ment. Because the HTML document’s URL is fixed (bookmarked, linked to, and so on), it’s necessary to ensure the response is not cached by the browser.

This is not the case for Ajax responses. The URL of the Ajax request is included inside the HTML document; it’s not bookmarked or linked to. Developers have the freedom to change the Ajax request’s URL when they generate the page. This allows developers to make Ajax responses cacheable. If an updated version of the Ajax data is available, the cached version is avoided by adding a

dynamic variable to the Ajax URL. For example, an Ajax request for the user’s address book could include the time it was last edited as a parameter in the URL, “&edit=1218050433.” As long as the user hasn’t edited their address book, the previously cached Ajax response can continue to be used, making for a faster page.

 

Performance analysis with yslow Evangelizing these performance best practices is a challenge. I was able to share this information through confer-

ences, training classes, consulting, and documentation. Even with the knowledge in hand, it would still take hours of loading pages in a packet sniffer and reading HTML to identify the appropriate set of performance improvements. A better alternative would be to codify this expertise in a tool that anyone could run, reducing the learning curve and increasing adoption of these performance best practices. This was the inspiration for YSlow.

YSlow ( http://developer.yahoo.com/ yslow/) is a performance analysis tool that answers the question posed in the introduction: “Why is this site so slow?” I created YSlow so that any Web developer could quickly and easily apply the performance rules to their site, and find out specifically what needed to be improved. It runs inside Firefox as an extension to Firebug (http://getfirebug. com/), the tool of choice for many Web developers.

The screenshot in Figure 3 shows Firefox with iGoogle loaded. Firebug is open in the lower portion of the window, with tabs for Console, HTML, CSS, Script, DOM, and Net. When YSlow is installed, the YSlow tab is added. Clicking YSlow’s Performance button initiates an analysis of the page against the set of rules, resulting in a weighted score for the page.

As shown in Figure 3, YSlow explains each rule’s results with details about what to fix. Each rule in the YSlo w screen is a link to the companion Web site, where additional information about the rule is available.

the next Performance
challenge: Javascript

Web 2.0 promises a future where developers can build Web applications that provide an experience similar to desktop apps. Web 2.0 apps are built using JavaScript, which presents significant performance challenges because JavaScript blocks downloads and rendering in the browser. To build faster Web 2.0 apps, developers should address these performance issues using the following guidelines:

˲ Split the initial payload

˲ Load scripts without blocking ˲ Don’t scatter scripts

figure 3: yslow.

split the initial Payload