UUID v4 vs v5
Both are 128-bit unique identifiers, but they are generated differently: v4 is random (collision is practically impossible) and v5 is deterministic (derived from the hash of a namespace + a name, so the same input always yields the same UUID). One prioritizes unpredictability; the other, reproducibility.
| Aspect | UUID v4 | UUID v5 |
|---|---|---|
| How it is generated | Random (CSPRNG) | Hash of namespace+name |
| Same input, same ID | No | Yes |
| Predictable | No | Yes (if you know the input) |
| Typical case | Primary keys, tokens | Dedup, idempotency |
When to use UUID v4
Use v4 for record IDs, primary keys, session tokens: you want them unpredictable and revealing nothing about the content.
When to use UUID v5
Use v5 when you need the same ID from the same data without storing a map (dedup, deriving a resource ID from its URL, idempotency).
In short: v4 for unpredictable uniqueness (the common case); v5 when the ID must be stable and derivable from a known input.
Tools for this
UUID generatorGenerate random UUIDs (v4 and v7), one or in bulk. Compatible with databases, APIs and testing. Runs entirely in your browser. UUID v5 generatorGenerate deterministic UUID v5 from a namespace and a name (SHA-1, RFC 4122). Same input, same UUID: great for reproducible IDs. Runs in your browser.