diff options
| author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-05-07 02:04:06 +0000 |
|---|---|---|
| committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-05-07 02:04:06 +0000 |
| commit | 5dff2d61cc1c27747ee398e04d8e02843aabb1f8 (patch) | |
| tree | a67c336b406c8227bac912beb74a1ad3cdc55100 /docs/manual/mod/mod_unique_id.html.en | |
| parent | Initial commit. (diff) | |
| download | apache2-5dff2d61cc1c27747ee398e04d8e02843aabb1f8.tar.xz apache2-5dff2d61cc1c27747ee398e04d8e02843aabb1f8.zip | |
Adding upstream version 2.4.38.upstream/2.4.38
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'docs/manual/mod/mod_unique_id.html.en')
| -rw-r--r-- | docs/manual/mod/mod_unique_id.html.en | 250 |
1 files changed, 250 insertions, 0 deletions
diff --git a/docs/manual/mod/mod_unique_id.html.en b/docs/manual/mod/mod_unique_id.html.en new file mode 100644 index 0000000..6c7625a --- /dev/null +++ b/docs/manual/mod/mod_unique_id.html.en @@ -0,0 +1,250 @@ +<?xml version="1.0" encoding="ISO-8859-1"?> +<!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=ISO-8859-1" http-equiv="Content-Type" /> +<!-- + XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX + This file is generated from xml source: DO NOT EDIT + XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX + --> +<title>mod_unique_id - Apache HTTP Server Version 2.4</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/directives.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 2.4</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 2.4</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"><a href="https://www.apache.org/foundation/contributing.html" class="badge"><img src="https://www.apache.org/images/SupportApache-small.png" alt="Support Apache!" /></a><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 + 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 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> +<script type="text/javascript"><!--//--><![CDATA[//><!-- +var comments_shortname = 'httpd'; +var comments_identifier = 'http://httpd.apache.org/docs/2.4/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 2019 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/directives.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>
\ No newline at end of file |