docs/manual/content-negotiation.html - httpd - Git at Google

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 <head>
 <title>Apache server Content arbitration: MultiViews and *.var files</title>
 </head>

 <body>
 <IMG SRC="../images/apache_sub.gif" ALT="">
 <h3>Content Arbitration:  MultiViews and *.var files</h3>

 The HTTP standard allows clients (i.e., browsers like Mosaic or
 Netscape) to specify what data formats they are prepared to accept.
 The intention is that when information is available in multiple
 variants (e.g., in different data formats), servers can use this
 information to decide which variant to send.  This feature has been
 supported in the CERN server for a while, and while it is not yet
 supported in the NCSA server, it is likely to assume a new importance
 in light of the emergence of HTML3 capable browsers. <p>

 The Apache module <A HREF="mod_negotiation.html">mod_negotiation</A> handles
 content negotiation in two different ways; special treatment for the
 pseudo-mime-type <code>application/x-type-map</code>, and the
 MultiViews per-directory Option (which can be set in srm.conf, or in
 .htaccess files, as usual).  These features are alternate user
 interfaces to what amounts to the same piece of code (in the new file
 <code>http_mime_db.c</code>) which implements the content negotiation
 portion of the HTTP protocol. <p>

 Each of these features allows one of several files to satisfy a
 request, based on what the client says it's willing to accept; the
 differences are in the way the files are identified:

 <ul>
   <li> A type map (i.e., a <code>*.var</code> file) names the files
        containing the variants explicitly
   <li> In a MultiViews search, the server does an implicit filename
        pattern match, and chooses from among the results.
 </ul>

 Apache also supports a new pseudo-MIME type,
 text/x-server-parsed-html3, which is treated as text/html;level=3
 for purposes of content negotiation, and as server-side-included HTML
 elsewhere.

 <h3>Type maps (*.var files)</h3>

 A type map is a document which is typed by the server (using its
 normal suffix-based mechanisms) as
 <code>application/x-type-map</code>.  Note that to use this feature,
 you've got to have an <code>AddType</code> some place which defines a
 file suffix as <code>application/x-type-map</code>; the easiest thing
 may be to stick a
 <pre>

   AddType application/x-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, e.g.,
 <pre>

   URI: foo; vary="type,language"

   URI: foo.en.html
   Content-type: text/html; level=2
   Content-language: en

   URI: foo.fr.html
   Content-type: text/html; level=2
   Content-language: fr

 </pre>
 If the variants have different qualities, that may be indicated by the
 "qs" parameter, as in this picture (available as jpeg, gif, or ASCII-art):
 <pre>

   URI: foo; vary="type,language"

   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>

 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 --- level may be specified, along with "qs".  These
        are often referred to as MIME types; typical media types are
        <code>image/gif</code>, <code>text/plain</code>, or
        <code>text/html;&nbsp;level=3</code>.
   <dt> <code>Content-language:</code>
   <dd> The language 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>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>&lt;Directory&gt;</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>httpd.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 *not* 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 by the globbing is a CGI script, it's not
 obvious what should happen.  My code gives that case gets 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.  This is the only jiggering of quality ratings done by the
 MultiViews code; aside from that, all Qualities in the synthesized
 type maps are 1.0.

 <p>

 <B>New as of 0.8:</B> Documents in multiple languages can also be resolved through the use
 of the <code>AddLanguage</code> and <code>LanguagePriority</code>
 directives:

 <pre>
 AddLanguage en .en
 AddLanguage fr .fr
 AddLanguage de .de
 AddLanguage da .da
 AddLanguage el .el
 AddLanguage it .it

 # LanguagePriority allows you to give precedence to some languages
 # in case of a tie during content negotiation.
 # Just list the languages in decreasing order of preference.

 LanguagePriority en fr de
 </pre>

 Here, a request for "foo.html" matched against "foo.html.en" and
 "foo.html.fr" would return an French document to a browser that
 indicated a preference for French, or an English document otherwise.
 In fact, a request for "foo" matched against "foo.html.en",
 "foo.html.fr", "foo.ps.en", "foo.pdf.de", and "foo.txt.it" would do
 just what you expect - treat those suffices as a database and compare
 the request to it, returning the best match.  The languages and data
 types share the same suffix name space.

 <p>

 Note that this machinery only comes into play if the file which the
 user attempted to retrieve does <em>not</em> exist by that name; if it
 does, it is simply retrieved as usual.  (So, someone who actually asks
 for <code>foo.jpeg</code>, as opposed to <code>foo</code>, never gets
 <code>foo.gif</code>).

 <P><HR><P>
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 </body> </html>
	<html>
	<head>
	<title>Apache server Content arbitration: MultiViews and *.var files</title>
	</head>

	<body>
	<IMG SRC="../images/apache_sub.gif" ALT="">
	<h3>Content Arbitration: MultiViews and *.var files</h3>

	The HTTP standard allows clients (i.e., browsers like Mosaic or
	Netscape) to specify what data formats they are prepared to accept.
	The intention is that when information is available in multiple
	variants (e.g., in different data formats), servers can use this
	information to decide which variant to send. This feature has been
	supported in the CERN server for a while, and while it is not yet
	supported in the NCSA server, it is likely to assume a new importance
	in light of the emergence of HTML3 capable browsers. <p>

	The Apache module <A HREF="mod_negotiation.html">mod_negotiation</A> handles
	content negotiation in two different ways; special treatment for the
	pseudo-mime-type <code>application/x-type-map</code>, and the
	MultiViews per-directory Option (which can be set in srm.conf, or in
	.htaccess files, as usual). These features are alternate user
	interfaces to what amounts to the same piece of code (in the new file
	<code>http_mime_db.c</code>) which implements the content negotiation
	portion of the HTTP protocol. <p>

	Each of these features allows one of several files to satisfy a
	request, based on what the client says it's willing to accept; the
	differences are in the way the files are identified:

	<ul>
	<li> A type map (i.e., a <code>*.var</code> file) names the files
	containing the variants explicitly
	<li> In a MultiViews search, the server does an implicit filename
	pattern match, and chooses from among the results.
	</ul>

	Apache also supports a new pseudo-MIME type,
	text/x-server-parsed-html3, which is treated as text/html;level=3
	for purposes of content negotiation, and as server-side-included HTML
	elsewhere.

	<h3>Type maps (*.var files)</h3>

	A type map is a document which is typed by the server (using its
	normal suffix-based mechanisms) as
	<code>application/x-type-map</code>. Note that to use this feature,
	you've got to have an <code>AddType</code> some place which defines a
	file suffix as <code>application/x-type-map</code>; the easiest thing
	may be to stick a
	<pre>

	AddType application/x-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, e.g.,
	<pre>

	URI: foo; vary="type,language"

	URI: foo.en.html
	Content-type: text/html; level=2
	Content-language: en

	URI: foo.fr.html
	Content-type: text/html; level=2
	Content-language: fr

	</pre>
	If the variants have different qualities, that may be indicated by the
	"qs" parameter, as in this picture (available as jpeg, gif, or ASCII-art):
	<pre>

	URI: foo; vary="type,language"

	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>

	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 --- level may be specified, along with "qs". 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 language 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>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>
	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>httpd.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 not 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 by the globbing is a CGI script, it's not
	obvious what should happen. My code gives that case gets 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. This is the only jiggering of quality ratings done by the
	MultiViews code; aside from that, all Qualities in the synthesized
	type maps are 1.0.

	<p>

	<B>New as of 0.8:</B> Documents in multiple languages can also be resolved through the use
	of the <code>AddLanguage</code> and <code>LanguagePriority</code>
	directives:

	<pre>
	AddLanguage en .en
	AddLanguage fr .fr
	AddLanguage de .de
	AddLanguage da .da
	AddLanguage el .el
	AddLanguage it .it

	# LanguagePriority allows you to give precedence to some languages
	# in case of a tie during content negotiation.
	# Just list the languages in decreasing order of preference.

	LanguagePriority en fr de
	</pre>

	Here, a request for "foo.html" matched against "foo.html.en" and
	"foo.html.fr" would return an French document to a browser that
	indicated a preference for French, or an English document otherwise.
	In fact, a request for "foo" matched against "foo.html.en",
	"foo.html.fr", "foo.ps.en", "foo.pdf.de", and "foo.txt.it" would do
	just what you expect - treat those suffices as a database and compare
	the request to it, returning the best match. The languages and data
	types share the same suffix name space.

	<p>

	Note that this machinery only comes into play if the file which the
	user attempted to retrieve does <em>not</em> exist by that name; if it
	does, it is simply retrieved as usual. (So, someone who actually asks
	for <code>foo.jpeg</code>, as opposed to <code>foo</code>, never gets
	<code>foo.gif</code>).

	<P><HR><P>
	<A HREF="../"><IMG SRC="../images/apache_home.gif" ALT="Home"></A>
	<A HREF="./"><IMG SRC="../images/apache_index.gif" ALT="Index"></A>

	</body> </html>