What Is a UUID?

A UUID (Universally Unique Identifier) is a 128-bit identifier that looks like 550e8400-e29b-41d4-a716-446655440000. You'll see them as database primary keys, distributed system IDs, API request IDs, and session tokens. UUID v4 is generated from random numbers — the collision probability is so low it's effectively zero. No coordination between servers needed, no auto-increment race conditions.

UUID v1 vs v4 — What's the Difference?

v1 combines your machine's MAC address with a timestamp, which means you can technically trace when and where it was created — some people consider that a privacy concern. v4 is pure randomness with no traceable information, and it's what most projects use today. There's also the newer v7, which uses a Unix timestamp plus random bytes — the upside is that IDs are naturally time-sorted, so database insert performance is better than v4. This tool generates the most widely used v4 format.

Common Use Cases

Database primary keys — especially in distributed systems where multiple nodes generate IDs independently. API request IDs for tracing requests through your logs when something goes wrong. File naming so uploaded files never overwrite each other. Idempotency keys for payment APIs to make sure the same transaction doesn't get charged twice. Basically, anywhere you need a guaranteed-unique identifier without a central authority handing out numbers.

UUID vs Auto-Increment IDs

Auto-increment IDs (1, 2, 3...) are simple and small. But in distributed systems, two servers generating IDs at the same time can collide. UUIDs don't need coordination — each machine generates its own. The tradeoff: UUIDs are longer (36 characters) and random v4 UUIDs have worse B-tree index write performance. A common compromise is to use auto-increment internally as the primary key and expose UUIDs as the public-facing ID. You get the best of both worlds — efficient indexing and safe external identifiers.