A Universally Unique Identifier (UUID) is a 128-bit identifier described in Internet Engineering Task Force RFC 4122: A Universally Unique IDentifier (UUID) URN Namespace .
Generators for versions 1,3,4 and 5 UUID's are provided. The value held in a UUID is represented by a specific hexadecimal format of the binary fields. An example UUID string representation is: F81D4FAE-7DEC-11D0-A765-00A0C91E6BF6.
A cautionary note: there is no standard regarding binary representation of a UUID other than its string format.
The version 4 UUID is UUID based on random bytes. We fill the 128-bits with random bits (6 of the bits are correspondingly set to flag the version and variant of the UUID). No special configuration or implementation decisions are required to generate version 4 UUID's.
Version 3 UUIDs are initialized using a name, a namespace, and the MD5 hashing algorithm.
Version 5 UUIDs are initialized using a name, a namespace, and the SHA-1 hashing algorithm.
The version 1 UUID is a combination of node identifier (MAC address), timestamp and a random seed. The version one generator uses the commons-discovery package to determine the implementation. The implementations are specified by system properties.
|org.apache.commons.id.uuid.config.resource.filename||[No default, you must explicitly configure this for each jvm instance.]|
The UUID draft specification calls for persisting generator state to stable non-volatile storage (provisions are made for systems that can not provide persistent storage.) Persisting state decreases the likelihood of duplicating time and random seed (clock sequence) values, which are two components of the version one identifier. When the previous clock sequence is unknown the generator must generate new random bytes for the clock sequence. The system time may be set backwards during normal operation of a system; accordingly the generator is required to change the clock sequence value.
The State interface in the
package provides the interface for persistent state used by the
VersionOneGenerator. Three implementations are provided to accommodate
different scenarios. The
InMemoryStateImpl follows the
recommendations of the specification for those instances when no persistent
storage is available and the hardware (MAC) address cannot be read. The
ReadOnlyResourceImpl implementation is useful for situations or
containers that allow resource loading, but forbid explicit I/O. The xml
state file contains the node identifier (hardware address) and is loaded as a
system resource. Finally, the
ReadWriteFileImpl extends the
ReadOnlyResourceImpl (both share the same loading of
configuration data); however the
ReadWriteFileImpl uses IO to
write the clock sequence and last timestamp used to file.
The following is an example configuration file xml (be certain to change the uuid.state file for each virtual machine instance):
<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE uuidstate [ <!ELEMENT uuidstate (node*)> <!ELEMENT node EMPTY> <!ATTLIST node id ID #REQUIRED> <!ATTLIST node clocksequence CDATA #IMPLIED> <!ATTLIST node lasttimestamp CDATA #IMPLIED> ]> <uuidstate synchInterval="3000"> <node id="AA-BB-CC-DD-EE-11" /> <node id="22-33-44-55-66-77" /> </uuidstate>
The "synchInterval" attribute is specified as the number of milliseconds between writes to the file to update the "clocksequence" and "lasttimestamp". This interval should be set large enough to provide adequate performance, yet attempt not to specify a time longer than the time needed to restart the virtual machine and generate the next UUID. See the IETF draft for more on this strategy (specification: "126.96.36.199 Writing stable storage".)
The UUID specification is written with the frame of reference that one or
more physical (MAC address) node identifiers belong to a machine. Java's
Virtual Machine concept is that a physical machine hosts a virtual machine.
implementations assume that each virtual machine instance
configuration file with distinct identifiers/addresses.
Without this assumption a system wide mutex or mutual exclusion object is
required to prevent multiple virtual machine instance (either different
jvm's or concurrent instances of the same jvm) from generating
duplicates at the same time using the same clock sequence and identifier.
Writing a custom implementation of the
allows one to change this assumption. Several means of locking the node
identifier are possible, such as file system locks, sockets, and more - but
not discussed here, as not all are appropriate for all application
Another obstacle in UUID generation for various systems is the time
resolution called for in the UUID draft is based on 100-nanosecond intervals
from the Gregorian changeover epoch. The Java language provides millisecond
precision when retrieving system time; however the actual time resolution is
operating system and chipset dependent. The issue is that calls for the
system time in rapid succession produce duplicate time values and
sub-millisecond resolution is only provided by performance counters,
interrupts, or otherwise. The UUID specification provides a means of
compensating for this - suggesting use of an artificial time produced from
the actual time and a counter that may not exceed the next interval of the
system's effective-resolution. The
org.apache.commons.id.clock.Clock interface provides the SPI for
uuid time stamps. The
SystemClockImpl implementation uses the
millisecond resolution of the
System.currentTimeMillis plus a count
up to 10,000.
Now assume your system has an effective resolution of 54 milliseconds (the
clock increments after 54 milliseconds). This would allow less than 200
UUID's to be generated per millisecond. In the case where greater
numbers must be generated, the
ThreadClockImpl is provided as
one potential solution. This implementation uses a threaded clock class to
increment on a scheduled interval and up to (10,000 multiplied by the
interval length) UUID's may be generated. Other methods to increase the
generator throughput are described in the UUID draft (such as adding more
node identifiers or pre-generating id's to deal with sporadic demand).
One final issue to consider in UUID generation is security. A version one uuid exposes the node identifier as part of its string format. This may be very undesirable during non-secure transmision of the identifier. Another aspect of the security concern relates to privacy given that the version one uuid may identify a time and place (machine address). Your security requirements may determine the uuid version, the source of the identifier and/or the state implementation you chose.