Releases

v2.21.1

Worker Pool

Ruby 3.4 will feature a new fiber scheduler hook, blocking_operation_wait which allows the scheduler to redirect the work given to rb_nogvl to a worker pool.

The Async scheduler optionally supports this feature using a worker pool, by using the following environment variable:

ASYNC_SCHEDULER_DEFAULT_WORKER_POOL=true

This will cause the scheduler to use a worker pool for general blocking operations, rather than blocking the event loop.

It should be noted that this isn't a net win, as the overhead of using a worker pool can be significant compared to the rb_nogvl work. As such, it is recommended to benchmark your application with and without the worker pool to determine if it is beneficial.

v2.20.0

Traces and Metrics Providers

Async now has traces and metrics providers for various core classes. This allows you to emit traces and metrics to a suitable backend (including DataDog, New Relic, OpenTelemetry, etc.) for monitoring and debugging purposes.

To take advantage of this feature, you will need to introduce your own config/traces.rb and config/metrics.rb. Async's own repository includes these files for testing purposes, you could copy them into your own project and modify them as needed.

v2.19.0

Async::Scheduler Debugging

Occasionally on issues, I encounter people asking for help and I need more information. Pressing Ctrl-C to exit a hung program is common, but it usually doesn't provide enough information to diagnose the problem. Setting the CONSOLE_LEVEL=debug environment variable will now print additional information about the scheduler when you interrupt it, including a backtrace of the current tasks.

> CONSOLE_LEVEL=debug bundle exec ruby ./test.rb
^C  0.0s    debug: Async::Reactor [oid=0x974] [ec=0x988] [pid=9116] [2024-11-08 14:12:03 +1300]
               | Scheduler interrupted: Interrupt
               | #<Async::Reactor:0x0000000000000974 1 children (running)>
               | 	#<Async::Task:0x000000000000099c /Users/samuel/Developer/socketry/async/lib/async/scheduler.rb:185:in `transfer' (running)>
               | 	→ /Users/samuel/Developer/socketry/async/lib/async/scheduler.rb:185:in `transfer'
               | 	  /Users/samuel/Developer/socketry/async/lib/async/scheduler.rb:185:in `block'
               | 	  /Users/samuel/Developer/socketry/async/lib/async/scheduler.rb:207:in `kernel_sleep'
               | 	  /Users/samuel/Developer/socketry/async/test.rb:7:in `sleep'
               | 	  /Users/samuel/Developer/socketry/async/test.rb:7:in `sleepy'
               | 	  /Users/samuel/Developer/socketry/async/test.rb:12:in `block in <top (required)>'
               | 	  /Users/samuel/Developer/socketry/async/lib/async/task.rb:197:in `block in run'
               | 	  /Users/samuel/Developer/socketry/async/lib/async/task.rb:420:in `block in schedule'
/Users/samuel/Developer/socketry/async/lib/async/scheduler.rb:317:in `select': Interrupt
... (backtrace continues) ...

This gives better visibility into what the scheduler is doing, and should help diagnose issues.

Console Shims

The async gem depends on console gem, because my goal was to have good logging by default without thinking about it too much. However, some users prefer to avoid using the console gem for logging, so I've added an experimental set of shims which should allow you to bypass the console gem entirely.

require 'async/console'
require 'async'

Async{raise "Boom"}

Will now use Kernel#warn to print the task failure warning:

#<Async::Task:0x00000000000012d4 /home/samuel/Developer/socketry/async/lib/async/task.rb:104:in `backtrace' (running)>
Task may have ended with unhandled exception.
(irb):4:in `block in <top (required)>': Boom (RuntimeError)
	from /home/samuel/Developer/socketry/async/lib/async/task.rb:197:in `block in run'
	from /home/samuel/Developer/socketry/async/lib/async/task.rb:420:in `block in schedule'

v2.18.0

Add support for Sync(annotation:), so that you can annotate the block with a description of what it does, even if it doesn't create a new task.

v2.17.0

Introduce Async::Queue#push and Async::Queue#pop for compatibility with ::Queue.

v2.16.0

Better Handling of Async and Sync in Nested Fibers

Interleaving bare fibers within Async and Sync blocks should not cause problems, but it presents a number of issues in the current implementation. Tracking the parent-child relationship between tasks, when they are interleaved with bare fibers, is difficult. The current implementation assumes that if there is no parent task, then it should create a new reactor. This is not always the case, as the parent task might not be visible due to nested Fibers. As a result, Async will create a new reactor, trying to stop the existing one, causing major internal consistency issues.

I encountered this issue when trying to use Async within a streaming response in Rails. The protocol-rack uses a normal fiber to wrap streaming responses, and if you try to use Async within it, it will create a new reactor, causing the server to lock up.

Ideally, Async and Sync helpers should work when any Fiber.scheduler is defined. Right now, it's unrealistic to expect Async::Task to work in any scheduler, but at the very least, the following should work:

reactor = Async::Reactor.new # internally calls Fiber.set_scheduler

# This should run in the above reactor, rather than creating a new one.
Async do
  puts "Hello World"
end

In order to do this, bare Async and Sync blocks should use Fiber.scheduler as a parent if possible.

See https://github.com/socketry/async/pull/340 for more details.