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You should set both attributes. When you run an experiment, you choose which attribute you want to use to split traffic.

ok and obviously sometimes the user_id will be null because the user isn't logged in, should I just send a null value to set attributes for that parameter?

Yeah, null is fine. If you choose user id as the attribute for an experiment, it will skip anyone with null (or another falsy value)

so if we want to split traffic on say all users, we'd just split based on the anon_id (which all users have)