[ruby-core:124930] [Ruby Feature#21930] Add Ractor#empty? method to check for pending messages without blocking

5 Mar 2026

      Issue #21930 has been updated by synacker (Mikhail Milovidov).

nobu (Nobuyoshi Nakada) wrote in #note-8:
...
synacker (Mikhail Milovidov) wrote in #note-4:
...
In this code:
*   The «check» (empty?) and the «use» (receive) are tightly coupled in a loop.
*   Even if a new message arrives after the empty? check but before the receive call, the loop will catch it on the next iteration.
*   The batch simply grows by one more message — no data is lost, and no invalid state is entered.
*   This pattern is by design: the goal is to collect all available messages at the moment of polling, not to make an atomic decision based on a single state snapshot.
It would work for **your code**, but may not for general purposes.
And if `main_task` finished immediately and no command is coming, it will be a busy loop.
I guess what you want is non-blocking (and maybe bulk) read, right?
Thank you for the feedback. Ractor#empty? isn’t a niche fix - it’s a general‑purpose primitive for efficient schedulers and Ractor‑Fiber integration. The code doesn’t cause a busy loop because ```Ractor.receive``` blocks when the queue is empty. This method enables non‑blocking batching, complementing my other PR (https://bugs.ruby-lang.org/issues/21869) to improve the Ractor API for using it with cooperative multitasking.

----------------------------------------
Feature #21930: Add Ractor#empty? method to check for pending messages without blocking
https://bugs.ruby-lang.org/issues/21930#change-116589

* Author: synacker (Mikhail Milovidov)
* Status: Feedback
----------------------------------------
**Summary**
In concurrent Ractor‑based architectures, there’s a critical need to check whether a Ractor has pending messages without blocking. Currently, this is not possible with the standard API

**Motivation**
The Ractor API provides a powerful mechanism for communication between system OS threads. However, in high‑load systems that use cooperative multitasking, the current Ractor#receive method presents limitations:

* It blocks the current thread until a message arrives.

* It doesn’t offer a non‑blocking way to check the message queue.

* This makes it difficult to integrate Ractors with cooperative scheduling frameworks (e.g., Async, Fiber‑based systems).

As a result, developers must either:

* Accept thread blocking (hurting responsiveness).

* Implement complex workarounds with timeouts or auxiliary queues.

**Proposed solution**
Add Ractor#empty? to the Ractor API. The method should:

* Return true if there are no pending messages in the Ractor’s main queue.

* Return false if there is at least one message available for processing.

* Not block the calling thread under any circumstances.

* Be safe to call from any Ractor (including the current one).

**Demonstration code**
Below is a proof‑of‑concept showing how Ractor#empty? enables cooperative multitasking with the Async gem:

```ruby
require 'async'

class TimeCommand
    attr_reader :id

    def initialize(id)
        @id = id
    end

    def task
        1.upto(3) do |i|      
            sleep(1)
            puts "[cmd #{@id}] step #{i} @ #{Time.now}"
        end
    end
end

class Worker
    def initialize
        @ractor = Ractor.new do
            loop do
                Sync do |task|
                    in_queue = Async::Queue.new
                    queue_task = task.async do |subtask|
                        while command = in_queue.dequeue
                            subtask.async do |child_task|
                                command.task
                            end
                        end
                    end

                    task.async(transient: true) do |main_task|
                        loop do
                            commands = []
                            if queue_task.children? || !in_queue.empty?
                                main_task.yield
                                commands.append Ractor.receive while !Ractor.current.empty?
                            else
                                commands.append Ractor.receive
                            end

                            unless commands.empty?
                                puts "Worker received batch of #{commands.size} commands."
                                commands.each { |command| in_queue.enqueue(command) }
                            end
                        end
                    end
                end
            end

        end
    end

    def send(command)
        @ractor.send(command, move: true)
    end

    def wait
        @ractor.join
    end
end

worker = Worker.new

1000.times do |i|
    100.times do |j|
        worker.send TimeCommand.new(i * 10 + j)
    end
    sleep(1)
end

worker.wait
```

**Key observations:**
With Ractor#empty?, developers can:

* Integrate Ractors with cooperative multitasking frameworks (e.g., Async) more naturally.

* Avoid thread blocking when checking for incoming messages.

* Batch process messages efficiently (collect all pending messages in one go).

* Improve responsiveness in high‑concurrency scenarios by yielding control back to the scheduler when no work is available.

**Benefits**

* Enables better integration with modern Ruby concurrency tools.

* Reduces need for complex workarounds.

* Improves performance in message‑driven architectures.

* Maintains Ractor’s thread‑safety guarantees.

-- 
https://bugs.ruby-lang.org/