Hi, I have a questions about user_id vs anonymous_ids, we use rudderstack to send experiment_viewed events to bigquery. Rudderstack sends a user_id (logged in user id from our system) and an anonymous_id as columns in the experiement_viewed row. What then should we send to Growth book via the javascript setAttributes() function as the id parameter? should we send the user_id (logged in user id) or the anonymous_id or both?

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)