Agent systems that search the Semantic Web are seen as killer applications for description logic (DL) inference engines. The guiding examples for the Semantic Web involve information and document retrieval tasks. The instance retrieval inference service of description logic inference engines can be used as a basic machinery for implementing agent-based retrieval systems. However, since information is permanently added to information sources, usually agents need to return to previously visited servers in order to get updates for their queries over time. In this paper we present a software architecture that allows agents to register instance retrieval queries at a certain inference server. We will see how agents are notified when the result set of registered queries grows over time. The paper describes new optimization techniques for incrementally computing answers for sets of registered instance retrieval queries and reports on first experiences with an implementation as part of the ...