At the simplest level, composable event loops combine the cheap synchrony of event loops with the composability of preempted threads.

Multithreading¶

In a traditional OS, asynchronous tasks are generally accomplished by multiple threads or processes. Each thread has its own stack and is preempted periodically to switch between active threads. Low-level synchronization primitives or synchronized queues are often used to communicate between running threads.

Example of a multithreaded teapot:

Benefits:

• Complicated logic can be executed linearly without worrying about delays
• Multiple threads can be composed with minimal impacting to the timing of existing threads
• Supports multiple cores
• Parallelism is transparent to the user

Event Loop¶

In the classic event loop model, a single process executes short-lived events sequentially. Only a single stack is used and no synchronization is needed between events. The user is responsible for ensuring that events return in a lively manner.

Example of an event loop based teapot:

Benefits:

• Synchronization is elided
• Reduced memory consumption with only a single stack
• No cost for context switching and synchronization
• Parallelism is explicit and controlled

Composable Event Loops¶

A composable event loop system is a model for structuring multitasking programs such that each module is contained in a single event loop. Multiple modules are separated into distinct parallel threads and can communicate through message passing in the form of registering events across modules.

Example of a teapot using composable event loops:

At minimum, composable event loops need three primitives:

- Modular event loops - dispatching of a module's queued events - Multithreading - isolation between multiple module's threads of execution - Synchronized event registration - message passing between modules in the form of enqueuing events

Example¶

An asynchronous teapot based on the simple events library:

/*  An asynchronous tea pot built on composable event loops
 */
class TeaPot {
private:
    // Internal event loop and set of events for the tea pot
    EventLoop loop;
    Event<void()> begin_event;
    Event<void(float)> read_event;
    
    // Other tea pot internals
    HeatingElement heater;
    AnalogIn sensor;
    FuncPtr<void()> done_callback;

public:
    // The publically-accessibly boil method can be called from other threads.
    // Registering with the internal event loop synchronizes the tea pot.
    void boil(FuncPtr<void()> callback) {
        done_callback = callback;

        begin_event.attach(&loop, this, &TeaPot::begin_callback);
        begin_event.trigger();
    }

private:
    // Event registered from the boil method in case the heating element is not thread-safe.
    void begin_callback() {
        heater.on();

        read_event.attach(&loop, this, &TeaPot::read_callback);
        sensor.read(&read_event);
    }

    // Event for handling read callbacks. This callback is issued from the AnalogIn sensor, 
    // but runs on the TeaPot's event loop. AnalogIn may or may-not use an event loop.
    void read_callback(float data) {
        // Hopefully this log call will be pretty quick. However, if the log call takes a while,
        // we only have to worry about it blocking our tea pot.
        log.log("got: %d\n", data);

        if (data > 110) {
            heater.off();

            // Since events are compatible function objects, triggering events that may be from
            // other event loops is as easy as a function call.
            if (done_callback) {
                done_callback();
            }
        } else {
            sensor.read(&read_event);
        }
    }
}

Benefits¶

The core benefit of composable event loops is the alignment of synchronization issues with the separation of concerns between individual modules. When creating a module, a developer needs to worry about external synchronization and internal timing.

Benefits:

• Modules are internally synchronized, removing the need for synchronize individual components or worry about the interactions between internal threads
• Multiple modules can be composed with minimal impact to the timing of existing modules
• The quantity of stacks is statically bounded by the number of modules
• Supports multiple cores bounded by the number of modules
• Easily integrated with interrupts by treating them as another module boundary
• Easily integrated with traditional multithreaded environments
• Parallelism is transparent across module boundaries but controlled inside modules