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diff --git a/docs/manual/mod/mod_unique_id.xml b/docs/manual/mod/mod_unique_id.xml deleted file mode 100755 index c01f65dc67..0000000000 --- a/docs/manual/mod/mod_unique_id.xml +++ /dev/null @@ -1,181 +0,0 @@ -<?xml version="1.0"?> -<!DOCTYPE modulesynopsis SYSTEM "../style/modulesynopsis.dtd"> -<?xml-stylesheet type="text/xsl" href="../style/manual.xsl"?> -<modulesynopsis> - -<name>mod_unique_id</name> -<description>Provides an environment variable with a unique -identifier for each request</description> -<status>Extension</status> -<sourcefile>mod_unique_id.c</sourcefile> -<identifier>unique_id_module</identifier> - -<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> |