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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<HTML>
<HEAD>
<TITLE>Apache Content Negotiation</TITLE>
</HEAD>
<BODY>
<!--#include virtual="header.html" -->
<h1>Content Negotiation</h1>
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>
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>
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 <b>resource</b> 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 <b>representations</b>
or <b>variants</b>. The ways in which the variants for a particular
resource vary are called the <b>dimensions</b> of negotiation.
<h2>Negotiation in Apache</h2>
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>
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>
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>
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:
<p>
<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>
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>
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 <i>no</i> 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 <i>not</i> applied, so requests from browsers
which send the correct information to start with work as expected.
<h3>Variants with no Language</h3>
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>
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 Caching</h2>
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>
|
