mirror of
https://github.com/apache/httpd.git
synced 2025-08-29 04:02:02 +03:00
1b19735a79
apache-site... No thirty. git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@81322 13f79535-47bb-0310-9956-ffa450edef68
528 lines
19 KiB
HTML
528 lines
19 KiB
HTML
<!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>
|