event-manager

The event-manager provides abstractions for implementing event based systems. For now, this crate only works on Linux and uses the epoll API to provide a mechanism for handling I/O notifications.

Design

This crate is built around two abstractions:

• Event Manager
• Event Subscriber

The subscriber defines and registers an interest list with the event manager. The interest list represents the events that the subscriber wants to monitor.

The Event Manager allows adding and removing subscribers, and provides APIs through which the subscribers can be updated in terms of events in their interest list. These actions are abstracted through the SubscriberOps trait.

To interface with the Event Manager, the Event Subscribers need to provide an initialization function, and a callback for when events in the interest list become ready. The subscribers can update their interest list when handling ready events. These actions are abstracted through the EventSubscriber and MutEventSubscriber traits. They contain the same methods, but the former only requires immutable self borrows, whereas the latter requires mutable borrows. Any type implementing EventSubscriber automatically implements MutEventSubscriber as well.

A typical event-based application creates the event manager, registers subscribers, and then calls into the event manager's run function in a loop. Behind the scenes, the event manager calls into epoll::wait and maps the file descriptors in the ready list to the subscribers it manages. The event manager calls the subscriber's process function (its registered callback). When dispatching the events, the event manager creates a specialized object and passes it in the callback function so that the subscribers can use it to alter their interest list.

Read more in the design document.

Implementing an Event Subscriber

The event subscriber has full control over the events that it monitors. The events need to be added to the event manager's loop as part of the init function. Adding events to the loop can return errors, and it is the responsibility of the subscriber to handle them.

Similarly, the event subscriber is in full control of the ready events. When an event becomes ready, the event manager will call into the subscriber process function. The subscriber SHOULD handle the following events which are always returned when they occur (they don't need to be registered):

• EventSet::ERROR - an error occurred on the monitor file descriptor.
• EventSet::HANG_UP - hang up happened on the associated fd.
• EventSet::READ_HANG_UP - hang up when the registered event is edge triggered.

For more details about the error cases, you can check the epoll_ctl documentation.

Initializing the Event Manager

The EventManager uses a generic type parameter which represents the subscriber type. The crate provides automatic implementations of EventSubscriber for Arc<T> and Rc<T> (for any T: EventSubscriber +?Sized), together with automatic implementations of MutEventSubscriber for Mutex<T> and RefCell<T> (for any T: MutEventSubscriber + ?Sized). The generic type parameter enables either static or dynamic dispatch.

This crate has no default features. The optional remote_endpoint feature enables interactions with the EventManager from different threads without the need of more intrusive synchronization.

Examples

For closer to real life use cases, please check the examples in tests.

Basic Single Thread Subscriber

Implementing a Basic Subscriber

use event_manager::{EventOps, Events, MutEventSubscriber};
use vmm_sys_util::{eventfd::EventFd, epoll::EventSet};

use std::os::unix::io::AsRawFd;
use std::fmt::{Display, Formatter, Result};

pub struct CounterSubscriber {
    event_fd: EventFd,
    counter: u64,
}

impl CounterSubscriber {
    pub fn new() -> Self {
        Self {
            event_fd: EventFd::new(0).unwrap(),
            counter: 0,
        }
    }
}

impl MutEventSubscriber for CounterSubscriber {
    fn process(&mut self, events: Events, event_ops: &mut EventOps) {
        match events.event_set() {
            EventSet::IN => {
                self.counter += 1;
            }
            EventSet::ERROR => {
                eprintln!("Got error on the monitored event.");
            }
            EventSet::HANG_UP => {
                event_ops.remove(events).unwrap_or(
                    eprintln!("Encountered error during cleanup")
                );
                panic!("Cannot continue execution. Associated fd was closed.");
            }
            _ => {}
        }
    }

    fn init(&mut self, ops: &mut EventOps) {
        ops.add(Events::new(&self.event_fd, EventSet::IN)).expect("Cannot register event.");
    }
}

Adding Subscribers to the Event Manager

struct App {
    event_manager: EventManager<CounterSubscriber>,
    subscribers_id: Vec<SubscriberId>,
}

impl App {
    fn new() -> Self {
        Self {
            event_manager: EventManager::<CounterSubscriber>::new().unwrap(),
            subscribers_id: vec![]
        }
    }

    fn add_subscriber(&mut self) {
        let counter_subscriber = CounterSubscriber::default();
        let id = self.event_manager.add_subscriber(counter_subscriber);
        self.subscribers_id.push(id);
    }

    fn run(&mut self) {
        let _ = self.event_manager.run_with_timeout(100);
    }
}

Development and Testing

The event-manager is tested using unit tests, Rust integration tests and performance benchmarks. It leverages rust-vmm-ci for continuous testing. All tests are run in the rustvmm/dev container.

More details on running the tests can be found in the development document.

License

This project is licensed under either of:

• Apache License, Version 2.0
• BSD-3-Clause License