Universally unique identifier

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A universally unique identifier (UUID) is an identifier standard used in software construction, standardized by the Open Software Foundation (OSF) as part of the Distributed Computing Environment (DCE).

The intent of UUIDs is to enable distributed systems to uniquely identify information without significant central coordination. In this context the word unique should be taken to mean "practically unique" rather than "guaranteed unique". Since the identifiers have a finite size it is possible for two differing items to share the same identifier. The identifier size and generation process need to be selected so as to make this sufficiently improbable in practice. Anyone can create a UUID and use it to identify something with reasonable confidence that the same identifier will never be unintentionally created by anyone to identify something else. Information labeled with UUIDs can therefore be later combined into a single database without needing to resolve name conflicts.

One widespread use of this standard is in Microsoft's globally unique identifiers (GUIDs). Other significant uses include Linux's ext2/ext3 filesystem, LUKS encrypted partitions, GNOME, KDE, and Mac OS X, all of which use implementations derived from the uuid library found in the e2fsprogs (Ext2 Filesystems Utilities[1]) package.

UUIDs are documented as part of ISO/IEC 11578:1996 "Information technology – Open Systems Interconnection – Remote Procedure Call (RPC)" and more recently in ITU-T Rec. X.667 | ISO/IEC 9834-8:2005. The IETF has published Standards Track RFC 4122 that is technically equivalent with ITU-T Rec. X.667 | ISO/IEC 9834-8.

Definition

A UUID is a 16-byte (128-bit) number. The number of theoretically possible UUIDs is therefore about 3 × 1038. In its canonical form, a UUID is represented by 32 hexadecimal digits, displayed in 5 groups separated by hyphens, in the form 8-4-4-4-12 for a total of 36 characters (32 digits and 4 hyphens). For example:

550e8400-e29b-41d4-a716-446655440000

A UUID may also be used with a specific identifier intentionally used repeatedly to identify the same thing in different contexts. For example, in Microsoft's Component Object Model, every component must implement the IUnknown interface, which is done by creating a UUID representing IUnknown. In all cases wherever IUnknown is used, whether it is being used by a process trying to access the IUnknown interface in a component, or by a component implementing the IUnknown interface, it is always referenced by the same identifier: 00000000-0000-0000-C000-000000000046.

Variants and versions

The variant indicates the layout of the UUID. The UUID specification covers one particular variant. Other variants are reserved or exist for backward compatibility reasons (e.g. for values assigned before the UUID specification was produced). An example of a UUID that is a different variant is the nil UUID, which is a UUID that has all 128 bits set to zero.

In the canonical representation, xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx, the most significant bits of N indicates the variant (depending on the variant; one, two or three bits are used). The variant covered by the UUID specification is indicated by the two most significant bits of N being 1 0 (i.e. the hexadecimal N will always be 8, 9, a, or b).

In the variant covered by the UUID specification, there are five versions. For this variant, the four bits of M indicates the UUID version (i.e. the hexadecimal M will either be 1, 2, 3, 4, or 5).

Version 1 (MAC address)

Conceptually, the original (version 1) generation scheme for UUIDs was to concatenate the UUID version with the MAC address of the computer that is generating the UUID, and with the number of 100-nanosecond intervals since the adoption of the Gregorian calendar in the West. This scheme has been criticized in that it is not sufficiently "opaque"; it reveals both the identity of the computer that generated the UUID and the time at which it did so.

Version 2 (DCE Security)

Version 2 UUIDs are similar to Version 1 UUIDs, with the upper byte of the clock sequence replaced by the identifier for a "local domain" (typically either the "POSIX UID domain" or the "POSIX GID domain") and the first 4 bytes of the timestamp replaced by the user's POSIX UID or GID (with the "local domain" identifier indicating which it is).[2][3]

Version 3 (MD5 hash)

Version 3 UUIDs use a scheme deriving a UUID via MD5 from a URL, a fully qualified domain name, an object identifier, a distinguished name (DN as used in Lightweight Directory Access Protocol), or on names in unspecified namespaces. Version 3 UUIDs have the form xxxxxxxx-xxxx-3xxx-xxxx-xxxxxxxxxxxx with hexadecimal digits x.

To determine the version 3 UUID of a given name, the UUID of the namespace, e.g. 6ba7b810-9dad-11d1-80b4-00c04fd430c8 for a domain, is transformed to a string of bytes corresponding to its hexadecimal digits, concatenated with the input name, hashed with MD5 yielding 128 bits. Six bits are replaced by fixed values, four of these bits indicate the version, 0011 for version 3. Finally the fixed hash is transformed back into the hexadecimal form with hyphens separating the parts relevant in other UUID versions.

Version 4 (random)

Version 4 UUIDs use a scheme relying only on random numbers. This algorithm sets the version number as well as two reserved bits. All other bits are set using a random or pseudorandom data source. Version 4 UUIDs have the form xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx where x is any hexadecimal digit and y is one of 8, 9, A, or B. e.g. f47ac10b-58cc-4372-a567-0e02b2c3d479.

Version 5 (SHA-1 hash)

Version 5 UUIDs use a scheme with SHA-1 hashing; otherwise it is the same idea as in version 3. RFC 4122 states that version 5 is preferred over version 3 name based UUIDs without giving a reason. Note that the 160 bit SHA-1 hash is truncated to 128 bits to make the length work out. An erratum addresses the example in appendix B of RFC 4122.

Implementations

ActionScript
CASA Lib provides a Version 4 UUID function as part of the StringUtil class.[4] Adobe Flex also provides a UUID implementation with the UIDUtil class.[5]
C
libuuid is part of the e2fsprogs package. The OSSP project provides a UUID library.[6]
C++
ooid implements a C++ UUID class. QUuid is part of the C++ Qt framework. Boost.Uuid is a header-only implementation under a non-reciprocal Open Source license.
Caché ObjectScript
UUID Version 4 implementation for Caché ObjectScript.
CakePHP
CakePHP will automatically generate UUIDs for new records if you specify a table's primary key as data type CHAR(36).[7]
Cocoa/Carbon (Mac OS X)
The Core Foundation class CFUUIDRef is used to produce and store UUIDs, as well as to convert them to and from CFString/NSString representations.[8]
CodeGear RAD Studio (Delphi/C++ Builder)
A new GUID can be generated by pressing Ctrl+Shift+G. For runtime functions see the "Free Pascal & Lazarus IDE" section.
ColdFusion
The createUUID() function provides a UUID in all versions, however the format generated is in 4 segments instead of 5 xxxxxxxx-xxxx-xxxx-xxxxxxxxxxxxxxxx (8-4-4-16).[9]
Common Lisp
A library is available to create UUIDs (v1, v3, v4 and v5) according to RFC 4122.[10]
D
The Tango standard library includes a module to create UUIDs (v3, v4, and v5) according to RFC 4122.[11]
Eiffel
A library is available to create UUIDs Generates uuids according to RFC 4122, Variant 1 0, Version 4. Source available at Eiffel UUID library
Firebird Server
Firebird has gen_uuid() from version 2.1[12] and uuid_to_char() and char_to_uuid() from version 2.5[13] as built-in functions.
Free Pascal & Lazarus IDE
In Free Pascal there is a class called TGUID that holds the structure of a UUID. Also in the SysUtils.pas unit there are methods to create, compare and convert UUID's. They are CreateGUID(), GUIDToString() and IsEqualGUID().[14] In the Lazarus IDE you can also generate a UUID by pressing Ctrl+Shift+G.
Haskell
The package uuid[15] directly implements most of RFC 4122. The package supports generation (v1, v3, v4 and v5) as well as serialization to and from string and binary formats. The package system-uuid[16] provides bindings to the native UUID generators on Windows, Linux and Mac OS X.
Java
The J2SE 5.0 release of Java provides a class that will produce 128-bit UUIDs, although it only implements version 3 and 4 generation methods, not the original method (due to lack of means to access MAC addresses using pure Java before version 6). The API documentation for the java.util.UUID class refers to ISO/IEC 11578:1996. Open source implementations supporting MAC addresses on several common operating systems are UUID – generate UUIDs (or GUIDs) in Java and Java Uuid Generator (JUG).
Javascript
Broofa.com has implemented a JavaScript function which generates version 4 UUIDs as defined in the RFC 4122 specification. An open source library UUID.js, which is available under the MIT license, generates version 4 and version 1 UUIDs according to RFC 4122.
Lasso
A custom tag for Lassoscript by Douglas Burchard, and an LJAPI-module by Steffan A. Cline.
Lua
There is a Lua module by Luiz Henrique de Figueiredo.
Linux
Command line utility uuidgen is commonly available from e2fsprogs package (RedHat). There is also a tool called simply "uuid" available, which has the same functionality.
Mac OS X
Command line utility uuidgen is available. In the Terminal application, type: uuidgen
MySQL
MySQL provides a UUID() function.[17]
.NET Framework
The .NET Framework also provides a structure System.Guid to generate and manipulate 128-bit UUIDs.[18]
Objective Caml (OCaml)
The uuidm library implements universally unique identifiers version 3, 5 (name based with MD5, SHA-1 hashing) and 4 (random based) according to RFC 4122.
Oracle Database
The Oracle Database provides a function SYS_GUID() to generate unique identifiers.[19]
Perl
The Data::UUID and Data::GUID modules from CPAN can be used to create UUIDs.[20] The UUID::Tiny module is a lightweight, low dependency Pure Perl module for UUID creation and testing. [21]
PHP
In PHP there are several modules for creating UUIDs[22], including Kohana PHP Framework, which supports the generation of version 3, 4, and 5 UUIDs according to RFC 4122 specifications using the UUID module. [23]
PostgreSQL
PostgreSQL contains a uuid data type. Also various generation functions as part of the uuid-ossp contrib module.[24]
Progress OpenEdge ABL
The GENERATE-UUID function in OpenEdge 10 provides a UUID which can be made printable using the GUID() or BASE64-ENCODE() functions.[25]
Python
The uuid module[26] (included in the standard library since Python 2.5) creates UUIDs to RFC 4122.
Revolution/RunRev
The libUUID library[27] A library that generates UUIDs of type 1 (time based), type 3 (name-based) and type 4 (random-based). Version 1.0. by Mark Smith. OSL 3.0
Ruby
There are several RFC4122 implementations for Ruby, the most updated ones being Ruby-UUID (fork here), UUID and UUIDTools. Ruby 1.9 includes a built-in version 4 uuid generator (SecureRandom.uuid).
SQL Server
Transact-SQL (2000 and 2005) provides a function called NEWID() to generate unique identifiers. SQL Server 2005 provides an additional function called NEWSEQUENTIALID() which generates a new GUID that is greater than any GUID previously created by the NEWSEQUENTIALID() function on a given computer.
Apache Solr
Solr contains a uuid data type.
Tcl
A Tcl implementation is provided in the TclLib package.[28]

Random UUID probability of duplicates

Randomly generated UUIDs like those generated by the java.util.UUID class have 122 random bits. There are 128 bits altogether with 4 bits being used for the version ('Randomly generated UUID'), and 2 bits for the variant ('Leach-Salz'). With random UUIDs, the chance of two having the same value can be calculated using probability theory (Birthday paradox). Using the approximation

these are the probabilities of an accidental clash after calculating n UUIDs, with x=2122:

nprobability
68,719,476,736 = 2360.0000000000000004 (4 × 10−16)
2,199,023,255,552 = 2410.0000000000004 (4 × 10−13)
70,368,744,177,664 = 2460.0000000004 (4 × 10−10)

To put these numbers into perspective, one's annual risk of being hit by a meteorite is estimated to be one chance in 17 billion,[29] that means the probability is about 0.00000000006 (6 × 10−11), equivalent to the odds of creating a few tens of trillions of UUIDs in a year and having one duplicate. In other words, only after generating 1 billion UUIDs every second for the next 100 years, the probability of creating just one duplicate would be about 50%. The probability of one duplicate would be about 50% if every person on earth owns 600 million UUIDs.

However, these probabilities only hold when the UUIDs are generated using sufficient entropy. Otherwise the probability of duplicates may be significantly higher, since the statistical dispersion may be lower.

History

The initial design of DCE UUIDs was based on UUIDs as defined in the Network Computing System,[30] whose design was in turn inspired by the (64-bit) unique identifiers defined and used pervasively in Domain/OS, the operating system designed by Apollo Computer, Inc.

See also

References

  1. ^ Theodore Ts'o. "Ext2 Filesystems Utilities".
  2. ^ The Open Group (1997). "CDE 1.1: Remote Procedure Call".
  3. ^ The Open Group (1997). "DCE 1.1: Authentication and Security Services".
  4. ^ Aaron Clinger and the CASA Lib Team. "CASA Lib's StringUtil Documentation".
  5. ^ Adobe Systems Incorporated. "mx.utils.UIDUtil".
  6. ^ Open Source Software Project. "Universally Unique Identifier (UUID)".
  7. ^ "Cake version 1.2 manual".
  8. ^ Apple Computer, Inc. "CFUUID Reference".
  9. ^ Adobe Systems Inc. "ColdFusion Functions:CreateUUID".
  10. ^ Boian Tzonev. "UUID".
  11. ^ "D/Tango UUID API document".
  12. ^ "Firebird 2.1 Release Notes".
  13. ^ "Firebird 2.5 Release Notes".
  14. ^ Free Pascal Documentation. "Reference for 'sysutils' unit".
  15. ^ Antoine Latter. "uuid".
  16. ^ Jason Dusek. "system-uuid".
  17. ^ MySQL AB. "MySQL 5.0 Reference Manual".
  18. ^ "Guid Structure". MSDN Library.
  19. ^ "SYS_GUID". Oracle Database SQL Reference. Oracle Corporation.
  20. ^ Signes, Ricardo (16 January 2009). "Data-GUID". CPAN.
  21. ^ Augustin, Christian (31 January 2010). "UUID-Tiny". CPAN.
  22. ^ Holzgraefe, Hartmut (1 April 2008). "uuid". PECL.
  23. ^ Gilk, Woody. "Kohana UUID module".
  24. ^ PostgreSQL Global Development Group. "PostgreSQL 8.3.x Documentation: UUID Type".
  25. ^ http://www.psdn.com/library/servlet/KbServlet/download/1927-102-2537/dvref.pdf
  26. ^ "Python Library Reference: uuid".
  27. ^ "Revolution Stuff: libUUID".
  28. ^ "Tcl Standard Library: uuid".
  29. ^ Old Farmer's Almanac 1994, 220-222, Taking your Chances: An Explanation of Risk
  30. ^ Zahn, Lisa (1990). Network Computing Architecture. Prentice Hall. p. 10. ISBN 0136116744.

External links
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