A __universally unique identifier__ (UUID) is a 128-bit number used to identify information in computer systems. The term __globally unique identifier__ (GUID) is also used - wikipedia 
The probability to find a duplicate within 103 trillion version 4 UUIDs is one in a billion.
# Node.js
To use uuids in node.js - stackoverflow
npm install uuid var uuid = require('uuid'); Then create some ids ... const uuidv1 = require('uuid/v1');uuidv1(); // -> '6c84fb90-12c4-11e1-840d-7b25c5ee775a' const uuidv4 = require('uuid/v4');uuidv4(); // -> '110ec58a-a0f2-4ac4-8393-c866d813b8d1'
- https://github.com/broofa/node-uuid - https://www.npmjs.com/package/uuid
# Uniqueness
When generated according to the standard methods, UUIDs are for practical purposes unique, without depending for their uniqueness on a central registration authority or coordination between the parties generating them, unlike most other numbering schemes. While the probability that a UUID will be duplicated is not zero, it is close enough to zero to be negligible.
Thus, anyone can create a UUID and use it to identify something with near certainty that the identifier does not duplicate one that has already been, or will be, created to identify something else. Information labeled with UUIDs by independent parties can therefore be later combined into a single database, or transmitted on the same channel, with a negligible probability of duplication.
Adoption of UUIDs and GUIDs is widespread, with many computing platforms providing support for generating them, and for parsing their textual representation.
# Format
In its canonical textual representation, the sixteen octets of a UUID are represented as 32 hexadecimal digits, displayed in five groups separated by hyphens.
123e4567-e89b-12d3-a456-426655440000
The 36 characters (32 alphanumeric and four hyphens) are arranged in the form `8-4-4-4-12` - wikipedia
RFC 4122 section 3 requires that the characters to be generated in lower case, while being case insensitive on input, though some commonly-used implementations violate this rule- rfc
# Uses
Significant uses include ext2/ext3/ext4 filesystem userspace tools (e2fsprogs uses libuuid provided by util-linux), LUKS encrypted partitions, GNOME, KDE, and macOS, most of which are derived from the original implementation by Theodore Ts'o - wikipedia
One of the uses of UUIDs in Solaris (operating system) (using Open Software Foundation implementation) is identification of a running operating system instance for the purpose of pairing crash dump data with Fault Management Event in the case of kernel panic.
# As database keys
UUIDs are commonly used as a unique key in database tables. The function, which generates standard version 1 UUIDs - wikipedia
The random nature of standard version 3, 4, and 5 UUIDs and the ordering of the fields within standard version 1 and 2 UUIDs may create problems with database locality of reference or performance when UUIDs are used as primary keys.
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 non-standard and significantly more likely to be duplicated, as Nilsson acknowledged, but Nilsson only required uniqueness within the application.
# Tools
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# Sections
# See also