Debugging

Debugging Techniques

Debugging with puts

The simplest way to debug an Async program is to use puts to print messages to the console. This is useful for understanding the flow of your program and the values of variables. However, it can be difficult to use puts to debug programs that use asynchronous code, as the output may be interleaved. To prevent this, wrap it in Fiber.blocking{}:

require 'async'

Async do
	3.times do |i|
		sleep i
		Fiber.blocking{puts "Slept for #{i} seconds."}
	end
end

Using Fiber.blocking{} prevents any context switching until the block is complete, ensuring that the output is not interleaved and that flow control is strictly sequential. You should not use Fiber.blocking{} in production code, as it will block the reactor.

Debugging with IRB

You ca use IRB to debug your Async program. In some cases, you will want to stop the world and inspect the state of your program. You can do this by wrapping binding.irb inside a Fiber.blocking{} block:

Async do
	3.times do |i|
		sleep i
		# The event loop will stop at this point and you can inspect the state of your program.
		Fiber.blocking{binding.irb}
	end
end

If you don't use Fiber.blocking{}, the event loop will continue to run and you will end up with three instances of binding.irb running.

Debugging with Async::Debug

The async-debug gem provides a visual debugger for Async programs. It is a powerful tool that allows you to inspect the state of your program and see the hierarchy of your program:

require 'async'
require 'async/debug'

Sync do
	debugger = Async::Debug.serve
	
	3.times do
		Async do |task|
			while true
				duration = rand
				task.annotate("Sleeping for #{duration} second...")
				sleep(duration)
			end
		end
	end
end

When you run this program, it will start a web server on http://localhost:9000. You can open this URL in your browser to see the state of your program.