UUID

UUID stands for Universally Unique Identifier, a 128-bit value designed to be unique across space and time. It is used to identify information in distributed systems, software components, and databases where a unique key is needed without a central authority.

UUIDs are typically represented as 36-character strings consisting of 32 hexadecimal digits displayed in five groups separated by hyphens, in the form 8-4-4-4-12. The high-order bits encode a version, and a variant field identifies the layout; most UUIDs follow the RFC 4122 specification. The term GUID is commonly used in Microsoft products and is functionally equivalent to UUID.

Several generation methods exist. Version 1 is time-based and uses the current timestamp and, often, the node's MAC address. Version 3 and version 5 are name-based, deriving the UUID from a namespace UUID and a name via MD5 (v3) or SHA-1 (v5). Version 4 is random or pseudo-random, providing high entropy. There is also version 2, a DCE security version, rarely used today.

RFC 4122 defines the standard for UUIDs, with libraries available across programming languages. In practice, UUIDs are stored as binary(16) or as text strings; indexing in databases is common to optimize lookups.

Use cases include identifying records in distributed databases, keys for objects in RESTful APIs, and correlation identifiers in logs. Because UUIDs are globally unique with extremely low collision probability, they are favored when a central ID generator is impractical.

Caveats include potential leakage of information in time-based UUIDs (such as timestamps and MAC addresses) and the fact that UUIDs are not guaranteed to be universally unique—only with extremely high probability. In security contexts, v4 is preferable for unpredictability, while v1 options should be avoided if privacy is a concern.