UUID Generator

A v4 is what you want in almost every case where you have not already decided otherwise.

// Built in to every current browser and to Node 19+.
crypto.randomUUID();

import uuid

uuid.uuid4()

import "github.com/google/uuid"

id := uuid.New()

// uuid = { version = "1", features = ["v4"] }
use uuid::Uuid;

Uuid::new_v4();

-- gen_random_uuid() is built in from Postgres 13.
SELECT gen_random_uuid();

A UUID is a Universally Unique IDentifier: a 128-bit number used to identify something without coordinating with anyone else. It is usually written as 32 hexadecimal characters in five hyphenated groups.

UUIDs come in several versions. v4 is random, v1 and v6 encode a timestamp and a MAC address, v3 and v5 hash a name inside a namespace, and v7 encodes a Unix millisecond timestamp followed by random bits.

UUIDs can be generated anywhere, by anyone, with no central authority, and still be safely assumed not to collide. That makes them useful for distributed systems, offline clients, and merging data from separate databases.

They are also instantly recognisable, which makes them easy to pick out of logs.

Some databases (for example Postgres) store a UUID as a 128-bit integer rather than a 36-character string, so using one as a primary key costs far less than it looks.

Uniqueness is probabilistic, not guaranteed. The odds of two v4 UUIDs colliding are negligible, but they are not zero.

Some versions leak information. A v1 or v6 UUID contains the MAC address of the machine that generated it and the moment it was generated, which is enough to correlate a device across systems. You can see exactly what a UUID gives away with the UUID decoder.

Time-ordered IDs are also partly guessable, so they are a poor choice for anything that doubles as a secret, such as a password reset token.