Pattern-Oriented Software Architecture, Volume 2, Patterns for Concurrent and Networked Objects

Chapter 3

Event Handling Patterns

“The power to guess the unseen from the seen, to trace the implications of things, to judge the whole piece by the pattern … this cluster of gifts may almost be said to constitute experience.”

Henry James, Jr. (1843-1916) — English Author

This chapter presents four patterns that describe how to initiate, receive, demultiplex, dispatch, and process events in networked systems: Reactor, Proactor, Asynchronous Completion Token, and Acceptor-Connector.

Event-driven architectures are becoming pervasive in networked software applications. The four patterns in this chapter help to simplify the development of flexible and efficient event-driven applications. The first pattern can be applied to develop synchronous service providers:

• The Reactor architectural pattern (179) allows event-driven applications to demultiplex and dispatch service requests that are delivered to an application from one or more clients. The structure introduced by the Reactor pattern ‘inverts’ the flow of control within an application, which is known as the Hollywood Principle—‘Don’t call us, we’ll call you’ [Vlis98a].

  It is the responsibility of a designated component, called reactor, not an application, to wait for indication events synchronously, demultiplex them to associated event handlers that are responsible for processing these events, and then dispatch the appropriate hook method on the event handler. In particular, a reactor dispatches event handlers that react to the ...