--- /dev/null
+<?xml version="1.0"?>
+<?xml-stylesheet type="text/xsl" href="../style/manual.xsl"?>
+<modulesynopsis>
+
+<name>mod_unique_id</name>
+<status>Extension</status>
+<identifier>unique_id_module</identifier>
+<sourcefile>mod_unique_id.c</sourcefile>
+<compatibility>Available in Apache 1.3 and later.</compatibility>
+
+<description>This module provides an environment variable with a unique
+identifier for each request.</description>
+
+<summary>
+
+ <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>
+</summary>
+
+<section>
+ <title>Theory</title>
+
+ <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 httpd processes.</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. 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 112-bit (32-bit IP address, 32 bit
+ pid, 32 bit time stamp, 16 bit counter) quadruple using the
+ alphabet <code>[A-Za-z0-9@-]</code> in a manner similar to MIME
+ base64 encoding, producing 19 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 and 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
+ 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>
+</section>
+
+
+</modulesynopsis>
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