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96 Chapter 14 CHAPTER 14 Rule 14: Make Ajax Cacheable 14 People frequently ask whether the performance rules in this book apply to Web 2.0 applications. They definitely do! The rule discussed in this chapter is, however, the first rule that resulted from working with Web 2.0 applications at Yahoo!. In this chapter, I describe what Web 2.0 means, how Ajax fits into Web 2.0, and an impor- tant performance improvement you can make to Ajax. Web 2.0, DHTML, and Ajax The relationship between Web 2.0, DHTML, and Ajax is illustrated in Figure 14-1. This figure doesn't show that Ajax is used only in DHTML or that DHTML is used only by Web 2.0 applications, but rather it is meant to show that Web 2.0 includes many concepts, one of which is DHTML, and that Ajax is one of the key technolo- gies in DHTML. A discussion of Web 2.0 and what it includes is a book (or more) by itself, but we do want to have a common understanding of these terms. Below, I give brief definitions with references for more information. Figure 14-1. Relationship between Web 2.0, DHTML, and Ajax Ajax DHTML Web 2.0