12.3 Content Negotiation


HTTP clients and servers support a variety of accept headers that indicate which kinds of content the client is prepared to receive. For example, this browser request indicates that the client prefers French but is willing to read English; can handle HTML, plain text, and JPEG images; knows how to decode gzipped data; and recognizes the ASCII, Latin-1, and UTF-8 character sets:

 GET /index.html HTTP/1.1 User-Agent: Mozilla/4.6 [en] (WinNT; I) Host: www.cafeaulait.org Accept: text/html, text/plain, image/jpeg Accept-Encoding: gzip Accept-Language: fr, en Accept-Charset: us-ascii, iso-8859-1,utf-8 Connection: close If-Modified-Since: Sun, 31 Oct 1999 19:22:07 GMT 

The server that receives this request uses these headers to decide which version of a resource to send to the client. The same URL can return different content depending on how these headers are set. In browsers, this is normally controlled through preferences, but XInclude allows documents to control two of these headers, Accept and Accept-language , by attributes. Each xi:include element can have an accept and/or accept-language attribute. The values of these attributes should be legal values for the corresponding HTTP header fields. If one or both of these attributes is present, then the XInclude processor will add the relevant accept headers to the HTTP request it sends to the server. For example, this xi:include element indicates you want to include the French HTML version of Google's home page:

 <xi:include  href="http://www.google.com" parse="text"  accept-language="fr"  accept="text/html  " /> 

This xi:include element indicates you want to include the English XML version of Google:

 <xi:include  href="http://www.google.com"  accept-language="en"  accept="application/xml  " /> 

Both accept and accept-language can be used with parse="xml " and parse="text ".

It's not necessarily true, of course, that any given URL will have a version with the language and content type you request. Most servers simply return the same page in the same language regardless of the accept headers. However, for those servers that do provide different translations and formats of the same resource, these two attributes enable you to specify which is preferred.

XML in a Nutshell
XML in a Nutshell, Third Edition
ISBN: 0596007647
EAN: 2147483647
Year: 2003
Pages: 232

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