Proactive and Reactive run time service discovery

Question

Hello everyone,

        I am expecting simple demo regarding run-time web-service discovery based on unavailability or malfunctioning of services, changes in the structure, functionality, quality, or context of the services
        Can you give me idea about how to the identification of services during the execution of service-based applications to replace services in them that are no longer available and/or fail to satisfy certain requirements is an important issue.

regards,
chandrakant.