What Is a UUID?

A UUID (Universally Unique Identifier) is a 128-bit value formatted as 8-4-4-4-12 hexadecimal groups: 550e8400-e29b-41d4-a716-446655440000. Version 4 UUIDs (v4) are generated from random data. The probability of collision across all v4 UUIDs ever generated is negligibly small — you would need to generate a billion UUIDs per second for 85 years to reach a 50% chance of one collision.

Common Use Cases

• Database primary keys — UUIDs let clients generate IDs before inserting to the database, enabling optimistic inserts and distributed writes without a central sequence.
• Idempotency keys — Send a UUID with payment or mutation requests so the server can deduplicate retries without processing twice.
• Request tracing — Assign a UUID to each request and propagate it through logs for correlation across services.
• File upload tokens — Generate a UUID as an upload destination URL before the upload starts, so the client knows where to PUT the file.

v4 vs. Other Versions

v1 UUIDs embed a timestamp and MAC address, making them sortable but potentially exposing hardware identity. v5 UUIDs are deterministic SHA-1 hashes of a namespace + name — useful for stable IDs from content (same input always produces the same UUID). v7 (recent) uses a time-ordered random format, which is sortable like v1 but without exposing MAC addresses — a better choice for database indexed columns. This tool generates v4.