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- --><title>mod_unique_id - Apache HTTP Server</title><link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" /><link href="../style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" /><link href="../style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" /><link href="../images/favicon.ico" rel="shortcut icon" /></head><body><div id="page-header"><p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p><p class="apache">Apache HTTP Server Version 2.1</p><img alt="" src="../images/feather.gif" /></div><div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div><div id="path"><a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs-project/">Documentation</a> > <a href="../">Version 2.1</a> > <a href="./">Modules</a></div><div id="page-content"><div id="preamble"><h1>Apache Module mod_unique_id</h1><table class="module"><tr><th><a href="module-dict.html#Description">Description:
- </a></th><td>Provides an environment variable with a unique
-identifier for each request</td></tr><tr><th><a href="module-dict.html#Status">Status:
- </a></th><td>Extension</td></tr><tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:
- </a></th><td>unique_id_module</td></tr><tr><th><a href="module-dict.html#SourceFile">Source File:
- </a></th><td>mod_unique_id.c</td></tr></table><h3>Summary</h3>
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+<div id="page-content">
+<div id="preamble"><h1>Apache Module mod_unique_id</h1>
+<div class="toplang">
+<p><span>Available Languages: </span><a href="../en/mod/mod_unique_id.html" title="English"> en </a> |
+<a href="../fr/mod/mod_unique_id.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
+<a href="../ja/mod/mod_unique_id.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
+<a href="../ko/mod/mod_unique_id.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
+</div>
+<table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td>Provides an environment variable with a unique
+identifier for each request</td></tr>
+<tr><th><a href="module-dict.html#Status">Status:</a></th><td>Extension</td></tr>
+<tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:</a></th><td>unique_id_module</td></tr>
+<tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>mod_unique_id.c</td></tr></table>
+<h3>Summary</h3>
+
<p>This module provides a magic token for each request which is
guaranteed to be unique across "all" requests under very
set to the identifier for each request. Unique identifiers are
useful for various reasons which are beyond the scope of this
document.</p>
-</div><div id="quickview"><h3 class="directives">Directives</h3><p>This module provides no directives.</p><h3>Topics</h3><ul id="topics"><li><img alt="" src="../images/down.gif" /> Theory</li></ul></div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div><div class="section"><h2>Theory</h2>
+</div>
+<div id="quickview"><h3>Topics</h3>
+<ul id="topics">
+<li><img alt="" src="../images/down.gif" /> <a href="#theory">Theory</a></li>
+</ul><h3 class="directives">Directives</h3>
+<p>This module provides no
+ directives.</p>
+<h3>Bugfix checklist</h3><ul class="seealso"><li><a href="https://www.apache.org/dist/httpd/CHANGES_2.4">httpd changelog</a></li><li><a href="https://bz.apache.org/bugzilla/buglist.cgi?bug_status=__open__&list_id=144532&product=Apache%20httpd-2&query_format=specific&order=changeddate%20DESC%2Cpriority%2Cbug_severity&component=mod_unique_id">Known issues</a></li><li><a href="https://bz.apache.org/bugzilla/enter_bug.cgi?product=Apache%20httpd-2&component=mod_unique_id">Report a bug</a></li></ul><h3>See also</h3>
+<ul class="seealso">
+<li><a href="#comments_section">Comments</a></li></ul></div>
+<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="theory" id="theory">Theory</a></h2>
<p>First a brief recap of how the Apache server works on Unix
process requests one at a time. Each child can serve multiple
requests in its lifetime. For the purpose of this discussion,
the children don't share any data with each other. We'll refer
- to the children as httpd processes.</p>
+ to the children as <dfn>httpd processes</dfn>.</p>
<p>Your website has one or more machines under your
administrative control, together we'll call them a cluster of
<p>Given those assumptions, at a single point in time we can
identify any httpd process on any machine in the cluster from
all other httpd processes. The machine's IP address and the pid
- of the httpd process are sufficient to do this. So in order to
+ of the httpd process are sufficient to do this. A httpd process
+ can handle multiple requests simultaneously if you use a
+ multi-threaded MPM. In order to identify threads, we use a thread
+ index Apache httpd uses internally. So in order to
generate unique identifiers for requests we need only
distinguish between different points in time.</p>
even still, if you're running NTP then your UTC time will be
correct very shortly after reboot.</p>
+
<p>The <code>UNIQUE_ID</code> environment variable is
- constructed by encoding the 112-bit (32-bit IP address, 32 bit
- pid, 32 bit time stamp, 16 bit counter) quadruple using the
+ constructed by encoding the 144-bit (32-bit IP address, 32 bit
+ pid, 32 bit time stamp, 16 bit counter, 32 bit thread index)
+ quadruple using the
alphabet <code>[A-Za-z0-9@-]</code> in a manner similar to MIME
- base64 encoding, producing 19 characters. The MIME base64
+ base64 encoding, producing 24 characters. The MIME base64
alphabet is actually <code>[A-Za-z0-9+/]</code> however
<code>+</code> and <code>/</code> need to be specially encoded
in URLs, which makes them less desirable. All values are
and can otherwise use the same alphabet and bit length. Since
the time stamps are essentially an increasing sequence, it's
sufficient to have a <em>flag second</em> in which all machines
- in the cluster stop serving and request, and stop using the old
+ in the cluster stop serving any request, and stop using the old
encoding format. Afterwards they can resume requests and begin
issuing the new encodings.</p>
<p>This we believe is a relatively portable solution to this
- problem. It can be extended to multithreaded systems like
- Windows NT, and can grow with future needs. The identifiers
+ problem. The identifiers
generated have essentially an infinite life-time because future
identifiers can be made longer as required. Essentially no
communication is required between machines in the cluster (only
shortened, but more information needs to be assumed (for
example the 32-bit IP address is overkill for any site, but
there is no portable shorter replacement for it). </p>
-</div></div><div id="footer"><p class="apache">Maintained by the <a href="http://httpd.apache.org/docs-project/">Apache HTTP Server Documentation Project</a></p><p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p></div></body></html>
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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 again 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 Freenode, or sent to our <a href="http://httpd.apache.org/lists.html">mailing lists</a>.</div>
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