mirror of
https://github.com/apache/httpd.git
synced 2025-04-18 22:24:07 +03:00
01055edef2
git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1907148 13f79535-47bb-0310-9956-ffa450edef68
250 lines
15 KiB
XML
250 lines
15 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
|
||
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
|
||
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head>
|
||
<meta content="text/html; charset=UTF-8" http-equiv="Content-Type" />
|
||
<!--
|
||
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
|
||
This file is generated from xml source: DO NOT EDIT
|
||
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
|
||
-->
|
||
<title>mod_unique_id - Apache HTTP Server Version 2.5</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 rel="stylesheet" type="text/css" href="../style/css/prettify.css" />
|
||
<script src="../style/scripts/prettify.min.js" type="text/javascript">
|
||
</script>
|
||
|
||
<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/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>
|
||
<p class="apache">Apache HTTP Server Version 2.5</p>
|
||
<img alt="" src="../images/feather.png" /></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/">Documentation</a> > <a href="../">Version 2.5</a> > <a href="./">Modules</a></div>
|
||
<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
|
||
specific conditions. The unique identifier is even unique
|
||
across multiple machines in a properly configured cluster of
|
||
machines. The environment variable <code>UNIQUE_ID</code> is
|
||
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>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> <a title="Permanent link" href="#theory" class="permalink">¶</a></h2>
|
||
|
||
|
||
<p>First a brief recap of how the Apache server works on Unix
|
||
machines. This feature currently isn't supported on Windows NT.
|
||
On Unix machines, Apache creates several children, the children
|
||
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 <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
|
||
machines. Each machine can possibly run multiple instances of
|
||
Apache. All of these collectively are considered "the
|
||
universe", and with certain assumptions we'll show that in this
|
||
universe we can generate unique identifiers for each request,
|
||
without extensive communication between machines in the
|
||
cluster.</p>
|
||
|
||
<p>The machines in your cluster should satisfy these
|
||
requirements. (Even if you have only one machine you should
|
||
synchronize its clock with NTP.)</p>
|
||
|
||
<ul>
|
||
<li>The machines' times are synchronized via NTP or other
|
||
network time protocol.</li>
|
||
|
||
<li>The machines' hostnames all differ, such that the module
|
||
can do a hostname lookup on the hostname and receive a
|
||
different IP address for each machine in the cluster.</li>
|
||
</ul>
|
||
|
||
<p>As far as operating system assumptions go, we assume that
|
||
pids (process ids) fit in 32-bits. If the operating system uses
|
||
more than 32-bits for a pid, the fix is trivial but must be
|
||
performed in the code.</p>
|
||
|
||
<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. 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>
|
||
|
||
<p>To distinguish time we will use a Unix timestamp (seconds
|
||
since January 1, 1970 UTC), and a 16-bit counter. The timestamp
|
||
has only one second granularity, so the counter is used to
|
||
represent up to 65536 values during a single second. The
|
||
quadruple <em>( ip_addr, pid, time_stamp, counter )</em> is
|
||
sufficient to enumerate 65536 requests per second per httpd
|
||
process. There are issues however with pid reuse over time, and
|
||
the counter is used to alleviate this issue.</p>
|
||
|
||
<p>When an httpd child is created, the counter is initialized
|
||
with ( current microseconds divided by 10 ) modulo 65536 (this
|
||
formula was chosen to eliminate some variance problems with the
|
||
low order bits of the microsecond timers on some systems). When
|
||
a unique identifier is generated, the time stamp used is the
|
||
time the request arrived at the web server. The counter is
|
||
incremented every time an identifier is generated (and allowed
|
||
to roll over).</p>
|
||
|
||
<p>The kernel generates a pid for each process as it forks the
|
||
process, and pids are allowed to roll over (they're 16-bits on
|
||
many Unixes, but newer systems have expanded to 32-bits). So
|
||
over time the same pid will be reused. However unless it is
|
||
reused within the same second, it does not destroy the
|
||
uniqueness of our quadruple. That is, we assume the system does
|
||
not spawn 65536 processes in a one second interval (it may even
|
||
be 32768 processes on some Unixes, but even this isn't likely
|
||
to happen).</p>
|
||
|
||
<p>Suppose that time repeats itself for some reason. That is,
|
||
suppose that the system's clock is screwed up and it revisits a
|
||
past time (or it is too far forward, is reset correctly, and
|
||
then revisits the future time). In this case we can easily show
|
||
that we can get pid and time stamp reuse. The choice of
|
||
initializer for the counter is intended to help defeat this.
|
||
Note that we really want a random number to initialize the
|
||
counter, but there aren't any readily available numbers on most
|
||
systems (<em>i.e.</em>, you can't use rand() because you need
|
||
to seed the generator, and can't seed it with the time because
|
||
time, at least at one second resolution, has repeated itself).
|
||
This is not a perfect defense.</p>
|
||
|
||
<p>How good a defense is it? Suppose that one of your machines
|
||
serves at most 500 requests per second (which is a very
|
||
reasonable upper bound at this writing, because systems
|
||
generally do more than just shovel out static files). To do
|
||
that it will require a number of children which depends on how
|
||
many concurrent clients you have. But we'll be pessimistic and
|
||
suppose that a single child is able to serve 500 requests per
|
||
second. There are 1000 possible starting counter values such
|
||
that two sequences of 500 requests overlap. So there is a 1.5%
|
||
chance that if time (at one second resolution) repeats itself
|
||
this child will repeat a counter value, and uniqueness will be
|
||
broken. This was a very pessimistic example, and with real
|
||
world values it's even less likely to occur. If your system is
|
||
such that it's still likely to occur, then perhaps you should
|
||
make the counter 32 bits (by editing the code).</p>
|
||
|
||
<p>You may be concerned about the clock being "set back" during
|
||
summer daylight savings. However this isn't an issue because
|
||
the times used here are UTC, which "always" go forward. Note
|
||
that x86 based Unixes may need proper configuration for this to
|
||
be true -- they should be configured to assume that the
|
||
motherboard clock is on UTC and compensate appropriately. But
|
||
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 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 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
|
||
encoded in network byte ordering so that the encoding is
|
||
comparable across architectures of different byte ordering. The
|
||
actual ordering of the encoding is: time stamp, IP address,
|
||
pid, counter. This ordering has a purpose, but it should be
|
||
emphasized that applications should not dissect the encoding.
|
||
Applications should treat the entire encoded
|
||
<code>UNIQUE_ID</code> as an opaque token, which can be
|
||
compared against other <code>UNIQUE_ID</code>s for equality
|
||
only.</p>
|
||
|
||
<p>The ordering was chosen such that it's possible to change
|
||
the encoding in the future without worrying about collision
|
||
with an existing database of <code>UNIQUE_ID</code>s. The new
|
||
encodings should also keep the time stamp as the first element,
|
||
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 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. 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
|
||
NTP synchronization is required, which is low overhead), and no
|
||
communication between httpd processes is required (the
|
||
communication is implicit in the pid value assigned by the
|
||
kernel). In very specific situations the identifier can be
|
||
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 class="bottomlang">
|
||
<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><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>
|
||
<script type="text/javascript"><!--//--><![CDATA[//><!--
|
||
var comments_shortname = 'httpd';
|
||
var comments_identifier = 'http://httpd.apache.org/docs/trunk/mod/mod_unique_id.html';
|
||
(function(w, d) {
|
||
if (w.location.hostname.toLowerCase() == "httpd.apache.org") {
|
||
d.write('<div id="comments_thread"><\/div>');
|
||
var s = d.createElement('script');
|
||
s.type = 'text/javascript';
|
||
s.async = true;
|
||
s.src = 'https://comments.apache.org/show_comments.lua?site=' + comments_shortname + '&page=' + comments_identifier;
|
||
(d.getElementsByTagName('head')[0] || d.getElementsByTagName('body')[0]).appendChild(s);
|
||
}
|
||
else {
|
||
d.write('<div id="comments_thread">Comments are disabled for this page at the moment.<\/div>');
|
||
}
|
||
})(window, document);
|
||
//--><!]]></script></div><div id="footer">
|
||
<p class="apache">Copyright 2023 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
|
||
<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></div><script type="text/javascript"><!--//--><![CDATA[//><!--
|
||
if (typeof(prettyPrint) !== 'undefined') {
|
||
prettyPrint();
|
||
}
|
||
//--><!]]></script>
|
||
</body></html> |