apache/docs/manual/developer/request.xml

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<manualpage metafile="request.xml.meta">
<parentdocument href="./">Developer Documentation</parentdocument>

<title>Request Processing in Apache 2.0</title>

<summary>
    <note type="warning"><title>Warning</title>
      <p>Warning - this is a first (fast) draft that needs further
      revision!</p>
    </note>

    <p>Several changes in Apache 2.0 affect the internal request
    processing mechanics. Module authors need to be aware of these
    changes so they may take advantage of the optimizations and
    security enhancements.</p>

    <p>The first major change is to the subrequest and redirect
    mechanisms. There were a number of different code paths in
    Apache 1.3 to attempt to optimize subrequest or redirect
    behavior. As patches were introduced to 2.0, these
    optimizations (and the server behavior) were quickly broken due
    to this duplication of code. All duplicate code has been folded
    back into <code>ap_process_request_internal()</code> to prevent
    the code from falling out of sync again.</p>

    <p>This means that much of the existing code was 'unoptimized'.
    It is the Apache HTTP Project's first goal to create a robust
    and correct implementation of the HTTP server RFC. Additional
    goals include security, scalability and optimization. New
    methods were sought to optimize the server (beyond the
    performance of Apache 1.3) without introducing fragile or
    insecure code.</p>
</summary>

<section id="processing"><title>The Request Processing Cycle</title>
    <p>All requests pass through <code>ap_process_request_internal()</code>
    in <code>request.c</code>, including subrequests and redirects. If a module
    doesn't pass generated requests through this code, the author is cautioned
    that the module may be broken by future changes to request
    processing.</p>

    <p>To streamline requests, the module author can take advantage
    of the hooks offered to drop out of the request cycle early, or
    to bypass core Apache hooks which are irrelevant (and costly in
    terms of CPU.)</p>
</section>

<section id="parsing"><title>The Request Parsing Phase</title>
    <section id="unescape"><title>Unescapes the URL</title>
      <p>The request's <code>parsed_uri</code> path is unescaped, once and only
      once, at the beginning of internal request processing.</p>

      <p>This step is bypassed if the proxyreq flag is set, or the
      <code>parsed_uri.path</code> element is unset. The module has no further
      control of this one-time unescape operation, either failing to
      unescape or multiply unescaping the URL leads to security
      reprecussions.</p>
    </section>

    <section id="strip"><title>Strips Parent and This Elements from the
    URI</title>
      <p>All <code>/../</code> and <code>/./</code> elements are
      removed by <code>ap_getparents()</code>. This helps to ensure
      the path is (nearly) absolute before the request processing
      continues.</p>

      <p>This step cannot be bypassed.</p>
    </section>

    <section id="inital-location-walk"><title>Initial URI Location Walk</title>
      <p>Every request is subject to an
      <code>ap_location_walk()</code> call. This ensures that
      <directive type="section" module="core">Location</directive> sections
      are consistently enforced for all requests. If the request is an internal
      redirect or a sub-request, it may borrow some or all of the processing
      from the previous or parent request's ap_location_walk, so this step
      is generally very efficient after processing the main request.</p>
    </section>

    <section id="translate_name"><title>translate_name</title>
      <p>Modules can determine the file name, or alter the given URI
      in this step. For example, <module>mod_vhost_alias</module> will
      translate the URI's path into the configured virtual host,
      <module>mod_alias</module> will translate the path to an alias path,
      and if the request falls back on the core, the <directive module="core"
      >DocumentRoot</directive> is prepended to the request resource.</p>

      <p>If all modules <code>DECLINE</code> this phase, an error 500 is
      returned to the browser, and a "couldn't translate name" error is logged
      automatically.</p>
    </section>

    <section id="map_to_storage"><title>Hook: map_to_storage</title>
      <p>After the file or correct URI was determined, the
      appropriate per-dir configurations are merged together. For
      example, <module>mod_proxy</module> compares and merges the appropriate
      <directive module="mod_proxy" type="section">Proxy</directive> sections.
      If the URI is nothing more than a local (non-proxy) <code>TRACE</code>
      request, the core handles the request and returns <code>DONE</code>.
      If no module answers this hook with <code>OK</code> or <code>DONE</code>,
      the core will run the request filename against the <directive
      module="core" type="section">Directory</directive> and <directive
      module="core" type="section">Files</directive> sections. If the request
      'filename' isn't an absolute, legal filename, a note is set for
      later termination.</p>
    </section>

    <section id="location-walk"><title>URI Location Walk</title>
      <p>Every request is hardened by a second
      <code>ap_location_walk()</code> call. This reassures that a
      translated request is still subjected to the configured
      <directive module="core" type="section">Location</directive> sections.
      The request again borrows some or all of the processing from its previous
      <code>location_walk</code> above, so this step is almost always very
      efficient unless the translated URI mapped to a substantially different
      path or Virtual Host.</p>
    </section>

    <section id="header_parser"><title>Hook: header_parser</title>
      <p>The main request then parses the client's headers. This
      prepares the remaining request processing steps to better serve
      the client's request.</p>
    </section>
</section>

<section id="security"><title>The Security Phase</title>
    <p>Needs Documentation. Code is:</p>

    <example><pre>
        if ((access_status = ap_run_access_checker(r)) != 0) {
            return decl_die(access_status, "check access", r);
        }

        if ((access_status = ap_run_check_user_id(r)) != 0) {
            return decl_die(access_status, "check user", r);
        }

        if ((access_status = ap_run_auth_checker(r)) != 0) {
            return decl_die(access_status, "check authorization", r);
        }
    </pre>
    </example>
</section>

<section id="preparation"><title>The Preparation Phase</title>
    <section id="type_checker"><title>Hook: type_checker</title>
      <p>The modules have an opportunity to test the URI or filename
      against the target resource, and set mime information for the
      request. Both <module>mod_mime</module> and
      <module>mod_mime_magic</module> use this phase to compare the file
      name or contents against the administrator's configuration and set the
      content type, language, character set and request handler. Some modules
      may set up their filters or other request handling parameters at this
      time.</p>

      <p>If all modules <code>DECLINE</code> this phase, an error 500 is
      returned to the browser, and a "couldn't find types" error is logged
      automatically.</p>
    </section>

    <section id="fixups"><title>Hook: fixups</title>
      <p>Many modules are 'trounced' by some phase above. The fixups
      phase is used by modules to 'reassert' their ownership or force
      the request's fields to their appropriate values. It isn't
      always the cleanest mechanism, but occasionally it's the only
      option.</p>
    </section>
</section>

<section id="handler"><title>The Handler Phase</title>
    <p>This phase is <strong>not</strong> part of the processing in
    <code>ap_process_request_internal()</code>. Many
    modules prepare one or more subrequests prior to creating any
    content at all. After the core, or a module calls
    <code>ap_process_request_internal()</code> it then calls
    <code>ap_invoke_handler()</code> to generate the request.</p>

    <section id="insert_filter"><title>Hook: insert_filter</title>
      <p>Modules that transform the content in some way can insert
      their values and override existing filters, such that if the
      user configured a more advanced filter out-of-order, then the
      module can move its order as need be.  There is no result code,
      so actions in this hook better be trusted to always succeed.</p>
    </section>

    <section id="hook_handler"><title>Hook: handler</title>
      <p>The module finally has a chance to serve the request in its
      handler hook. Note that not every prepared request is sent to
      the handler hook. Many modules, such as <module>mod_autoindex</module>,
      will create subrequests for a given URI, and then never serve the
      subrequest, but simply lists it for the user. Remember not to
      put required teardown from the hooks above into this module,
      but register pool cleanups against the request pool to free
      resources as required.</p>
    </section>
</section>
</manualpage>