The Basic Idea: An Event Loop

This lesson discusses the basic idea behind the event loop.

The basic approach we’ll use, as stated in the last lesson, is called event-based concurrency. The approach is quite simple: you simply wait for something (i.e., an “event”) to occur; when it does, you check what type of event it is and do the small amount of work it requires, which may include issuing I/O requests, or scheduling other events for future handling, etc. That’s it!

Before getting into the details, let’s first examine what a canonical event-based server looks like. Such applications are based around a simple construct known as the event loop. Pseudocode for an event loop looks like this:

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while (1) {
    events = getEvents();
    for (e in events)
        processEvent(e);
}

It’s really that simple. The main loop simply waits for something to do (by calling getEvents() in the code above) and then, for each event returned, processes them, one at a time; the code that processes each event is known as an event handler. Importantly, when a handler processes an event, it is the only activity taking place in the system; thus, deciding which event to handle next is equivalent to scheduling. This explicit control over scheduling is one of the fundamental advantages of the event-based approach.

But this discussion leaves us with a bigger question: how exactly does an event-based server determine which events are taking place, in particular with regards to network and disk I/O? Specifically, how can an event server tell if a message has arrived for it?

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