[ruby-core:117108] [Ruby master Feature#20276] Introduce Fiber interfaces for Blocking operations on Ractors

12 Mar 2024

      Issue #20276 has been updated by forthoney (Seong-Heon Jung).

ioquatix (Samuel Williams) wrote in #note-4:
...
I support this proposal.
A simple way to deal with this right now might be the following code (I have not tested it):
```ruby
Thread.new do
  other_ractor.take
end.value
```
In general, the `block`/`unblock` operations should be sufficient, but it might require the ability for `Ractor` to invoke functionality across it's boundary OR we might need to implement some RPC mechanism (this is common in Actor based concurrency).
Tested on locally and it seems to work, at least with Async. Here's my code.
```ruby
Async do |task|
  1.upto(3) do
    r = Ractor.new do
      Ractor.recv
      fib = ->(x) { x < 2 ? 1 : fib.call(x - 2) + fib.call(x - 1) }
      puts "fin"
    end
    task.async do 
      Thread.new do
        r.send(nil) # ractor start
        r.take
      end.value
    end
  end
end
```
Not very scientific but the results are printed at roughly the same time.

----------------------------------------
Feature #20276: Introduce Fiber interfaces for Blocking operations on Ractors
https://bugs.ruby-lang.org/issues/20276#change-107183

* Author: forthoney (Seong-Heon Jung)
* Status: Feedback
----------------------------------------
### Motivation
I am trying to build a web server with Ractors. The lifecycle for a request in the current implementation is 

1. main ractor sends request to worker ractor
2. worker ractor handles response
3. worker ractor sends response to main ractor
4. main ractor writes response
5. repeat

The main ractor utilizes the Async gem (specifically async-http) to handle connections concurrently, meaning each request is handled on a separate fiber.
The issue I am running into is after I send a request to a worker ractor, I need to do a blocking wait until I receive a response.
While I am waiting for the response, I cannot take any more connections.

### Solution
If the fiber scheduler had a hook for `Ractor.receive` or `Ractor#take` (both of which are blocking), the main ractor can send the message, handle other connections while the worker processes the request. When the worker produces a message, it will then take the reqeust and write it in the socket. Specifically, I think the `block` and `unblock` hooks should be implemented for Ractors, considering Threads and Mutexes already use them.

-- 
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