RFC 9562 / START HERE
UUID identifiers, before the details
UUID: Universally Unique Identifier
A UUID is a 128-bit identifier designed to distinguish things without a single central numbering service. Different versions use different generation methods.
The idea in one picture
A new object needs an identifier
A UUID is generated
Systems use it to refer to the object
A simplified picture of the idea; the chapters explain the actual mechanisms.
Why it exists
Independent systems often need to create identifiers before they can coordinate with each other. A UUID's uniqueness properties depend on how it is generated.
A familiar example
A notes app assigns an identifier to a new note created offline and later synchronizes it with a server.
Helpful background
You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.
What you’ll learn next
Begin with chapter one, or choose a topic.
- UUID fields and textA UUID is 128 bits, commonly written in five hexadecimal groups.→
- Random UUID version 4UUIDv4 fills the available layout with random data around fixed version and variant bits.→
- Time-ordered UUID version 7UUIDv7 begins with a 48-bit Unix-epoch millisecond timestamp.→
- Name-based and custom layoutsName-based UUIDs combine a namespace with a name; other versions serve different goals.→
- Generation and privacy limitsUniqueness, ordering, privacy, and unguessability are separate properties.→
This introduction explains RFC 9562 (2024). Read the original RFC ↗