Fibers are primitives for implementing light weight cooperative concurrency in Ruby. Basically they are a means of creating code blocks that can be paused and resumed, much like threads. The main difference is that they are never preempted and that the scheduling must be done by the programmer and not the VM.

As opposed to other stackless light weight concurrency models, each fiber comes with a stack. This enables the fiber to be paused from deeply nested function calls within the fiber block. See the ruby(1) manpage to configure the size of the fiber stack(s).

When a fiber is created it will not run automatically. Rather it must be explicitly asked to run using the Fiber#resume method. The code running inside the fiber can give up control by calling Fiber.yield in which case it yields control back to caller (the caller of the Fiber#resume).

Upon yielding or termination the Fiber returns the value of the last executed expression

For instance:

fiber = do
  Fiber.yield 1

puts fiber.resume
puts fiber.resume
puts fiber.resume


FiberError: dead fiber called

The Fiber#resume method accepts an arbitrary number of parameters, if it is the first call to resume then they will be passed as block arguments. Otherwise they will be the return value of the call to Fiber.yield


fiber = do |first|
  second = Fiber.yield first + 2

puts fiber.resume 10
puts fiber.resume 1_000_000
puts fiber.resume "The fiber will be dead before I can cause trouble"


FiberError: dead fiber called

Non-blocking Fibers

Since Ruby 3.0, the concept of non-blocking fiber was introduced. Non-blocking fiber, when reaching any potentially blocking operation (like sleep, wait for another process, wait for I/O data to be ready), instead of just freezing itself and all execution in the thread, yields control to other fibers, and allows the scheduler to handle waiting and waking (resuming) the fiber when it can proceed.

For Fiber to behave as non-blocking, it should be created in with blocking: false (which is the default now), and Fiber.scheduler should be set with Fiber.set_scheduler. If Fiber.scheduler is not set in the current thread, blocking and non-blocking fiber's behavior is identical.

Ruby doesn't provide a scheduler class: it is expected to be implemented by the user and correspond to Fiber::SchedulerInterface.

There is also Fiber.schedule method, which is expected to immediately perform passed block in a non-blocking manner (but its actual implementation is up to the scheduler).

Class Methods

Returns false if the current fiber is non-blocking. Fiber is non-blocking if it was created via passing blocking: false to, or via Fiber.schedule.

If the current Fiber is blocking, the method, unlike usual predicate methods, returns a number of blocking fibers currently running (TBD: always 1?).

Note, that even if the method returns false, Fiber behaves differently only if Fiber.scheduler is set in the current thread.

See the “Non-blocking fibers” section in class docs for details.

Returns the current fiber. You need to require 'fiber' before using this method. If you are not running in the context of a fiber this method will return the root fiber.

Creates new Fiber. Initially, fiber is not running, but can be resumed with resume. Arguments to the first resume call would be passed to the block:

f = do |initial|
   current = initial
   loop do
     puts "current: #{current.inspect}"
     current = Fiber.yield
f.resume(100)     # prints: current: 100
f.resume(1, 2, 3) # prints: current: [1, 2, 3]
f.resume          # prints: current: nil
# ... and so on ...

if blocking: false is passed to the, and current thread has Fiber.scheduler defined, the Fiber becomes non-blocking (see “Non-blocking fibers” section in class docs).

The method is expected to immediately run the provided block of code in a separate non-blocking fiber.

puts "Go to sleep!"


Fiber.schedule do
  puts "Going to sleep"
  puts "I slept well"

puts "Wakey-wakey, sleepyhead"

Assuming MyScheduler is properly implemented, this program will produce:

Go to sleep!
Going to sleep
Wakey-wakey, sleepyhead
...1 sec pause here...
I slept well

…e.g. on the first blocking operation inside the Fiber (sleep(1)), the control is yielded at the outside code (main fiber), and at the end of the execution, the scheduler takes care of properly resuming all the blocked fibers.

Note that the behavior described above is how the method is expected to behave, actual behavior is up to the current scheduler's implementation of Fiber::SchedulerInterface#fiber method. Ruby doesn't enforce this method to behave in any particular way.

If the scheduler is not set, the method raises RuntimeError (No scheduler is available!).

Fiber scheduler, set in the current thread with Fiber.set_scheduler. If the scheduler is nil (which is the default), non-blocking fibers behavior is the same as blocking. (see “Non-blocking fibers” section in class docs for details about the scheduler concept).

Sets Fiber scheduler for the current thread. If the scheduler is set, non-blocking fibers (created by with blocking: false, or by Fiber.schedule) call that scheduler's hook methods on potentially blocking operations, and the current thread will call scheduler's close method on finalization (allowing the scheduler to properly manage all non-finished fibers).

scheduler can be an object of any class corresponding to Fiber::SchedulerInterface. Its implementation is up to the user.

See also the “Non-blocking fibers” section in class docs.

Yields control back to the context that resumed the fiber, passing along any arguments that were passed to it. The fiber will resume processing at this point when resume is called next. Any arguments passed to the next resume will be the value that this Fiber.yield expression evaluates to.

Instance Methods

Returns true if the fiber can still be resumed (or transferred to). After finishing execution of the fiber block this method will always return false. You need to require 'fiber' before using this method.

Returns the current execution stack of the fiber. start, count and end allow to select only parts of the backtrace.

def level3

def level2

def level1

f = { level1 }

# It is empty before the fiber started
#=> []


#=> ["test.rb:2:in `yield'", "test.rb:2:in `level3'", "test.rb:6:in `level2'", "test.rb:10:in `level1'", "test.rb:13:in `block in <main>'"]
p f.backtrace(1) # start from the item 1
#=> ["test.rb:2:in `level3'", "test.rb:6:in `level2'", "test.rb:10:in `level1'", "test.rb:13:in `block in <main>'"]
p f.backtrace(2, 2) # start from item 2, take 2
#=> ["test.rb:6:in `level2'", "test.rb:10:in `level1'"]
p f.backtrace(1..3) # take items from 1 to 3
#=> ["test.rb:2:in `level3'", "test.rb:6:in `level2'", "test.rb:10:in `level1'"]


# It is nil after the fiber is finished
#=> nil

Like backtrace, but returns each line of the execution stack as a Thread::Backtrace::Location. Accepts the same arguments as backtrace.

f = { Fiber.yield }
loc = f.backtrace_locations.first
loc.label  #=> "yield"
loc.path   #=> "test.rb"
loc.lineno #=> 1

Returns true if fiber is blocking and false otherwise. Fiber is non-blocking if it was created via passing blocking: false to, or via Fiber.schedule.

Note, that even if the method returns false, Fiber behaves differently only if Fiber.scheduler is set in the current thread.

See the “Non-blocking fibers” section in class docs for details.

An alias for to_s

Raises an exception in the fiber at the point at which the last Fiber.yield was called. If the fiber has not been started or has already run to completion, raises FiberError. If the fiber is yielding, it is resumed. If it is transferring, it is transferred into. But if it is resuming, raises FiberError.

With no arguments, raises a RuntimeError. With a single String argument, raises a RuntimeError with the string as a message. Otherwise, the first parameter should be the name of an Exception class (or an object that returns an Exception object when sent an exception message). The optional second parameter sets the message associated with the exception, and the third parameter is an array of callback information. Exceptions are caught by the rescue clause of begin...end blocks.

Resumes the fiber from the point at which the last Fiber.yield was called, or starts running it if it is the first call to resume. Arguments passed to resume will be the value of the Fiber.yield expression or will be passed as block parameters to the fiber's block if this is the first resume.

Alternatively, when resume is called it evaluates to the arguments passed to the next Fiber.yield statement inside the fiber's block or to the block value if it runs to completion without any Fiber.yield

Returns fiber information string.

Transfer control to another fiber, resuming it from where it last stopped or starting it if it was not resumed before. The calling fiber will be suspended much like in a call to Fiber.yield. You need to require 'fiber' before using this method.

The fiber which receives the transfer call is treats it much like a resume call. Arguments passed to transfer are treated like those passed to resume.

The two style of control passing to and from fiber (one is resume and Fiber::yield, another is transfer to and from fiber) can't be freely mixed.

  • If the Fiber's lifecycle had started with transfer, it will never be able to yield or be resumed control passing, only finish or transfer back. (It still can resume other fibers that are allowed to be resumed.)

  • If the Fiber's lifecycle had started with resume, it can yield or transfer to another Fiber, but can receive control back only the way compatible with the way it was given away: if it had transferred, it only can be transferred back, and if it had yielded, it only can be resumed back. After that, it again can transfer or yield.

If those rules are broken FiberError is raised.

For an individual Fiber design, yield/resume is more easy to use style (the Fiber just gives away control, it doesn't need to think about who the control is given to), while transfer is more flexible for complex cases, allowing to build arbitrary graphs of Fibers dependent on each other.


require 'fiber'

manager = nil # For local var to be visible inside worker block

# This fiber would be started with transfer
# It can't yield, and can't be resumed
worker = { |work|
  puts "Worker: starts"
  puts "Worker: Performed #{work.inspect}, transferring back"
  # Fiber.yield     # this would raise FiberError: attempt to yield on a not resumed fiber
  # manager.resume  # this would raise FiberError: attempt to resume a resumed fiber (double resume)

# This fiber would be started with resume
# It can yield or transfer, and can be transferred
# back or resumed
manager = {
  puts "Manager: starts"
  puts "Manager: transferring 'something' to worker"
  result = worker.transfer('something')
  puts "Manager: worker returned #{result.inspect}"
  # worker.resume    # this would raise FiberError: attempt to resume a transferring fiber
  Fiber.yield        # this is OK, the fiber transferred from and to, now it can yield
  puts "Manager: finished"

puts "Starting the manager"
puts "Resuming the manager"
# manager.transfer  # this would raise FiberError: attempt to transfer to a yielding fiber


Starting the manager
Manager: starts
Manager: transferring 'something' to worker
Worker: starts
Worker: Performed "something", transferring back
Manager: worker returned "Something"
Resuming the manager
Manager: finished