What is a UUID?

A UUID (Universally Unique Identifier) is a 128-bit number used to uniquely identify information in computer systems. They are represented as 32 hexadecimal digits displayed in five groups separated by hyphens.

What's the difference between UUID versions?

UUID v1: Based on timestamp and MAC address. Sequential but can leak information about when and where it was generated.

UUID v3: Based on MD5 hash of namespace and name. Deterministic - same input always produces same UUID.

UUID v4: Completely random (122 random bits). Most commonly used, no information leakage.

UUID v5: Based on SHA-1 hash of namespace and name. Like v3 but uses stronger hash algorithm.

How unique are UUIDs?

UUID v4 has 122 random bits, giving 5.3 × 10³⁶ possible values. The probability of generating duplicate UUIDs is negligible - you'd need to generate 1 billion UUIDs per second for 85 years to have a 50% chance of collision.

When should I use UUIDs?

• Database primary keys in distributed systems
• Unique file or object identifiers
• Session IDs and tokens
• Tracking IDs for analytics
• API request IDs for tracing
• Message queue identifiers

What are namespace UUIDs?

Namespace UUIDs (v3 and v5) generate consistent UUIDs from a namespace and name combination. They're useful when you need deterministic UUIDs - the same namespace + name will always produce the same UUID. Common namespaces include DNS, URL, OID, and X.500 DN.

UUID Format Explained

Standard UUID format: xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx

Where M indicates version (1-5) and N indicates variant (8, 9, A, or B for RFC 4122).

Example: 550e8400-e29b-41d4-a716-446655440000