CHAPTER 5 PERMUTATIONS PATTERN in a time-dependent
CHAPTER 5 PERMUTATIONS PATTERN in a time-dependent fashion (for example, http://mydomain.com/news/column/jack/current for current news and http://mydomain.com/news/column/jack/2005-10-10 for an archived news item)? You must remember that the URL represents a resource that the HTTP server is responsible for converting into a representation. The client is not responsible for knowing what technologies or files are stored on the server side, because that is a complete dependency of the HTTP server. Identifying the Resource and Representation Taking a closer look at the URL http://mydomain.com/books/[ISBN], let s work through how it would be implemented. The URL refers to a specific book with the identified ISBN number. When the URL is sent to an HTTP server, a response is generated. The problem is determining which content the server should send to the client. Separating the resource from the representation means that a single URL will have separate representations. The representation that is sent depends on the value of the HTTP Accept-* header, but that header need not be the only one. As was just mentioned, the user using a query variable could specify the representation. More about other HTTP headers will be discussed shortly. For now, let s focus on the Accept HTTP header and consider the following HTTP conversation that returns some content. Request: GET /books/3791330942 HTTP/1.1 Host: 192.168.1.242:8100 User-Agent: Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; . en-US; rv:1.7.8) Gecko/20050511 Accept: text/xml,application/xml,application/xhtml+xml, . text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 Accept-Language: en-us,en;q=0.5 Accept-Encoding: gzip,deflate Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7 Keep-Alive: 300 Connection: keep-alive Response: HTTP/1.1 200 OK Date: Sun, 21 Aug 2005 14:51:40 GMT Server: Apache/2.0.53 (Ubuntu) PHP/4.3.10-10ubuntu4 Last-Modified: Wed, 11 May 2005 17:43:45 GMT ETag: “41419c-45-438fd340″ Accept-Ranges: bytes Content-Length: 69 Keep-Alive: timeout=15, max=100 Connection: Keep-Alive Content-Type: text/html; charset=UTF-8 The request is an HTTP GET, which means the HTTP server needs to retrieve the data associated with the resource. The operation becomes specific due to the request-provided HTTP headers Accept, Accept-Language, Accept-Encoding, and AcceptCharset. These HTTP headers are accepted by the HTTP server and indicate what content to send.
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