UUID Generator

What a UUID gives you

A UUID is a 128-bit identifier written as 32 hexadecimal digits in five dash-separated groups, like 550e8400-e29b-41d4-a716-446655440000. The point of a UUID is that you can generate one anywhere, on any machine, without a central coordinator, and still be confident it will not clash with a UUID generated somewhere else.

How version 4 works

Version 4 UUIDs are almost entirely random. Of the 128 bits, 6 are fixed to mark the version and variant, leaving 122 random bits. That is an enormous space: you would need to generate billions of UUIDs before the chance of a single collision becomes meaningful. You can tell a v4 UUID because the 13th hex digit is always 4 and the 17th is one of 8, 9, a, or b.

When to use one

UUIDs shine as primary keys you want to assign before a row reaches the database, as idempotency keys for API requests, as filenames that must not collide, and as correlation ids threaded through logs. The usual downside is that random v4 values make poor clustered-index keys because inserts land in random spots; if write ordering matters, a time-sortable scheme may suit you better.

Example

550e8400-e29b-41d4-a716-446655440000
                ^            ^
            version 4   variant (8/9/a/b)

FAQ

Can two v4 UUIDs ever be the same? In theory yes, in practice the odds are so small they are ignored for normal workloads. The risk only grows if your random source is weak.

Are UUIDs secret or hard to guess? A v4 is hard to guess, but do not treat it as a security token on its own. Use it for uniqueness, not for authorization.

Why does mine always start with the same digits? The version and variant nibbles are fixed by the spec, so patterns at those positions are expected, not a bug.