GET /citieshtml HTTP/11 If-Modified-Since: Mon, 3 Apr 2001 18:00:00 GMT in Java

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GET /citieshtml HTTP/11 If-Modified-Since: Mon, 3 Apr 2001 18:00:00 GMT
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If the originating server determines that the requested object has been modified since this date, it sends the object to the client, just as if it had been requested for the first time If the requested
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object hasn't been modified, the originating server can return an HTTP 304 message, indicating that the cached object is still up-to-date, and thus avoid a redundant data transfer:
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HTTP/11 304 OK
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Subsequently, the browser can use its cached copy to resolve the request, which is what the most recent versions of the Netscape and Microsoft browsers do today The other conditional GET header fields act in a similar or predictable fashion For example, IfUnmodified-Since works in reverse of If-Modified-Since: If the document requested has been modified since the time specified, the specified operation (GET in our case) shouldn't be performed Obviously, this is of much less use than If-Modified-Since from the GET perspective, and it actually makes more sense when it comes to HTTP update requests (perform the requested update only if the document hasn't changed since this time)
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If-Match and If-None-Match are similar to If-Modified-Since and IfUnmodified-Since and will be discussed in detail later in this chapter The main point is that the GET request can be augmented with these parameters to leverage intermediate caches
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We should note the part that proxy servers (both client-side and server-side) play in brokering client/server communication In general, they can extend the client-side caching model so that multiple users can benefit from the same local cache In this way, the proxies act as intermediate caches and obey caching logic just as a browser might (because they're shared, RFC 2616 does make some important distinctions about how they handle certain cache directives) Content distribution networks also play a role in reducing the latencies associated with the transfer of static content by replicating and distributing relatively static and bandwidth-hungry content (such as images) While such replication obviously benefits the client because it reduces latencies and improves the perception of server availability, it also greatly benefits the server because dramatically fewer connections and server-side bandwidth are required Caching Dynamic GET Requests As we've seen, the GET method requests that the Web server return the information specified Often this is simply a static page, in which case it's always an exact copy of the page stored on the server Sometimes the specified URI is a so-called "data-producing process" (ie, a CGI program), in which case the information produced by this process is returned In such a scenario, it's also possible to pass parameters from the client to the server The primary mechanism for this is the HTML FORM element, which enables client input to be sent to the server Based on the URI requested (the process) and the client-side parameters, a custom HTTP reply (Web page) is generated To understand this more clearly, let's return to our earlier example of a Web page that lists the cities of the world We'll extend it to include an initial page that displays a list box containing the major cities and their countries On this page, shown in Figure 5-3, the user chooses a particular city from the list box A server-side CGI-bin program called cityquerycgi processes this request to return dynamic results When the user clicks the Submit button, the following GET request is sent to the server:
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GET /cityquerycgi city=Paris%2C+France HTTP/11
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Figure 5-3 Querying a specific city
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Thus, we see that the request that the client made is a combination of the FORM target and the data associated with the list box selection We can also see that there's some encoding and reformatting of the data when the request is made The main point here, however, is that a dynamic page can be generated through a GET request You probably already know how GET requests are constructed from HTML forms and how parameters are encoded But did you realize that the responses to GET requests with parameters can be cached In fact, they can, and this has obvious important performance implications As long as the server properly indicates it as cacheable, a dynamically generated GET response can be delivered faster to the requestor, without incurring additional server load This support is built in to HTTP 11 As an application designer, all you need to do is make sure that the HTTP responses include the proper header information to indicate to the client and the intermediate network elements that the data can be cached The way to do this is with the Expires HTTP header field For example, a server can return a result that includes the following information:
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HTTP/11 200 OK < other header fields > Expires: Fri, 1 Jan 2010 00:00:00 GMT <HTML> <HEAD></HEAD> <BODY>
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