Async is a composable asynchronous I/O framework for Ruby based on nio4r and timers.

> “Lately I’ve been looking into async, as one of my projects – tus-ruby-server – would really benefit from non-blocking I/O. It’s really beautifully designed.” – janko

Motivation

Several years ago, I was hosting websites on a server in my garage. Back then, my ADSL modem was very basic, and I wanted to have a DNS server which would resolve to an internal IP address when the domain itself resolved to my public IP. Thus was born RubyDNS. This project was originally built on top of EventMachine, but a lack of support for IPv6 at the time and other problems, meant that I started looking for other options. Around that time Celluloid was picking up steam. I had not encountered actors before and I wanted to learn more about it. So, I reimplemented RubyDNS on top of Celluloid and this eventually became the first stable release.

Moving forward, I refactored the internals of RubyDNS into Celluloid::DNS. This rewrite helped solidify the design of RubyDNS and to a certain extent it works. However, unfixed bugs and design problems in Celluloid meant that RubyDNS 2.0 was delayed by almost 2 years. I wasn’t happy releasing it with known bugs and problems. After working on the issues for a while, and thinking about possible solutions, I decided to build a small event reactor using nio4r and timers, the core parts of Celluloid::IO which made it work so well. The result is this project.

One observation I made when looking at existing gems for asynchronous IO was a tendency to try and do everything within a single code-base. The design of this core library is deliberately simple. Additional libraries provide asynchronous networking, process management, etc. It’s likely you will prefer to depend on async-io for actual wrappers around IO and Socket. This helps to ensure a clean separation of concerns.

In designing this library, I also built a similarly designed C++ library of the same name. These two libraries share similar design principles.

Installation

Add this line to your application’s Gemfile:

gem "async"

And then execute:

$ bundle

Or install it yourself as:

$ gem install async

Usage

Please try the interactive online tutorial.

Tasks

An Async::Task runs using a Fiber and blocking operations e.g. sleep, read, write yield control until the operation can complete. There are two main methods to create tasks.

Async{...}

The highest level entry point is Async{...}. It’s useful if you are building a library and you want well defined asynchronous semantics. This internally invokes Async::Reactor.run{...}.

def run_server
    Async do |task|
        # ... acccept connections
    end
end

If Async(&block) happens within an existing reactor, it will schedule an asynchronous task and return. If Async(&block) happens outside of an existing reactor, it will create a reactor, schedule the asynchronous task, and block until it completes. The task is scheduled by calling Async::Reactor#async(&block).

This allows the caller to have either blocking or non-blocking behaviour.

require 'async'

def sleepy(duration = 1)
    Async do |task|
        task.sleep duration
        puts "I'm done sleeping, time for action!"
    end
end

# Synchronous operation:
sleepy

# Asynchronous operation:
Async do
    # These two functions will sleep simultaneously.
    sleepy
    sleepy
end

The cost of using Async{...} is minimal for initialization/server setup, but is not ideal for per-connection tasks.

Async::Task#async

If you can guarantee you are running within a task, and have access to it (e.g. via an argument), you can efficiently schedule new tasks using the Async::Task#async(&block) method.

require 'async'

def nested_sleepy(task: Async::Task.current)
    # Block caller
    task.sleep 0.1

    # Schedule nested task:
    subtask = task.async do |subtask|
        puts "I'm going to sleep..."
        subtask.sleep 1.0
    ensure
        puts "I'm waking up!"
    end
end

Async do |task|
    subtask = nested_sleepy(task: task)
end

This example creates a child subtask from the given parent task. It’s the most efficient way to schedule a task. The task is executed until the first blocking operation, at which point it will yield control and #async will return. The result of this method is the task itself.

Waiting for Results

Like promises, Async::Task produces results. In order to wait for these results, you must invoke Async::Task#wait:

require 'async'

task = Async do
    rand
end

puts task.wait

Stopping Tasks

Use Async::Task#stop to stop tasks. This function raises Async::Stop on the target task and all descendent tasks.

require 'async'

Async do
    sleepy = Async do |task|
        task.sleep 1000
    end

    sleepy.stop
end

When you design a server, you should return the task back to the caller. They can use this task to stop the server if needed, independently of any other unrelated tasks within the reactor, and it will correctly clean up all related tasks.

Reactors

Async::Reactor is the top level IO reactor, and runs multiple tasks asynchronously. The reactor itself is not thread-safe, so you’d typically have one reactor per thread or process.

Hierarchy

Async::Reactor and Async::Task form nodes in a tree. Reactors and tasks can spawn children tasks. When you invoke Async::Reactor#async, the parent task is determined by calling Async::Task.current? which uses fiber local storage. A slightly more efficient method is to use Async::Task#async, which uses self as the parent task.

require 'async'

def sleepy(duration, task: Async::Task.current)
    task.async do |subtask|
        subtask.annotate "I'm going to sleep #{duration}s..."
        subtask.sleep duration
        puts "I'm done sleeping!"
    end
end

def nested_sleepy(task: Async::Task.current)
    task.async do |subtask|
        subtask.annotate "Invoking sleepy 5 times..."
        5.times do |index|
            sleepy(index, task: subtask)
        end
    end
end

Async do |task|
    task.annotate "Invoking nested_sleepy..."
    subtask = nested_sleepy

    # Print out all running tasks in a tree:
    task.print_hierarchy($stderr)

    # Kill the subtask
    subtask.stop
end

Stopping Reactors

Async::Reactor#run will run until the reactor runs out of work to do or is explicitly stopped.

require 'async'

Async.logger.debug!
reactor = Async::Reactor.new

# Run the reactor for 1 second:
reactor.run do |task|
    task.sleep 1
    reactor.stop
end

You can use this approach to embed the reactor in another event loop. Async::Reactor#stop is can be called safely from a different thread.

Resource Management

In order to ensure your resources are cleaned up correctly, make sure you wrap resources appropriately, e.g.:

Async::Reactor.run do
    socket = connect(remote_address) # May raise Async::Stop

    begin
        socket.write(...) # May raise Async::Stop
        socket.read(...) # May raise Async::Stop
    ensure
        socket.close
    end
end

As tasks run synchronously until they yield back to the reactor, you can guarantee this model works correctly. While in theory IO#autoclose allows you to automatically close file descriptors when they go out of scope via the GC, it may produce unpredictable behavour (exhaustion of file descriptors, flushing data at odd times), so it’s not recommended.

Exception Handling

Async::Task captures and logs exceptions. All unhandled exceptions will cause the enclosing task to enter the :failed state. Non-StandardError exceptions are re-raised immediately and will generally cause the reactor to fail. This ensures that exceptions will always be visible and cause the program to fail appropriately.

require 'async'

task = Async do
    # Exception will be logged and task will be failed.
    raise "Boom"
end

puts task.status # failed
puts task.result # raises RuntimeError: Boom

Propagating Exceptions

If a task has finished due to an exception, calling Task#wait will re-raise the exception.

require 'async'

Async do
    task = Async do
        raise "Boom"
    end

    begin
        task.wait # Re-raises above exception.
    rescue
        puts "It went #{$!}!"
    end
end

Timeouts

You can wrap asynchronous operations in a timeout. This ensures that malicious services don’t cause your code to block indefinitely.

require 'async'

Async do |task|
    task.with_timeout(1) do
        task.sleep 100
    rescue Async::TimeoutError
        puts "I timed out!"
    end
end

Reoccurring Timers

Sometimes you need to do some periodic work in a loop.

require 'async'

Async do |task|
    while true
        puts Time.now
        task.sleep 1
    end
end

Caveats

Enumerators

Due to limitations within Ruby and the nature of this library, it is not possible to use to_enum on methods which invoke asynchronous behavior. We hope to fix this issue in the future.

Blocking Methods in Standard Library

Blocking Ruby methods such as pop in the Queue class require access to their own threads and will not yield control back to the reactor which can result in a deadlock. As a substitute for the standard library Queue, the Async::Queue class can be used.

Contributing

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request

See Also

• async-io — Asynchronous networking and sockets.
• async-http — Asynchronous HTTP client/server.
• async-process — Asynchronous process spawning/waiting.
• async-websocket — Asynchronous client and server websockets.
• async-dns — Asynchronous DNS resolver and server.
• async-rspec — Shared contexts for running async specs.

Projects Using Async

• ciri - An Ethereum implementation written in Ruby.
• falcon — A rack compatible server built on top of async-http.
• rubydns — A easy to use Ruby DNS server.
• slack-ruby-bot — A client for making slack bots.

License

Released under the MIT license.

Copyright, 2017, by Samuel G. D. Williams.

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the “Software”), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.