The ECho Event Delivery System

ECho is an event delivery middleware system developed at Georgia Tech. Super cially, the semantics and organization of structures in ECho are similar to the Event Channels described by the CORBA Event Services speci cation[COSEvents]. Like most event systems, what ECho implements can be viewed as an anonymous group communication mechanism. In contrast with more familiar one-to-one send-receive communication models, data senders in anonymous group communication are unaware of the number or identity of data receivers. Instead, message sends are delivered to receivers according to the rules of the communication mechanism. In this case, event channels provide the mechanism for matching senders and receivers. Messages (or events) are sent via sources into channels which may have zero or more subscribers (or sinks). The locations of the sinks, which may be on the same machine or process as the sender, or anywhere else in the network, are immaterial to the sender. A program or system may create or use multiple event channels, and each subscriber receives only the messages sent to the channel to which it is subscribed. The network tra c for multiple channels is multiplexed over shared communications links, and channels themselves impose relatively low overhead. Instead of doing explicit read() operations, sink subscribers specify a subroutine (or handler) to be run whenever a message arrives. In this sense, event delivery is asynchronous and passive for the application.