML schemas change over time, for the same reasons that library APIs evolve in programming language. Over time, the schema designers introduce new content, and change or remove existing content, as they acquire greater familiarity with the domain model and the use cases, as they add additional functionality, and as they fix design bugs.

Often it is necessary to maintain instances of old versions alongside instances of new versions. (I’ll discuss why this is necessary.) An example is XSLT, where XSLT 1.0 and XSLT 2.0 documents may be present on the same file system. Other examples are XHTML, and SVG. The presence of instances of multiple schema versions frequently requires the presence of descriptions of multiple versions, and the problem of maintaining these multiple schema versions arises.

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Aspect-Oriented Programming with mod_rewrite

August 17, 2004 in Essays, Programming | Comments

I spent part of my vacation last month working on my web site. One change I wanted to make was to put a banner on every page, and a directory-specific column on the left. Nothing fancy by web design standards, but an adventure for a hobbiest without a CMS.

The first time I had to make a number of pages with a common template was in 1995, and I wrote a Lisp program to generate them. The next time I made a web site, I used Barry Warsaw’s Python program to create some pages. (I still didn’t have access to a server with server-side includes.)

Last year when I moved my site to a modern server, I used PHP include to add navigation elements to some of the pages. But this time I didn’t relish editing a number of HTML files, and, more to the point, I was ready to try something new.

The Problem

Let’s say that software/index.html looks like this:

<h1>This is my software page</h1>
<p>Welcome to my software page</p>				Read the rest of this entry »

Unqualified Imports for XML

August 4, 2004 in Essays, OpenLaszlo, XML

An A Fresh Canvas I argued that there’s a human-factors advantage to allowing an XML document to contain names from multiple namespaces without requiring namespace prefixes on the elements from all but one of them. In What’s in a Namespace I looked at how namespaces and namespace imports work in programming languages, which generally allow both qualified imports (like XML Namespace) and unqualified imports as well.

I also said that I would demonstrate that unqualified imports could be added to XML in a well-defined way, if certain conditions were met. (The conditions are that the set of names in each namespace is known when the document is processed. This is the same condition that programming languages such as C++ and Java, that resolve the namespace of unqualified names at compile time, impose.) Here’s where I make good on that promise.

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A Taxonomy of Comments

August 31, 2003 in Essays, Programming | Comments

Christian Sepulveda writes about comments in source code:

Not all comments are bad. But they are generally deodorant; they cover up mistakes in the code. Each time a comment is written to explain what the code is doing, the code should be re-written to be more clean and self explanatory.

This statement is provocative and interesting, but wrong. There are more good uses for comments than bad ones. (The rest of Sepulveda’s posting is more nuanced, and much of what I write here expands on points he makes.)

Comments are an escape hatch for expressing everything about a program that the programming language can’t. Comments therefore don’t fall into a single natural category.

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Rethinking MVC

August 4, 2003 in Essays, OpenLaszlo | Comments

In the Model-View-Controller architecture, the Model is decoupled from information about the user interface. In a Data-Driven Presentation, the data contains all the information necessary to assemble the user-interface elements. These design patterns appear at first to be exclusive mutually exclusive: either the data contains presentation information, or it doesn’t. This apparent conflict is because of a confusion between the Model of MVC, and the Data in DDP.
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Two Architectures


MVC

The Model-View-Controller (MVC) architecture consists of a Model, which simulates a real-world or “business” object, and a View, which represents the user interface of the object. (There’s also a Controller, but I’m not going to talk about that here.)

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