I have requirement, where the authentication to my Pega Application is going to be handled by an external system.

Once user authenticates themselves in the external system they should see the Pega User Portal.

They only catch here is external system will be sending an userid which we need to capture and display cases specific to that user in Pega User portal.

Can someone guide me on how to achieve this? Should I for a Rest/SOAP  Service or an Authentication service.