Hi @kashev,
Thanks for your questions. Let me see if I can answer them.
contentId is a long in the database (well, last time I looked it was). So they can get pretty big. Really big. Although they are returned as a String type in the json, unless you need to do math on them I’d recommend keeping them as a String, and the max length would be what the max length of a Java long would be.
They are unique per instance, so https://mycompany.atlassian.net/wiki will have unique contentId values. Content includes pages, blog posts, comments. They stay the same when content is moved. E.g. you’ll find some magic happening when a new version of a page is created, that the new pages gets the same contentId but the previous version will have a new contentId.
A different instance, https://myothercompany.atlassian.net/wiki will have its own set of contentId values. If you want a distinct key to use, then clientKey and contentId combined will give you that.
Hope this helps.
Regards,
James.