This is not an existing class, but documentation of the interface that Scheduler object should comply in order to be used as Fiber.scheduler and handle non-blocking fibers. See also the “Non-blocking fibers” section in Fiber class docs for explanations of some concepts.

Scheduler's behavior and usage are expected to be as follows:

  • When the execution in the non-blocking Fiber reaches some blocking operation (like sleep, wait for a process, or a non-ready I/O), it calls some of the scheduler's hook methods, listed below.

  • Scheduler somehow registers what the current fiber is waited for, and yields control to other fibers with Fiber.yield (so the fiber would be suspended while expecting its wait to end, and other fibers in the same thread can perform)

  • At the end of the current thread execution, the scheduler's method close is called

  • The scheduler runs into a wait loop, checking all the blocked fibers (which it has registered on hook calls) and resuming them when the awaited resource is ready (I/O ready, sleep time passed).

A typical implementation would probably rely for this closing loop on a gem like EventMachine or Async.

This way concurrent execution will be achieved in a way that is transparent for every individual Fiber's code.

Hook methods are:

When not specified otherwise, the hook implementations are mandatory: if they are not implemented, the methods trying to call hook will fail. To provide backward compatibility, in the future hooks will be optional (if they are not implemented, due to the scheduler being created for the older Ruby version, the code which needs this hook will not fail, and will just behave in a blocking fashion).

It is also strongly suggested that the scheduler implement the fiber method, which is delegated to by Fiber.schedule.

Sample toy implementation of the scheduler can be found in Ruby's code, in test/fiber/scheduler.rb


Instance Methods


Invoked by methods like Thread.join, and by Mutex, to signify that current Fiber is blocked till further notice (e.g. unblock) or till timeout will pass.

blocker is what we are waiting on, informational only (for debugging and logging). There are no guarantees about its value.

Expected to return boolean, specifying whether the blocking operation was successful or not.

Called when the current thread exits. The scheduler is expected to implement this method in order to allow all waiting fibers to finalize their execution.

The suggested pattern is to implement the main event loop in the close method.

Implementation of the Fiber.schedule. The method is expected to immediately run passed block of code in a separate non-blocking fiber, and to return that Fiber.

Minimal suggested implementation is:

def fiber(&block)
  Fiber.new(blocking: false, &block).tap(&:resume)
end

Invoked by IO#wait, IO#wait_readable, IO#wait_writable to ask whether the specified descriptor is ready for specified events within the specified timeout.

events is a bit mask of IO::READABLE, IO::WRITABLE, and IO::PRIORITY.

Suggested implementation should register which Fiber is waiting for which resources and immediately calling Fiber.yield to pass control to other fibers. Then, in the close method, the scheduler might dispatch all the I/O resources to fibers waiting for it.

Expected to return the subset of events that are ready immediately.

Invoked by Kernel#sleep and Mutex#sleep and is expected to provide an implementation of sleeping in a non-blocking way. Implementation might register the current fiber in some list of “what fiber waits till what moment”, call Fiber.yield to pass control, and then in close resume the fibers whose wait period have ended.

Invoked by Process::Status.wait in order to wait for a specified process. See that method description for arguments description.

Suggested minimal implementation:

Thread.new do
  Process::Status.wait(pid, flags)
end.value

This hook is optional: if it is not present in the current scheduler, Process::Status.wait will behave as a blocking method.

Expected to returns a Process::Status instance.

Invoked to wake up Fiber previously blocked with block (for example, Mutex#lock calls block and Mutex#unlock calls unblock). The scheduler should use the fiber parameter to understand which fiber is unblocked.

blocker is what was awaited for, but it is informational only (for debugging and logging), and it is not guaranteed to be the same value as the blocker for block.