How can I allow the client to auto-generate the distinct_id from $device_id and $user_id?

[billsbooth]

Mixpanel Support is telling me to set the $device_id and $user_id, and let the client decide on the $distinct_id.... but all your APIs in this library expect me to provide the distinct_id. So what is the right solution?

Seems like I have three options:

1. Fetch $distinct_id from my mobile app, and use it on backend.
2. Use either device_id or user_id as distinct_id
3. Use empty string for distinct_id, and set $device_id and $user_id in the event properties.

What is the right approach there?