I am using C# .NET, AngularJS and other technology that are quite loosely coupled. I need to implement Segment and Mixpanel. I have implemented Alias and Identify in the back-end upon registration and login respectively. I even put Track, Page calls where required.
I need to pass user_id to client side on login to pass this value with each event as it is required in the documentation. This (How do I connect Segment.io server and client side events to the same anonymous user?) is quite close to what I need, and it did not work for me.
The point here was that I need anyway to pass userId for Identify call on login to both back-end and front-end libraries. It should be regardless of where Alias is call. That is in accord with Segment Support responses.