Apparently using the URL is no good - why is this the case, and how do you generate a good one?
Mark Pilgrim's article How to make a good ID in Atom is good. Here's part of it:
Why you shouldn’t use your permalink as an Atom ID
It’s valid to use your permalink URL as your <id>, but I discourage it because it can create confusion about which element should be treated as the permalink. Developers who don’t read specs will look at your Atom feed, and they see two identical pieces of information, and they pick one and use it as the permalink, and some of them will pick incorrectly. Then they go to another feed where the two elements are not identical, and they get confused.
In Atom, <link rel="alternate"> is always the permalink of the entry. <id> is always a unique identifier for the entry. Both are required, but they serve different purposes. An entry ID should never change, even if the permalink changes.
“Permalink changes”? Yes, permalinks are not as permanent as you might think. Here’s an example that happened to me. My permalink URLs were automatically generated from the title of my entry, but then I updated an entry and changed the title. Guess what, the “permanent” link just changed! If you’re clever, you can use an HTTP redirect to redirect visitors from the old permalink to the new one (and I did). But you can’t redirect an ID.
The ID of an Atom entry must never change! Ideally, you should generate the ID of an entry once, and store it somewhere. If you’re auto-generating it time after time from data that changes over time, then the entry’s ID will change, which defeats the purpose.
Use a GUID for the ID.
depends what language you use, but you could use
System.Guid for .NET.
来源：https://stackoverflow.com/questions/150741/how-do-you-generate-a-good-id-in-atom-documents