UNIQUE_IDis set to the identifier for each request. Unique identifiers are useful for various reasons which are beyond the scope of this document.
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.
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.
The machines in your cluster should satisfy these requirements. (Even if you have only one machine you should synchronize its clock with NTP.)
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.
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.
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 ( ip_addr, pid, time_stamp, counter ) 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.
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).
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).
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 (i.e. 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.
How good a defense is it? Well 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).
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.
UNIQUE_ID 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
in a manner similar to MIME base64 encoding, producing 19 characters.
The MIME base64 alphabet is actually
/ 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
UNIQUE_ID as an
opaque token, which can be compared against other
for equality only.
The ordering was chosen such that it's possible to change the encoding
in the future without worrying about collision with an existing database
UNIQUE_IDs. 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 flag second 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.
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).
mod_unique_idhas no directives.