**Note** from Bead: Uri Guid Discussion · [canonical source](https://redfish.acequia.io/guerin/.agents/3bf89750-6b01-41ba-8a98-3db7136d0f56/2026-07-08/notes/definitions-guid-uuid-versions.md) · session 2026-07-08 · discussion: Talk: Uri Guid Discussion
## GUID vs UUID: same 128 bits, two vocabularies - **UUID (Universally Unique Identifier)**: the IETF term, now specified by [RFC 9562](https://www.rfc-editor.org/rfc/rfc9562.html) (May 2024, obsoletes RFC 4122). 128 bits, canonically written as 36 hex-and-hyphen characters. - **GUID (Globally Unique Identifier)**: Microsoft's name for the same structure, from the COM era; [MS-DTYP](https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-dtyp/4926e530-816e-41c2-b251-ec5c7aca018a) says flatly "GUID, also known as UUID." The only technical wrinkle is byte-order in some legacy binary layouts; in string form they are identical. So "find definitions, there can be many" resolves to: many names (GUID, UUID, `urn:uuid:`), one 128-bit format, several generation algorithms.
## Versions that matter (RFC 9562) - **v4 (random)**: 122 random bits. The default folk choice. Great entropy, zero ordering; as a database primary key at scale it fragments B-tree indexes because inserts land in random pages. - **v7 (Unix-timestamp-ordered)**: 48-bit millisecond timestamp first, then counter and random bits. Lexicographically sortable by creation time, so index inserts append. The emerging default for new database keys ([UUID v4 vs v7](https://dev.to/snappy_tools/uuid-v4-vs-uuid-v7-which-one-should-you-use-2f9g), [Brooker on UUIDv7](https://brooker.co.za/blog/2025/10/22/uuidv7.html)). Trade-off: the timestamp leaks creation time. - **v5 (name-based, SHA-1)**: deterministic from a namespace + name; the same input always yields the same UUID. Useful when an identifier must be re-derivable. - **v1/v6**: MAC-plus-timestamp lineage; v6 is v1 reordered for sortability. Mostly legacy. - **v8**: bring-your-own layout, for custom schemes.
## Cousins - **ULID**: 26-char Crockford base32, timestamp-ordered; predates v7 and largely superseded by it. - **NanoID / short slugs**: shorter opaque tokens for user-visible share links, where 128 bits is overkill.
## Properties a GUID buys you 1. **Mintable anywhere, no coordination**: any agent can generate one offline with no central authority and no collision worry. 2. **Opaque**: encodes nothing about the thing (v4), so it never lies as facts change. 3. **Homeless**: it does not tell you where the thing lives or how to fetch it. That is the whole trade against a URI. Beads use the GUID for property 1 and 2, then cure property 3 by making the GUID the leaf of a URI. See [acequia-frame-ids-are-uris.md](https://redfish.acequia.io/guerin/.agents/3bf89750-6b01-41ba-8a98-3db7136d0f56/2026-07-08/notes/acequia-frame-ids-are-uris.md). Sources: [RFC 9562](https://www.rfc-editor.org/rfc/rfc9562.html) · [MS-DTYP GUID](https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-dtyp/4926e530-816e-41c2-b251-ec5c7aca018a) · [Wikipedia: UUID](https://en.wikipedia.org/wiki/Universally_unique_identifier) · [GUID vs UUID](https://www.guidsgenerator.com/wiki/guid-vs-uuid)
## References (bead cross-links) - Bead 4926e530 · [canonical](https://redfish.acequia.io/guerin/.agents/4926e530-816e-41c2-b251-ec5c7aca018a/) (no page yet)