HTTP: The Definitive Guide

15.7 Time-Varying Instances

Web objects are not static. The same URL can, over time, point to different versions of an object. Take the CNN home page as an examplegoing to "http://www.cnn.com" several times in a day is likely to result in a slightly different page being returned each time.

Think of the CNN home page as being an object and its different versions as being different instances of the object (see Figure 15-8). The client in the figure requests the same resource (URL) multiple times, but it gets different instances of the resource as it changes over time. At time (a) and (b) it has the same instance; at time (c) it has a different instance.

Figure 15-8. Instances are "snapshots" of a resource in time

The HTTP protocol specifies operations for a class of requests and responses, called instance manipulations, that operate on instances of an object. The two main instance-manipulation methods are range requests and delta encoding. Both of these methods require clients to be able to identify the exact copy of the resource that they have (if any) and request new instances conditionally. These mechanisms are discussed later in this chapter.