Cannot run background services independently in Falcon - service blocks Rack app

[MateuszMydlarz]

Hello,

I’m trying to run a background service in Falcon using async together with a Rack app. The goal is to have a long-running task (e.g. subscribing to Redis with async-redis) that runs alongside the Rack endpoint handler without blocking it.

What I expected:
Falcon should start the Rack app (so HTTP endpoints work as usual).
A background service (e.g. an async task) should run independently and not block the Rack server.

What happens instead:
When I run a background service using Async do ... end, the service blocks Falcon from starting the Rack app.
If I wait for the background task (.wait), the HTTP app never boots.
If I don’t wait for it, the task never seems to run.

Essentially, I can’t get a service to run in the background concurrently with Falcon's Rack integration.

Result:
The Redis subscription service runs, but Falcon does not start the Rack application.
If I remove the Async block part, Falcon runs the Rack app normally, but task never start.
I can’t figure out how to make both work together in the same Falcon process.

falcon.rb:

require_relative 'app'

load :rack, :supervisor

rack 'localhost' do
  endpoint Async::HTTP::Endpoint
             .parse('http://0.0.0.0:8013')
  config 'config.ru'
end

service "task1" do |task|
    start_task1(task)
end

app.rb:

require 'async/redis'

def start_task1(task)
  url = "redis://:yourpassword@redis-server:6379/1"
  uri = Async::Redis::Endpoint.parse(url)
  client = Async::Redis::Client.new(uri)

  Async do |task|
    task.async do |subtask|
      client.subscribe 'status' do |context|
        while event = context.listen
          pp event
        end
      end
    end
  end
  
  class MyRackApp
  def call(env)
    [
      200,
      {"Content-Type" => "text/plain"},
      ["Hello World"]
    ]
  end
end

[ioquatix]

Hopefully this gets you on the right track:

#!/usr/bin/env falcon-host

require "falcon/environment/rack"
require "async/redis"

service 'localhost' do
	include Falcon::Environment::Rack
	endpoint {Async::HTTP::Endpoint.parse('http://0.0.0.0:8013')}
end

class MyService < Async::Service::Generic
	def setup(container)
		container.spawn do |instance|
			evaluator = @environment.evaluator
			Console.info(self, "Connecting to Redis at", evaluator.redis_endpoint)
			
			Async do
				client = Async::Redis::Client.new(evaluator.redis_endpoint)

				instance.ready!
				
				client.subscribe 'status' do |context|
					Console.info(self, "Subscribed to Redis channel 'status'.")
					while response = context.listen
						Console.info(self, "Received event:", response)
					end
				end
			end
		end
	end
end

service "myservice" do
	service_class MyService
	
	redis_endpoint {Async::Redis::Endpoint.local}
end

[MateuszMydlarz]

It looks great and works perfectly — thank you!

I have a few more questions, if you don’t mind:

1. What is the best practice for passing data between a service and a Rack app instance? Should I use Async::Notification / Async::Condition, Async::Queue or something else?
2. I’m not sure where to find information about the overall service architecture — specifically when to use Async::Service::Generic, evaluator, etc. Is there any guide or documentation available?

[ioquatix]

1. Async::Service is not in the business of IPC, so you need to decide what makes the most sense for your use case (and reliability requirements). For example, you could use Async::Redis, a database, or UNIX domain sockets IPC. I added a simple example of the latter here: https://github.com/socketry/async-service/tree/main/examples/ipc
2. I wrote documentation for you: https://github.com/socketry/async-service?tab=readme-ov-file#usage - let me know if it's insufficient.