I. Introduction
Content-based communication is a communication service whereby the flow of messages from senders to receivers: is driven by the content of the messages, rather than by explicit addresses assigned by senders and attached to the messages [4]. Using a content-based communication service, receivers declare their interests by means of selection predicates, while senders simply publish' messages. The service consists of delivering to any and all receivers each message that matches the selection predicates declared by those receivers.