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by 8organicbits 109 days ago
You're talking about the hash-based UUIDv3/v5? I haven't found examples of those being used, but I'm curious.

Using MD5 or 122 bits of a SHA1 hash seems questionable now that both algorithms have known collisions. Using 122 bits of a SHA2/3 seems pretty limited too. Maybe if you've got trusted inputs?
3 comments
buffalobuffalo 108 days ago
I use these a lot. My favorite use case is templates, especially ones that were not initially planned in the architecture.

Let's say i have some entity like an "organization" that has data that spans several different tables. I want to use that organization as a "parent" in such a way where i can clone them to create new "child" organizations structured the same way they are. I also want to periodically be able to pull changes from the parent organization down into the child organization.

If the primary keys for all tables involved are UUIDs, I can accomplish this very easily by mapping all IDs in the relevant tables `id => uuid5(id, childOrgId)`. This can be done to all join tables, foreign keys, etc. The end result is a perfect "child" clone of the organization with all data relations still in place. This data can be refreshed from the parent organization any time simply by repeating the process.

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eureka7 109 days ago
I remember using them in a massive SQL query that needed to generate a GIS data set from multiple tables with an ungodly amount of JOINs and sub-queries to achieve ID stability. Don't ask :p

For those ~~curious~~ worried, no, this was not a security sensitive context.

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zadikian 108 days ago
Common one is if you want two structs deemed "equivalent" based on a few fields to get the same ID, and you're only concerned about accidental collision. There are valid use cases for that, but I've also seen it misused often.

v7 rough ordering also helps as a PK in certain sharded DBs, while others want random, or nonsharded ones usually just serial int.

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8organicbits 108 days ago
Have you seen UUIDv3/v5 used there though? I've seen lots of md5 historically and sha variants recently, but not the UUID approach.
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zadikian 108 days ago
Yeah, I've seen both 3 and 5 used, not just hashes in some custom format. That way it works with Postgres uuid type etc.
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