HTTP Operational Model and Client/Server Communication
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The Impact of Caching on HTTP Communication
The normal HTTP communication model is changed through the application of caching to client requests. Caching is employed by various devices on the Web to store recently-retrieved resources so they can be quickly supplied in reply to a request. The client itself will cache recently-accessed Web documents so that if the user asks for them again they can be displayed without even making a request to a server. If a request is in fact required, any intermediary device can satisfy a request for a file if the file is in its cache.
When a cache is used, the device that has the cached resource requested returns it directly, short-circuiting the normal HTTP communication process. In the example above, if intermediary 1 has the file the client needs, it will supply it back to the client directly, and intermediary 2 and the real Web server that the client was trying to reach originally will not even be aware that a request was ever made; the topic on HTTP caching discusses the subject in much more detail.
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