Principles and Applications of Distributed Event-Based Systems
Knowledge-Based Networking, which is built on-top of Content-based Networking (CBN), involves the forwarding of events across a network of brokers based on subscription filters applied to some semantics of the data and associated metadata of the events contents. Knowledge-based Networks (KBN) therefore support the efficient filtered dissemination of semantically enriched knowledge over a large, loosely coupled network of distributed heterogeneous agents. This is achieved by incorporating
... ical semantics into event messages, allowing subscribers to define semantic filters, and providing a subscription brokering and routing mechanism. The KBN used for this work provides ontological concepts as an additional message attribute type, onto which subsumption relationships, equivalence, type queries and arbitrary ontological relationships can be applied. It also provides a bag type to be used that supports bags equivalence, sub-bag and super-bag relationships to be used in subscription filters, composed with traditional CBN subscription operators or the ontological operators. When combined with the benefits of Content-based Networking, this allows subscribers to easily express meaningful subscription interests and receive results in a more expressive and flexible distributed event system than heretofore. Within this chapter the detailed analysis of ontological operators and their application to a publish/subscribe (pub/sub) domain will be fully explored and evaluated. Many of the additional capabilities of a KBN over a CBN could be achieved with complicated and unwieldy CBN subscriptions, which become unmanageable within both the network and with regard to the human creating the subscription. In addition, by extending the expressiveness of the subscription language we risk adversely affecting the potential for subscription aggregation in the broker network. However we have shown that more expressive subscriptions lead to more accurate and more concise subscriptions (Keeney et. al., 2007a) , thereby improving performance and making a KBN more flexible and applicable in a wide range of use-cases.