About a month ago¹, I was onboarding a friend into one of my side project codebases and she asked me why I was using a particular type of UUID. I'd heard about this type while working on that project, and it's really neat. So instead of hogging that knowledge for just us, here it is: some good uses for different versions of UUID.

What are the different versions?

Usually when we have multiple numbered versions, the higher numbers are newer and presumed to be better. In contrast, there are 8 UUID versions (v1 through v8) which are different and all defined in the standard.

Here, I'll provide some explanation of what they are at a high level, linking to the specific section of the RFC in case you want more details.

• UUID Version 1 (v1) is generated from timestamp, monotonic counter, and a MAC address.
• UUID Version 2 (v2) is reserved for security IDs with no known details².
• UUID Version 3 (v3) is generated from MD5 hashes of some data you provide. The RFC suggests DNS and URLs among the candidates for data.
• UUID Version 4 (v4) is generated from entirely random data. This is probably what most people think of and run into with UUIDs.
• UUID Version 5 (v5) is generated from SHA1 hahes of some data you provide. As with v3, the RFC suggests DNS or URLs as candidates.
• UUID Version 6 (v6) is generated from timestamp, monotonic counter, and a MAC address. These are the same data as Version 1, but they change the order so that sorting them will sort by creation time.
• UUID Version 7 (v7) is generated from a timestamp and random data.
• UUID Version 8 (v8) is entirely custom (besides the required version/variant fields that all versions contain).

When should you use them?

With eight different versions, which should you use? There are a few common use cases that dictate which you should use, and some have been replaced by others.

You'll usually be picking between two of them: v4 or v7. There are also some occasions to pick v5 or v8.

• Use v4 when you just want a random ID. This is a good default choice.
• Use v7 if you're using the ID in a context where you want to be able to sort. For example, consider using v7 if you are using UUIDs as database keys.
• v5 or v8 are used if you have your own data you want in the UUID, but generally, you will know if you need it.

What about the other ones?

• Per the RFC, v7 improves on v1 and v6 and should be used over those if possible. So you usually won't want v1 or v6. If you do want one of those, you can use v6.
• v2 is reserved for unspecified security things. If you are using these, you probably can't tell me or anyone else about it, and you're probably not reading this post to figure out more about them.
• v3 is superceded by v5, which uses a stronger hash. This one is one where you probably know if you need it.