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diff --git a/docs/manual/content-negotiation.html.en b/docs/manual/content-negotiation.html.en deleted file mode 100644 index 3f9d60ecf2..0000000000 --- a/docs/manual/content-negotiation.html.en +++ /dev/null @@ -1,527 +0,0 @@ -<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"> -<HTML> -<HEAD> -<TITLE>Apache Content Negotiation</TITLE> -</HEAD> - -<!-- Background white, links blue (unvisited), navy (visited), red (active) --> -<BODY - BGCOLOR="#FFFFFF" - TEXT="#000000" - LINK="#0000FF" - VLINK="#000080" - ALINK="#FF0000" -> -<!--#include virtual="header.html" --> -<H1 ALIGN="CENTER">Content Negotiation</H1> - -<P> -Apache's support for content negotiation has been updated to meet the -HTTP/1.1 specification. It can choose the best representation of a -resource based on the browser-supplied preferences for media type, -languages, character set and encoding. It is also implements a -couple of features to give more intelligent handling of requests from -browsers which send incomplete negotiation information. <P> - -Content negotiation is provided by the -<A HREF="mod/mod_negotiation.html">mod_negotiation</A> module, -which is compiled in by default. - -<HR> - -<H2>About Content Negotiation</H2> - -<P> -A resource may be available in several different representations. For -example, it might be available in different languages or different -media types, or a combination. One way of selecting the most -appropriate choice is to give the user an index page, and let them -select. However it is often possible for the server to choose -automatically. This works because browsers can send as part of each -request information about what representations they prefer. For -example, a browser could indicate that it would like to see -information in French, if possible, else English will do. Browsers -indicate their preferences by headers in the request. To request only -French representations, the browser would send - -<PRE> - Accept-Language: fr -</PRE> - -<P> -Note that this preference will only be applied when there is a choice -of representations and they vary by language. -<P> - -As an example of a more complex request, this browser has been -configured to accept French and English, but prefer French, and to -accept various media types, preferring HTML over plain text or other -text types, and preferring GIF or JPEG over other media types, but also -allowing any other media type as a last resort: - -<PRE> - Accept-Language: fr; q=1.0, en; q=0.5 - Accept: text/html; q=1.0, text/*; q=0.8, image/gif; q=0.6, - image/jpeg; q=0.6, image/*; q=0.5, */*; q=0.1 -</PRE> - -Apache 1.2 supports 'server driven' content negotiation, as defined in -the HTTP/1.1 specification. It fully supports the Accept, -Accept-Language, Accept-Charset and Accept-Encoding request headers. -<P> - -The terms used in content negotiation are: a <STRONG>resource</STRONG> is an -item which can be requested of a server, which might be selected as -the result of a content negotiation algorithm. If a resource is -available in several formats, these are called <STRONG>representations</STRONG> -or <STRONG>variants</STRONG>. The ways in which the variants for a particular -resource vary are called the <STRONG>dimensions</STRONG> of negotiation. - -<H2>Negotiation in Apache</H2> - -<P> -In order to negotiate a resource, the server needs to be given -information about each of the variants. This is done in one of two -ways: - -<UL> - <LI> Using a type map (i.e., a <CODE>*.var</CODE> file) which - names the files containing the variants explicitly - <LI> Or using a 'MultiViews' search, where the server does an implicit - filename pattern match, and chooses from among the results. -</UL> - -<H3>Using a type-map file</H3> - -<P> -A type map is a document which is associated with the handler -named <CODE>type-map</CODE> (or, for backwards-compatibility with -older Apache configurations, the mime type -<CODE>application/x-type-map</CODE>). Note that to use this feature, -you've got to have a <CODE>SetHandler</CODE> some place which defines a -file suffix as <CODE>type-map</CODE>; this is best done with a -<PRE> - - AddHandler type-map var - -</PRE> -in <CODE>srm.conf</CODE>. See comments in the sample config files for -details. <P> - -Type map files have an entry for each available variant; these entries -consist of contiguous RFC822-format header lines. Entries for -different variants are separated by blank lines. Blank lines are -illegal within an entry. It is conventional to begin a map file with -an entry for the combined entity as a whole (although this -is not required, and if present will be ignored). An example -map file is: -<PRE> - - URI: foo - - URI: foo.en.html - Content-type: text/html - Content-language: en - - URI: foo.fr.de.html - Content-type: text/html; charset=iso-8859-2 - Content-language: fr, de -</PRE> - -If the variants have different source qualities, that may be indicated -by the "qs" parameter to the media type, as in this picture (available -as jpeg, gif, or ASCII-art): -<PRE> - URI: foo - - URI: foo.jpeg - Content-type: image/jpeg; qs=0.8 - - URI: foo.gif - Content-type: image/gif; qs=0.5 - - URI: foo.txt - Content-type: text/plain; qs=0.01 - -</PRE> -<P> - -qs values can vary between 0.000 and 1.000. Note that any variant with -a qs value of 0.000 will never be chosen. Variants with no 'qs' -parameter value are given a qs factor of 1.0. <P> - -The full list of headers recognized is: - -<DL> - <DT> <CODE>URI:</CODE> - <DD> uri of the file containing the variant (of the given media - type, encoded with the given content encoding). These are - interpreted as URLs relative to the map file; they must be on - the same server (!), and they must refer to files to which the - client would be granted access if they were to be requested - directly. - <DT> <CODE>Content-type:</CODE> - <DD> media type --- charset, level and "qs" parameters may be given. These - are often referred to as MIME types; typical media types are - <CODE>image/gif</CODE>, <CODE>text/plain</CODE>, or - <CODE>text/html; level=3</CODE>. - <DT> <CODE>Content-language:</CODE> - <DD> The languages of the variant, specified as an Internet standard - language code (e.g., <CODE>en</CODE> for English, - <CODE>kr</CODE> for Korean, etc.). - <DT> <CODE>Content-encoding:</CODE> - <DD> If the file is compressed, or otherwise encoded, rather than - containing the actual raw data, this says how that was done. - For compressed files (the only case where this generally comes - up), content encoding should be - <CODE>x-compress</CODE>, or <CODE>x-gzip</CODE>, as appropriate. - <DT> <CODE>Content-length:</CODE> - <DD> The size of the file. Clients can ask to receive a given media - type only if the variant isn't too big; specifying a content - length in the map allows the server to compare against these - thresholds without checking the actual file. -</DL> - -<H3>Multiviews</H3> - -<P> -This is a per-directory option, meaning it can be set with an -<CODE>Options</CODE> directive within a <CODE><Directory></CODE>, -<CODE><Location></CODE> or <CODE><Files></CODE> -section in <CODE>access.conf</CODE>, or (if <CODE>AllowOverride</CODE> -is properly set) in <CODE>.htaccess</CODE> files. Note that -<CODE>Options All</CODE> does not set <CODE>MultiViews</CODE>; you -have to ask for it by name. (Fixing this is a one-line change to -<CODE>http_core.h</CODE>). - -<P> - -The effect of <CODE>MultiViews</CODE> is as follows: if the server -receives a request for <CODE>/some/dir/foo</CODE>, if -<CODE>/some/dir</CODE> has <CODE>MultiViews</CODE> enabled, and -<CODE>/some/dir/foo</CODE> does <EM>not</EM> exist, then the server reads the -directory looking for files named foo.*, and effectively fakes up a -type map which names all those files, assigning them the same media -types and content-encodings it would have if the client had asked for -one of them by name. It then chooses the best match to the client's -requirements, and forwards them along. - -<P> - -This applies to searches for the file named by the -<CODE>DirectoryIndex</CODE> directive, if the server is trying to -index a directory; if the configuration files specify -<PRE> - - DirectoryIndex index - -</PRE> then the server will arbitrate between <CODE>index.html</CODE> -and <CODE>index.html3</CODE> if both are present. If neither are -present, and <CODE>index.cgi</CODE> is there, the server will run it. - -<P> - -If one of the files found when reading the directive is a CGI script, -it's not obvious what should happen. The code gives that case -special treatment --- if the request was a POST, or a GET with -QUERY_ARGS or PATH_INFO, the script is given an extremely high quality -rating, and generally invoked; otherwise it is given an extremely low -quality rating, which generally causes one of the other views (if any) -to be retrieved. - -<H2>The Negotiation Algorithm</H2> - -After Apache has obtained a list of the variants for a given resource, -either from a type-map file or from the filenames in the directory, it -applies a algorithm to decide on the 'best' variant to return, if -any. To do this it calculates a quality value for each variant in each -of the dimensions of variance. It is not necessary to know any of the -details of how negotiation actually takes place in order to use Apache's -content negotiation features. However the rest of this document -explains in detail the algorithm used for those interested. <P> - -In some circumstances, Apache can 'fiddle' the quality factor of a -particular dimension to achieve a better result. The ways Apache can -fiddle quality factors is explained in more detail below. - -<H3>Dimensions of Negotiation</H3> - -<TABLE> -<TR><TH>Dimension -<TH>Notes -<TR><TD>Media Type -<TD>Browser indicates preferences on Accept: header. Each item -can have an associated quality factor. Variant description can also -have a quality factor. -<TR><TD>Language -<TD>Browser indicates preferences on Accept-Language: header. Each -item -can have a quality factor. Variants can be associated with none, one -or more languages. -<TR><TD>Encoding -<TD>Browser indicates preference with Accept-Encoding: header. -<TR><TD>Charset -<TD>Browser indicates preference with Accept-Charset: header. Variants -can indicate a charset as a parameter of the media type. -</TABLE> - -<H3>Apache Negotiation Algorithm</H3> - -<P> -Apache uses an algorithm to select the 'best' variant (if any) to -return to the browser. This algorithm is not configurable. It operates -like this: - -<OL> -<LI> -Firstly, for each dimension of the negotiation, the appropriate -Accept header is checked and a quality assigned to this each -variant. If the Accept header for any dimension means that this -variant is not acceptable, eliminate it. If no variants remain, go -to step 4. - -<LI>Select the 'best' variant by a process of elimination. Each of -the following tests is applied in order. Any variants not selected at -each stage are eliminated. After each test, if only one variant -remains, it is selected as the best match. If more than one variant -remains, move onto the next test. - -<OL> -<LI>Multiply the quality factor from the Accept header with the - quality-of-source factor for this variant's media type, and select - the variants with the highest value - -<LI>Select the variants with the highest language quality factor - -<LI>Select the variants with the best language match, using either the - order of languages on the <CODE>LanguagePriority</CODE> directive (if - present), - else the order of languages on the Accept-Language header. - -<LI>Select the variants with the highest 'level' media parameter - (used to give the version of text/html media types). - -<LI>Select only unencoded variants, if there is a mix of encoded - and non-encoded variants. If either all variants are encoded - or all variants are not encoded, select all. - -<LI>Select only variants with acceptable charset media parameters, - as given on the Accept-Charset header line. Charset ISO-8859-1 - is always acceptable. Variants not associated with a particular - charset are assumed to be in ISO-8859-1. - -<LI>Select the variants with the smallest content length - -<LI>Select the first variant of those remaining (this will be either the -first listed in the type-map file, or the first read from the directory) -and go to stage 3. - -</OL> - -<LI>The algorithm has now selected one 'best' variant, so return - it as the response. The HTTP response header Vary is set to indicate the - dimensions of negotiation (browsers and caches can use this - information when caching the resource). End. - -<LI>To get here means no variant was selected (because non are acceptable - to the browser). Return a 406 status (meaning "No acceptable representation") - with a response body consisting of an HTML document listing the - available variants. Also set the HTTP Vary header to indicate the - dimensions of variance. - -</OL> - -<H2><A NAME="better">Fiddling with Quality Values</A></H2> - -<P> -Apache sometimes changes the quality values from what would be -expected by a strict interpretation of the algorithm above. This is to -get a better result from the algorithm for browsers which do not send -full or accurate information. Some of the most popular browsers send -Accept header information which would otherwise result in the -selection of the wrong variant in many cases. If a browser -sends full and correct information these fiddles will not -be applied. -<P> - -<H3>Media Types and Wildcards</H3> - -<P> -The Accept: request header indicates preferences for media types. It -can also include 'wildcard' media types, such as "image/*" or "*/*" -where the * matches any string. So a request including: -<PRE> - Accept: image/*, */* -</PRE> - -would indicate that any type starting "image/" is acceptable, -as is any other type (so the first "image/*" is redundant). Some -browsers routinely send wildcards in addition to explicit types they -can handle. For example: -<PRE> - Accept: text/html, text/plain, image/gif, image/jpeg, */* -</PRE> - -The intention of this is to indicate that the explicitly -listed types are preferred, but if a different representation is -available, that is ok too. However under the basic algorithm, as given -above, the */* wildcard has exactly equal preference to all the other -types, so they are not being preferred. The browser should really have -sent a request with a lower quality (preference) value for *.*, such -as: -<PRE> - Accept: text/html, text/plain, image/gif, image/jpeg, */*; q=0.01 -</PRE> - -The explicit types have no quality factor, so they default to a -preference of 1.0 (the highest). The wildcard */* is given -a low preference of 0.01, so other types will only be returned if -no variant matches an explicitly listed type. -<P> - -If the Accept: header contains <EM>no</EM> q factors at all, Apache sets -the q value of "*/*", if present, to 0.01 to emulate the desired -behavior. It also sets the q value of wildcards of the format -"type/*" to 0.02 (so these are preferred over matches against -"*/*". If any media type on the Accept: header contains a q factor, -these special values are <EM>not</EM> applied, so requests from browsers -which send the correct information to start with work as expected. - -<H3>Variants with no Language</H3> - -<P> -If some of the variants for a particular resource have a language -attribute, and some do not, those variants with no language -are given a very low language quality factor of 0.001.<P> - -The reason for setting this language quality factor for -variant with no language to a very low value is to allow -for a default variant which can be supplied if none of the -other variants match the browser's language preferences. - -For example, consider the situation with three variants: - -<UL> -<LI>foo.en.html, language en -<LI>foo.fr.html, language en -<LI>foo.html, no language -</UL> - -<P> -The meaning of a variant with no language is that it is -always acceptable to the browser. If the request Accept-Language -header includes either en or fr (or both) one of foo.en.html -or foo.fr.html will be returned. If the browser does not list -either en or fr as acceptable, foo.html will be returned instead. - -<H2>Note on hyperlinks and naming conventions</H2> - -<P> -If you are using language negotiation you can choose between -different naming conventions, because files can have more than one -extension, and the order of the extensions is normally irrelevant -(see <A HREF="mod/mod_mime.html">mod_mime</A> documentation for details). -<P> -A typical file has a mime-type extension (e.g. <SAMP>html</SAMP>), -maybe an encoding extension (e.g. <SAMP>gz</SAMP> and of course a -language extension (e.g. <SAMP>en</SAMP>) when we have different -language variants of this file. - -<P> -Examples: -<UL> -<LI>foo.en.html -<LI>foo.html.en -<LI>foo.en.html.gz -</UL> - -<P> -Here some more examples of filenames together with valid and invalid -hyperlinks: -</P> - -<TABLE BORDER=1 CELLPADDING=8 CELLSPACING=0> -<TR> - <TH>Filename</TH> - <TH>Valid hyperlink</TH> - <TH>Invalid hyperlink</TH> -</TR> -<TR> - <TD><EM>foo.html.en</EM></TD> - <TD>foo<BR> - foo.html</TD> - <TD>-</TD> -</TR> -<TR> - <TD><EM>foo.en.html</EM></TD> - <TD>foo</TD> - <TD>foo.html</TD> -</TR> -<TR> - <TD><EM>foo.html.en.gz</EM></TD> - <TD>foo<BR> - foo.html</TD> - <TD>foo.gz<BR> - foo.html.gz</TD> -</TR> -<TR> - <TD><EM>foo.en.html.gz</EM></TD> - <TD>foo</TD> - <TD>foo.html<BR> - foo.html.gz<BR> - foo.gz</TD> -</TR> -<TR> - <TD><EM>foo.gz.html.en</EM></TD> - <TD>foo<BR> - foo.gz<BR> - foo.gz.html</TD> - <TD>foo.html</TD> -</TR> -<TR> - <TD><EM>foo.html.gz.en</EM></TD> - <TD>foo<BR> - foo.html<BR> - foo.html.gz</TD> - <TD>foo.gz</TD> -</TR> -</TABLE> - -<P> -Looking at the table above you will notice that it is always possible to -use the name without any extensions in an hyperlink (e.g. <SAMP>foo</SAMP>). -The advantage is that you can hide the actual type of a -document rsp. file and can change it later, e.g. from <SAMP>html</SAMP> -to <SAMP>shtml</SAMP> or <SAMP>cgi</SAMP> without changing any -hyperlink references. - -<P> -If you want to continue to use a mime-type in your hyperlinks (e.g. -<SAMP>foo.html</SAMP>) the language extension (including an encoding extension -if there is one) must be on the right hand side of the mime-type extension -(e.g. <SAMP>foo.html.en</SAMP>). - - -<H2>Note on Caching</H2> - -<P> -When a cache stores a document, it associates it with the request URL. -The next time that URL is requested, the cache can use the stored -document, provided it is still within date. But if the resource is -subject to content negotiation at the server, this would result in -only the first requested variant being cached, and subsequent cache -hits could return the wrong response. To prevent this, -Apache normally marks all responses that are returned after content negotiation -as non-cacheable by HTTP/1.0 clients. Apache also supports the HTTP/1.1 -protocol features to allow caching of negotiated responses. <P> - -For requests which come from a HTTP/1.0 compliant client (either a -browser or a cache), the directive <TT>CacheNegotiatedDocs</TT> can be -used to allow caching of responses which were subject to negotiation. -This directive can be given in the server config or virtual host, and -takes no arguments. It has no effect on requests from HTTP/1.1 -clients. - -<!--#include virtual="footer.html" --> -</BODY> -</HTML> |