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diff --git a/docs/manual/mod/mod_unique_id.xml b/docs/manual/mod/mod_unique_id.xml new file mode 100755 index 0000000000..6c4166c1d8 --- /dev/null +++ b/docs/manual/mod/mod_unique_id.xml @@ -0,0 +1,182 @@ +<?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> |