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It's common to have two user ids. An anonymous_id stored in a cookie or localStorage a user_id from your auth system. Every visitor would have an anonymous_id attribute, but only logged-in users would have a user_id attribute. When creating an experiment rule for a feature, you specify which attribute you want to use to split traffic. So if the experiment is before the auth flow, you would choose

anonymous_id

, otherwise

user_id

.

for the anon/device ID, it can work quite well to set as a domain-wide cookie too, so e.g. www.example.com, example.com and app.example.com would have the same ID