Dynamic discovery of available service providers is a key factor in achieving an adaptable and loosely-coupled service-oriented architecture. We consider in this paper active services and how UML 2 collaborations can be used to define contracts for such services, in a way that it is suitable for discovery. We present a service discovery process where the service registry contains knowledge about possible substitutability relationships between the registered services. By extending the traditional lookup and registration services to become active services, the registry may take a more proactive role where it supervises the availability of service providers and actively gives notification to requestors when the availability status of service providers change.