Editing Universally unique identifier (section)

== Uses ==

=== Filesystems ===

Significant uses include [[ext2]]/[[ext3]]/[[ext4]] filesystem userspace tools ([[e2fsprogs]] uses libuuid provided by [[util-linux]]), [[Logical Volume Manager (Linux)|LVM]], [[LUKS]] encrypted partitions, [[GNOME]], [[KDE]], and [[macOS]],<ref>[https://opensource.apple.com/source/Libc/Libc-391/uuid/uuidsrc/gen_uuid.c gen_uuid.c in Apple's Libc-391, corresponding to Mac OS X 10.4]</ref> most of which are derived from the original implementation by [[Theodore Ts'o]].<ref name="e2fsprogs">{{cite web|url=https://git.kernel.org/?p=fs/ext2/e2fsprogs.git;a=blob;f=lib/uuid/gen_uuid.c;hb=1bbfec624c4bbe767060a13762aa9a656536a4fd|title=ext2/e2fsprogs.git - Ext2/3/4 filesystem userspace utilities|website=Kernel.org|access-date=9 January 2017}}</ref>
The "partition label" and the "partition UUID" are both stored in the [[Superblock (file system)|superblock]]. They are both part of the file system rather than of the partition. For example, ext2–4 contain a UUID, while NTFS or FAT32 do not. The superblock is a part of the file system, thus fully contained within the partition, hence doing {{code|1=dd if=/dev/sda1 of=/dev/sdb1}} leaves both sda1 and sdb1 with the same label and UUID.

=== Partition tables ===

The [[GUID Partition Table]] (GUID) is one example that utilised GUIDs to label partition types.

=== Remoting ===

There are several flavors of GUIDs used in Microsoft's [[Component Object Model]] (COM):

* {{mono|IID}} – interface identifier; (The ones that are registered on a system are stored in the [[Windows Registry]] at {{code|[HKEY_CLASSES_ROOT\Interface]}}<ref>{{cite web|url=https://msdn.microsoft.com/en-us/library/windows/desktop/ms688484(v=vs.85).aspx|title=Interface Pointers and Interfaces|website=Windows Dev Center - Desktop app technologies|publisher=[[Microsoft]]|access-date=15 December 2015|quote=You reference an interface at run time with a globally unique interface identifier ({{mono|IID}}). This {{mono|IID}}, which is a specific instance of a globally unique identifier ({{mono|GUID}}) supported by COM, allows a client to ask an object precisely whether it supports the semantics of the interface, without unnecessary overhead and without the confusion that could arise in a system from having multiple versions of the same interface with the same name.}}</ref> )
* {{mono|CLSID}} {{anchor|CLSID}} – class identifier; (Stored at {{code|[HKEY_CLASSES_ROOT\CLSID]}}). In practice it is not entirely separate from the {{mono|IID}} space, because remoting the interface can require a [[Stub (distributed computing)|proxy/stub object]] which some toolsets used to create with a {{mono|CLSID}} equal to the interface's {{mono|IID}}.
* {{mono|LIBID}} – type library identifier; (Stored at {{code|[HKEY_CLASSES_ROOT\TypeLib]}}<ref>{{cite web|url=https://msdn.microsoft.com/en-us/library/windows/desktop/ms221610(v=vs.85).aspx|title=Registering a Type Library|website=[[Microsoft Developer Network]]|publisher=[[Microsoft]]|access-date=15 December 2015}}</ref>)
* {{mono|CATID}} – category identifier; (its presence on a class identifies it as belonging to certain class categories, listed at {{code|[HKEY_CLASSES_ROOT\Component Categories]}}<ref>{{cite web|url=https://msdn.microsoft.com/en-us/library/windows/desktop/ms682451(v=vs.85).aspx|title=Categorizing by Component Capabilities|website=Windows Dev Center - Desktop app technologies|publisher=[[Microsoft]]|access-date=15 December 2015|quote=A listing of the CATIDs and the human-readable names is stored in a well-known location in the registry.}}</ref>)

=== Databases ===

UUIDs are commonly used as a [[unique key]] in [[database]] tables. The {{mono|NEWID}} function in [[Microsoft SQL Server]] version 4 [[Transact-SQL]] returns standard random version-4 UUIDs, while the {{mono|NEWSEQUENTIALID}} function returns 128-bit identifiers similar to UUIDs which are committed to ascend in sequence until the next system reboot.<ref>{{cite web |url=http://msdn.microsoft.com/en-us/library/ms189786.aspx |title=NEWSEQUENTIALID (Transact-SQL) |website=[[Microsoft Developer Network]] |publisher=[[Microsoft]] |date=2015-08-08 |access-date=2017-01-14}}</ref> The [[Oracle Database]] {{mono|SYS_GUID}} function does not return a standard GUID, despite the name.  Instead, it returns a 16-byte 128-bit RAW value based on a host identifier and a process or thread identifier, somewhat similar to a GUID.<ref>{{cite web |url=https://docs.oracle.com/cd/B12037_01/server.101/b10759/functions153.htm |publisher=[[Oracle Corporation|Oracle]] |title=Oracle Database SQL Reference}}</ref> [[PostgreSQL]] contains a {{mono|UUID}} datatype<ref>{{cite web |url=https://www.postgresql.org/docs/9.4/static/datatype-uuid.html |work=PostgreSQL 9.4.10 Documentation |title=Section 8.12 UUID Type |date=13 February 2020 |publisher=PostgreSQL Global Development Group}}</ref> and can generate most versions of UUIDs through the use of functions from modules.<ref>{{cite web |url=https://www.postgresql.org/docs/9.6/static/uuid-ossp.html |work=PostgreSQL: Documentation: 9.6 |title=uuid-ossp |date=12 August 2021 |publisher=PostgreSQL Global Development Group}}</ref><ref>{{cite web |url=https://www.postgresql.org/docs/9.6/static/pgcrypto.html |work=PostgreSQL: Documentation: 9.6 |title=pgcrypto |date=12 August 2021 |publisher=PostgreSQL Global Development Group}}</ref> [[MySQL]] provides a {{mono|UUID}} function, which generates standard version-1 UUIDs.<ref>{{cite book |chapter-url=http://dev.mysql.com/doc/refman/5.7/en/miscellaneous-functions.html |title=MySQL 5.7 Reference Manual |chapter=Section 13.20 Miscellaneous Functions |publisher=[[Oracle Corporation]]}}</ref>

==== Combined Time-GUID ====

The random nature of standard UUIDs of versions 3, 4, and 5, and the ordering of the fields within standard versions 1 and 2 may create problems with database [[locality of reference|locality]] or performance when UUIDs are used as [[primary key]]s. For example, in 2002 Jimmy Nilsson reported a significant improvement in performance with Microsoft SQL Server when the version-4 UUIDs being used as keys were modified to include a non-random suffix based on system time. This so-called "COMB" (combined time-GUID) approach made the UUIDs significantly more likely to be duplicated, as Nilsson acknowledged, but Nilsson only required uniqueness within the application.<ref>{{cite web |last=Nilsson |first=Jimmy |url=http://www.informit.com/articles/article.aspx?p=25862 |title=The Cost of GUIDs as Primary Keys |website=InformIT |access-date=2012-06-20|date=2002-03-08 }}</ref> By reordering and encoding version 1 and 2 UUIDs so that the timestamp comes first, insertion performance loss can be averted.<ref>{{cite web |url=https://percona.com/blog/2014/12/19/store-uuid-optimized-way/ |publisher=Percona |title=Storing UUID Values in MySQL |access-date=2021-02-10 |archive-url=https://web.archive.org/web/20201129230946/https://www.percona.com/blog/2014/12/19/store-uuid-optimized-way/ |archive-date=2020-11-29| date=2014-12-19}}</ref>

COMB-like arrangements of UUID payloads were eventually standardized in RFC 9562{{r|RFC 9562}} as UUIDv6 and UUIDv7.