This class defines a UUID (Universally Unique IDentifier), also known as GUIDs (Globally Unique IDentifier).
A UUID is 128 bits long, and if generated according to this algorithm, is either guaranteed to be different from all other UUIDs/GUIDs generated until 3400 A.D. or extremely likely to be different. UUIDs were originally used in the Network Computing System (NCS) and later in the Open Software Foundation's (OSF) Distributed Computing Environment (DCE).
- Note
- In the way this UUID is constructed, when used outside of their original concept (NCS), they are actually not Globally unique and indeed multiple distinct concurrent processes are actually likely to generate the same UUID. Technically this is because the UUID is constructed only from the node information and time information. To make a globally unique number, this needs to be combined with TProcessUUID.
Structure of universal unique IDs (UUIDs).
Depending on the network data representation, the multi- octet unsigned integer fields are subject to byte swapping when communicated between dissimilar endian machines.
+-----------------------------------+
| low 32 bits of time | 0-3 .fTimeLow
+-------------------------------+----
| mid 16 bits of time | 4-5 .fTimeMid
+-------+-----------------------+
| vers. |
hi 12 bits of time | 6-7 .fTimeHiAndVersion
+-------+-------+---------------+
|Res | clkSeqHi | 8 .fClockSeqHiAndReserved
+---------------+
| clkSeqLow | 9 .fClockSeqLow
+---------------+------------------+
| node ID | 10-15 .fNode
+----------------------------------+
The adjusted time stamp is split into three fields, and the clockSeq is split into two fields.
The timestamp is a 60-bit value. For UUID version 1, this is represented by Coordinated Universal Time (UTC/GMT) as a count of 100-nanosecond intervals since 00:00:00.00, 15 October 1582 (the date of Gregorian reform to the Christian calendar).
The version number is multiplexed in the 4 most significant bits of the 'fTimeHiAndVersion' field. There are two defined versions:
MSB <---
Version 4-Bit Code Description
------------------------------------------------------------
| 1 0 0 0 1 DCE version, as specified herein.
| 2 0 0 1 0 DCE Security version, with
| embedded POSIX UIDs.
| 3 0 0 1 1 node
id is
a random value
------------------------------------------------------------
Clock Sequence
The clock sequence value must be changed whenever:
The UUID generator detects that the local value of UTC has gone backward; this may be due to re-syncing of the system clock.
While a node is operational, the UUID service always saves the last UTC used to create a UUID. Each time a new UUID is created, the current UTC is compared to the saved value and if either the current value is less or the saved value was lost, then the clock sequence is incremented modulo 16,384, thus avoiding production of duplicated UUIDs.
The clock sequence must be initialized to a random number to minimize the correlation across system. This provides maximum protection against node identifiers that may move or switch from system to system rapidly.
Clock Adjustment
UUIDs may be created at a rate greater than the system clock resolution. Therefore, the system must also maintain an adjustment value to be added to the lower-order bits of the time. Logically, each time the system clock ticks, the adjustment value is cleared. Every time a UUID is generated, the current adjustment value is read and incremented, and then added to the UTC time field of the UUID.
Clock Overrun
The 100-nanosecond granularity of time should prove sufficient even for bursts of UUID production in the next generation of high-performance multiprocessors. If a system overruns the clock adjustment by requesting too many UUIDs within a single system clock tick, the UUID generator will stall until the system clock catches up.
Definition at line 42 of file TUUID.h.
