In this report, we show how to build a distributed system allowing objects to register interest in and receive notifications of events in other objects. The system is built on top of a pair of interfaces that are interesting only in their extreme simplicity. We then present a simple and efficient implementation of these interfaces.

Next, we show how more complex functionality can be introduced to the system by adding third-party services. These services can be added without changing the simple interfaces, and without changing the objects in the system that do not need the functionality of those services.

Finally, we note a number of open issues that remain, and attempt to draw some conclusions based on the work.