Problem Statement

Event is a synchronization object. An event has two states signaled and reset. A task may await for the event to enter the desired state, usually the signaled state. It is released once the state is entered. Releasing waiting tasks is called event notification. Programmatically controlled events can be set by a task into one of its states. In concurrent programming event also refers to a notification that some state has been reached through an asynchronous activity. The source of the event can be: Event is a low-level synchronization mechanism. It neither identify the state that caused it signaled, nor the source of, nor who is the subject of notification. Events augmented by data and/or publisher-subscriber schemes are often referred as messages, signals etc. In the context of general programming event-driven architecture refers to a design that deploy events in order to synchronize tasks with the asynchronous activities they must be aware of. The opposite approach is polling sometimes called busy waiting, when the synchronization is achieved by an explicit periodic querying the state of the activity. As the name suggests busy waiting consumes system resources even when the external activity does not change its state. Event-driven architectures are widely used in GUI design and SCADA systems. They are flexible and have relatively short response times. At the same time event-driven architectures suffer to the problems related to their unpredictability. They face race condition, deadlocking, live locks and priority inversion. For this reason real-time systems tend to polling schemes, trading performance for predictability in the worst case scenario.

Let's start with the solution:

record Event(cond, value)

procedure main()
    event := Event(condvar())
    t1 := thread {
        write("Task one waiting for event....")
        critical event.cond: while /(event.value) do wait(event.cond)
        write("Task one received event.")
        }
    t2 := thread {
        write("Task two waiting for event....")
        critical event.cond: while /(event.value) do wait(event.cond)
        write("Task two received event.")
        }
    delay(1000)                   # Let main thread post the event.
    event.value := "yes"
    write("Signalling event.")
    signal(event.cond,0)
    every wait(t1|t2)
end