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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<title>Request Processing in the Apache HTTP Server 2.x - Apache HTTP Server Version 2.5</title>
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<link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" />
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<link href="../style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" />
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<script src="../style/scripts/prettify.min.js" type="text/javascript">
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</script>
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<link href="../images/favicon.ico" rel="shortcut icon" /></head>
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<body id="manual-page"><div id="page-header">
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<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/quickreference.html">Directives</a> | <a href="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
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<p class="apache">Apache HTTP Server Version 2.5</p>
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<img alt="" src="../images/feather.png" /></div>
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<div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
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<div id="path">
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<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.5</a> > <a href="./">Developer Documentation</a></div><div id="page-content"><div id="preamble"><h1>Request Processing in the Apache HTTP Server 2.x</h1>
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<div class="toplang">
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<p><span>Available Languages: </span><a href="../en/developer/request.html" title="English"> en </a></p>
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</div>
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<div class="warning"><h3>Warning</h3>
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<p>Warning - this is a first (fast) draft that needs further
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revision!</p>
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</div>
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<p>Several changes in 2.0 and above affect the internal request
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processing mechanics. Module authors need to be aware of these
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changes so they may take advantage of the optimizations and
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security enhancements.</p>
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<p>The first major change is to the subrequest and redirect
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mechanisms. There were a number of different code paths in
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the Apache HTTP Server 1.3 to attempt to optimize subrequest
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or redirect behavior. As patches were introduced to 2.0, these
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optimizations (and the server behavior) were quickly broken due
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to this duplication of code. All duplicate code has been folded
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back into <code>ap_process_request_internal()</code> to prevent
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the code from falling out of sync again.</p>
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<p>This means that much of the existing code was 'unoptimized'.
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It is the Apache HTTP Project's first goal to create a robust
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and correct implementation of the HTTP server RFC. Additional
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goals include security, scalability and optimization. New
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methods were sought to optimize the server (beyond the
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performance of 1.3) without introducing fragile or
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insecure code.</p>
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</div>
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<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#processing">The Request Processing Cycle</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#parsing">The Request Parsing Phase</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#security">The Security Phase</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#preparation">The Preparation Phase</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#handler">The Handler Phase</a></li>
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</ul><h3>See also</h3><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
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<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="processing" id="processing">The Request Processing Cycle</a> <a title="Permanent link" href="#processing" class="permalink">¶</a></h2>
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<p>All requests pass through <code>ap_process_request_internal()</code>
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in <code>server/request.c</code>, including subrequests and redirects. If a module
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doesn't pass generated requests through this code, the author is cautioned
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that the module may be broken by future changes to request
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processing.</p>
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<p>To streamline requests, the module author can take advantage
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of the <a href="./modguide.html#hooking">hooks offered</a> to drop
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out of the request cycle early, or to bypass core hooks which are
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irrelevant (and costly in terms of CPU.)</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="parsing" id="parsing">The Request Parsing Phase</a> <a title="Permanent link" href="#parsing" class="permalink">¶</a></h2>
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<h3><a name="unescape" id="unescape">Unescapes the URL</a></h3>
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<p>The request's <code>parsed_uri</code> path is unescaped, once and only
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once, at the beginning of internal request processing.</p>
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<p>This step is bypassed if the proxyreq flag is set, or the
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<code>parsed_uri.path</code> element is unset. The module has no further
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control of this one-time unescape operation, either failing to
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unescape or multiply unescaping the URL leads to security
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repercussions.</p>
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<h3><a name="strip" id="strip">Strips Parent and This Elements from the
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URI</a></h3>
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<p>All <code>/../</code> and <code>/./</code> elements are
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removed by <code>ap_getparents()</code>, as well as any trailing
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<code>/.</code> or <code>/..</code> element. This helps to ensure
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the path is (nearly) absolute before the request processing
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continues. (See RFC 1808 section 4 for further discussion.)</p>
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<p>This step cannot be bypassed.</p>
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<h3><a name="inital-location-walk" id="inital-location-walk">Initial URI Location Walk</a></h3>
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<p>Every request is subject to an
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<code>ap_location_walk()</code> call. This ensures that
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<code class="directive"><a href="../mod/core.html#location"><Location></a></code> sections
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are consistently enforced for all requests. If the request is an internal
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redirect or a sub-request, it may borrow some or all of the processing
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from the previous or parent request's ap_location_walk, so this step
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is generally very efficient after processing the main request.</p>
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<h3><a name="translate_name" id="translate_name">translate_name</a></h3>
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<p>Modules can determine the file name, or alter the given URI
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in this step. For example, <code class="module"><a href="../mod/mod_vhost_alias.html">mod_vhost_alias</a></code> will
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translate the URI's path into the configured virtual host,
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<code class="module"><a href="../mod/mod_alias.html">mod_alias</a></code> will translate the path to an alias path,
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and if the request falls back on the core, the <code class="directive"><a href="../mod/core.html#documentroot">DocumentRoot</a></code> is prepended to the request resource.</p>
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<p>If all modules <code>DECLINE</code> this phase, an error 500 is
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returned to the browser, and a "couldn't translate name" error is logged
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automatically.</p>
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<h3><a name="map_to_storage" id="map_to_storage">Hook: map_to_storage</a></h3>
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<p>After the file or correct URI was determined, the
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appropriate per-dir configurations are merged together. For
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example, <code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code> compares and merges the appropriate
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<code class="directive"><a href="../mod/mod_proxy.html#proxy"><Proxy></a></code> sections.
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If the URI is nothing more than a local (non-proxy) <code>TRACE</code>
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request, the core handles the request and returns <code>DONE</code>.
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If no module answers this hook with <code>OK</code> or <code>DONE</code>,
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the core will run the request filename against the <code class="directive"><a href="../mod/core.html#directory"><Directory></a></code> and <code class="directive"><a href="../mod/core.html#files"><Files></a></code> sections. If the request
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'filename' isn't an absolute, legal filename, a note is set for
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later termination.</p>
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<h3><a name="location-walk" id="location-walk">URI Location Walk</a></h3>
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<p>Every request is hardened by a second
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<code>ap_location_walk()</code> call. This reassures that a
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translated request is still subjected to the configured
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<code class="directive"><a href="../mod/core.html#location"><Location></a></code> sections.
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The request again borrows some or all of the processing from its previous
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<code>location_walk</code> above, so this step is almost always very
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efficient unless the translated URI mapped to a substantially different
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path or Virtual Host.</p>
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<h3><a name="header_parser" id="header_parser">Hook: header_parser</a></h3>
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<p>The main request then parses the client's headers. This
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prepares the remaining request processing steps to better serve
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the client's request.</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="security" id="security">The Security Phase</a> <a title="Permanent link" href="#security" class="permalink">¶</a></h2>
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<p>Needs Documentation. Code is:</p>
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<pre class="prettyprint lang-c">if ((access_status = ap_run_access_checker(r)) != 0) {
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return decl_die(access_status, "check access", r);
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}
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if ((access_status = ap_run_check_user_id(r)) != 0) {
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return decl_die(access_status, "check user", r);
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}
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if ((access_status = ap_run_auth_checker(r)) != 0) {
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return decl_die(access_status, "check authorization", r);
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}</pre>
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="preparation" id="preparation">The Preparation Phase</a> <a title="Permanent link" href="#preparation" class="permalink">¶</a></h2>
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<h3><a name="type_checker" id="type_checker">Hook: type_checker</a></h3>
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<p>The modules have an opportunity to test the URI or filename
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against the target resource, and set mime information for the
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request. Both <code class="module"><a href="../mod/mod_mime.html">mod_mime</a></code> and
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<code class="module"><a href="../mod/mod_mime_magic.html">mod_mime_magic</a></code> use this phase to compare the file
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name or contents against the administrator's configuration and set the
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content type, language, character set and request handler. Some modules
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may set up their filters or other request handling parameters at this
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time.</p>
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<p>If all modules <code>DECLINE</code> this phase, an error 500 is
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returned to the browser, and a "couldn't find types" error is logged
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automatically.</p>
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<h3><a name="fixups" id="fixups">Hook: fixups</a></h3>
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<p>Many modules are 'trounced' by some phase above. The fixups
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phase is used by modules to 'reassert' their ownership or force
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the request's fields to their appropriate values. It isn't
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always the cleanest mechanism, but occasionally it's the only
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option.</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="handler" id="handler">The Handler Phase</a> <a title="Permanent link" href="#handler" class="permalink">¶</a></h2>
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<p>This phase is <strong>not</strong> part of the processing in
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<code>ap_process_request_internal()</code>. Many
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modules prepare one or more subrequests prior to creating any
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content at all. After the core, or a module calls
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<code>ap_process_request_internal()</code> it then calls
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<code>ap_invoke_handler()</code> to generate the request.</p>
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<h3><a name="insert_filter" id="insert_filter">Hook: insert_filter</a></h3>
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<p>Modules that transform the content in some way can insert
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their values and override existing filters, such that if the
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user configured a more advanced filter out-of-order, then the
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module can move its order as need be. There is no result code,
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so actions in this hook better be trusted to always succeed.</p>
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<h3><a name="hook_handler" id="hook_handler">Hook: handler</a></h3>
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<p>The module finally has a chance to serve the request in its
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handler hook. Note that not every prepared request is sent to
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the handler hook. Many modules, such as <code class="module"><a href="../mod/mod_autoindex.html">mod_autoindex</a></code>,
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will create subrequests for a given URI, and then never serve the
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subrequest, but simply lists it for the user. Remember not to
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put required teardown from the hooks above into this module,
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but register pool cleanups against the request pool to free
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resources as required.</p>
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</div></div>
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<div class="bottomlang">
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<p><span>Available Languages: </span><a href="../en/developer/request.html" title="English"> en </a></p>
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</div><div class="top"><a href="#page-header"><img src="../images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Libera.chat, or sent to our <a href="https://httpd.apache.org/lists.html">mailing lists</a>.</div>
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