› Async › Source › Async › Task

class Task

A sequence of instructions, defined by a block, which is executed sequentially and managed by the scheduler. A task can be in one of the following states: initialized, running, completed, failed, cancelled or stopped.

stateDiagram-v2 [*] --> Initialized Initialized --> Running : Run Running --> Completed : Return Value Running --> Failed : Exception Completed --> [*] Failed --> [*] Running --> Stopped : Stop Stopped --> [*] Completed --> Stopped : Stop Failed --> Stopped : Stop Initialized --> Stopped : Stop

Example

require 'async'

# Create an asynchronous task that sleeps for 1 second:
Async do |task|
	sleep(1)
end

Signature
public

Since Async v1.

Nested

Definitions

def self.yield

  • deprecated

Signature
deprecated

With no replacement.

Implementation

 
def self.yield
	warn("`Async::Task.yield` is deprecated with no replacement.", uplevel: 1, category: :deprecated) if $VERBOSE
	
	Fiber.scheduler.transfer
end

def yield

Yield back to the reactor and allow other fibers to execute.

Implementation

 
def yield
	Fiber.scheduler.yield
end

def self.run(scheduler, *arguments, **options, &block)

Run the given block of code in a task, asynchronously, in the given scheduler.

Implementation

 
def self.run(scheduler, *arguments, **options, &block)
	self.new(scheduler, **options, &block).tap do |task|
		task.run(*arguments)
	end
end

def run(*arguments)

Begin the execution of the task.

Signature
raises　RuntimeError

If the task is already running.

Implementation

 
def run(*arguments)
	if @status == :initialized
		@status = :running
		
		schedule do
			@block.call(self, *arguments)
		rescue => error
			# I'm not completely happy with this overhead, but the alternative is to not log anything which makes debugging extremely difficult. Maybe we can introduce a debug wrapper which adds extra logging.
			if @finished.nil?
				warn(self, "Task may have ended with unhandled exception.", exception: error)
			end
			
			raise
		end
	else
		raise RuntimeError, "Task already running!"
	end
end

def initialize(parent = Task.current?, finished: nil, **options, &block)

Create a new task.

Signature
parameter　reactor　Reactor

the reactor this task will run within.

parameter　parent　Task

the parent task.

Implementation

 
def initialize(parent = Task.current?, finished: nil, **options, &block)
	super(parent, **options)
	
	# These instance variables are critical to the state of the task.
	# In the initialized state, the @block should be set, but the @fiber should be nil.
	# In the running state, the @fiber should be set.
	# In a finished state, the @block should be nil, and the @fiber should be nil.
	@block = block
	@fiber = nil
	
	@status = :initialized
	@result = nil
	@finished = finished
	
	@defer_stop = nil
end

def reactor

Signature
returns　Scheduler

The scheduler for this task.

Implementation

 
def reactor
	self.root
end

def backtrace(*arguments)

Signature
returns　Array(Thread::Backtrace::Location) | Nil

The backtrace of the task, if available.

Implementation

 
def backtrace(*arguments)
	@fiber&.backtrace(*arguments)
end

def annotate(annotation, &block)

Annotate the task with a description.

This will internally try to annotate the fiber if it is running, otherwise it will annotate the task itself.

Signature
parameter　annotation　String

The description to annotate the task with.

Implementation

 
def annotate(annotation, &block)
	if @fiber
		@fiber.annotate(annotation, &block)
	else
		super
	end
end

def annotation

Signature
returns　Object

The annotation of the task.

Implementation

 
def annotation
	if @fiber
		@fiber.annotation
	else
		super
	end
end

def to_s

Signature
returns　String

A description of the task and it's current status.

Implementation

 
def to_s
	"\#<#{self.description} (#{@status})>"
end

def sleep(duration = nil)

  • deprecated

Signature
deprecated

Prefer Kernel#sleep except when compatibility with stable-v1 is required.

Implementation

 
def sleep(duration = nil)
	Kernel.warn("`Async::Task#sleep` is deprecated, use `Kernel#sleep` instead.", uplevel: 1, category: :deprecated) if $VERBOSE
	
	super
end

def with_timeout(duration, exception = TimeoutError, message = "execution expired", &block)

Execute the given block of code, raising the specified exception if it exceeds the given duration during a non-blocking operation.

Implementation

 
def with_timeout(duration, exception = TimeoutError, message = "execution expired", &block)
	Fiber.scheduler.with_timeout(duration, exception, message, &block)
end

attr :fiber

Signature
attribute　Fiber

The fiber which is being used for the execution of this task.

def alive?

Signature
returns　Boolean

Whether the internal fiber is alive, i.e. it is actively executing.

Implementation

 
def alive?
	@fiber&.alive?
end

def finished?

Whether we can remove this node from the reactor graph.

Signature
returns　Boolean

Implementation

 
def finished?
	# If the block is nil and the fiber is nil, it means the task has finished execution. This becomes true after `finish!` is called.
	super && @block.nil? && @fiber.nil?
end

def running?

Signature
returns　Boolean

Whether the task is running.

Implementation

 
def running?
	@status == :running
end

def failed?

Signature
returns　Boolean

Whether the task failed with an exception.

Implementation

 
def failed?
	@status == :failed
end

def stopped?

Signature
returns　Boolean

Whether the task has been stopped.

Implementation

 
def stopped?
	@status == :stopped
end

def completed?

Signature
returns　Boolean

Whether the task has completed execution and generated a result.

Implementation

 
def completed?
	@status == :completed
end

def complete?

Alias for #completed?.

Implementation

 
def complete?
	self.completed?
end

attr :status

Signature
attribute　Symbol

The status of the execution of the task, one of :initialized, :running, :complete, :stopped or :failed.

def async(*arguments, **options, &block)

  • public
  • asynchronous

Run an asynchronous task as a child of the current task.

Signature
public

Since Async v1.

asynchronous

May context switch immediately to the new task.

yields　{|task| ...}

in the context of the new task.

raises　FinishedError

If the task has already finished.

returns　Task

The child task.

Implementation

 
def async(*arguments, **options, &block)
	raise FinishedError if self.finished?
	
	task = Task.new(self, **options, &block)
	
	# When calling an async block, we deterministically execute it until the first blocking operation. We don't *have* to do this - we could schedule it for later execution, but it's useful to:
	#
	# - Fail at the point of the method call where possible.
	# - Execute determinstically where possible.
	# - Avoid scheduler overhead if no blocking operation is performed.
	#
	# There are different strategies (greedy vs non-greedy). We are currently using a greedy strategy.
	task.run(*arguments)
	
	return task
end

def wait

Retrieve the current result of the task. Will cause the caller to wait until result is available. If the task resulted in an unhandled error (derived from StandardError), this will be raised. If the task was stopped, this will return nil.

Conceptually speaking, waiting on a task should return a result, and if it throws an exception, this is certainly an exceptional case that should represent a failure in your program, not an expected outcome. In other words, you should not design your programs to expect exceptions from #wait as a normal flow control, and prefer to catch known exceptions within the task itself and return a result that captures the intention of the failure, e.g. a TimeoutError might simply return nil or false to indicate that the operation did not generate a valid result (as a timeout was an expected outcome of the internal operation in this case).

Signature
raises　RuntimeError

If the task's fiber is the current fiber.

returns　Object

The final expression/result of the task's block.

Implementation

 
def wait
	raise "Cannot wait on own fiber!" if Fiber.current.equal?(@fiber)
	
	# `finish!` will set both of these to nil before signaling the condition:
	if @block || @fiber
		@finished ||= Condition.new
		@finished.wait
	end
	
	if @status == :failed
		raise @result
	else
		return @result
	end
end

attr :result

Access the result of the task without waiting. May be nil if the task is not completed. Does not raise exceptions.

def stop(later = false, cause: $!)

Stop the task and all of its children.

If later is false, it means that stop has been invoked directly. When later is true, it means that stop is invoked by stop_children or some other indirect mechanism. In that case, if we encounter the "current" fiber, we can't stop it right away, as it's currently performing #stop. Stopping it immediately would interrupt the current stop traversal, so we need to schedule the stop to occur later.

Signature
parameter　later　Boolean

Whether to stop the task later, or immediately.

parameter　cause　Exception

The cause of the stop operation.

Implementation

 
def stop(later = false, cause: $!)
	# If no cause is given, we generate one from the current call stack:
	unless cause
		cause = Stop::Cause.for("Stopping task!")
	end
	
	if self.stopped?
		# If the task is already stopped, a `stop` state transition re-enters the same state which is a no-op. However, we will also attempt to stop any running children too. This can happen if the children did not stop correctly the first time around. Doing this should probably be considered a bug, but it's better to be safe than sorry.
		return stopped!
	end
	
	# If the fiber is alive, we need to stop it:
	if @fiber&.alive?
		# As the task is now exiting, we want to ensure the event loop continues to execute until the task finishes.
		self.transient = false
		
		# If we are deferring stop...
		if @defer_stop == false
			# Don't stop now... but update the state so we know we need to stop later.
			@defer_stop = cause
			return false
		end
		
		if self.current?
			# If the fiber is current, and later is `true`, we need to schedule the fiber to be stopped later, as it's currently invoking `stop`:
			if later
				# If the fiber is the current fiber and we want to stop it later, schedule it:
				Fiber.scheduler.push(Stop::Later.new(self, cause))
			else
				# Otherwise, raise the exception directly:
				raise Stop, "Stopping current task!", cause: cause
			end
		else
			# If the fiber is not curent, we can raise the exception directly:
			begin
				# There is a chance that this will stop the fiber that originally called stop. If that happens, the exception handling in `#stopped` will rescue the exception and re-raise it later.
				Fiber.scheduler.raise(@fiber, Stop, cause: cause)
			rescue FiberError
				# In some cases, this can cause a FiberError (it might be resumed already), so we schedule it to be stopped later:
				Fiber.scheduler.push(Stop::Later.new(self, cause))
			end
		end
	else
		# We are not running, but children might be, so transition directly into stopped state:
		stop!
	end
end

def defer_stop

  • public

Defer the handling of stop. During the execution of the given block, if a stop is requested, it will be deferred until the block exits. This is useful for ensuring graceful shutdown of servers and other long-running tasks. You should wrap the response handling code in a defer_stop block to ensure that the task is stopped when the response is complete but not before.

You can nest calls to defer_stop, but the stop will only be deferred until the outermost block exits.

If stop is invoked a second time, it will be immediately executed.

Signature
yields　{}

The block of code to execute.

public

Since Async v1.

Implementation

 
def defer_stop
	# Tri-state variable for controlling stop:
	# - nil: defer_stop has not been called.
	# - false: defer_stop has been called and we are not stopping.
	# - true: defer_stop has been called and we will stop when exiting the block.
	if @defer_stop.nil?
		begin
			# If we are not deferring stop already, we can defer it now:
			@defer_stop = false
			
			yield
		rescue Stop
			# If we are exiting due to a stop, we shouldn't try to invoke stop again:
			@defer_stop = nil
			raise
		ensure
			defer_stop = @defer_stop
			
			# We need to ensure the state is reset before we exit the block:
			@defer_stop = nil
			
			# If we were asked to stop, we should do so now:
			if defer_stop
				raise Stop, "Stopping current task (was deferred)!", cause: defer_stop
			end
		end
	else
		# If we are deferring stop already, entering it again is a no-op.
		yield
	end
end

def stop_deferred?

Signature
returns　Boolean

Whether stop has been deferred.

Implementation

 
def stop_deferred?
	!!@defer_stop
end

def self.current

Lookup the class Async::Task for the current fiber. Raise RuntimeError if none is available.

Signature
returns　Task

Implementation

 
def self.current
	Fiber.current.async_task or raise RuntimeError, "No async task available!"
end

def self.current?

Check if there is a task defined for the current fiber.

Signature
returns　Interface(:async) | Nil

Implementation

 
def self.current?
	Fiber.current.async_task
end

def current?

Signature
returns　Boolean

Whether this task is the currently executing task.

Implementation

 
def current?
	Fiber.current.equal?(@fiber)
end

def finish!

Finish the current task, moving any children to the parent.

Implementation

 
def finish!
	# Don't hold references to the fiber or block after the task has finished:
	@fiber = nil
	@block = nil # If some how we went directly from initialized to finished.
	
	# Attempt to remove this node from the task tree.
	consume
	
	# If this task was being used as a future, signal completion here:
	if @finished
		@finished.signal(self)
		@finished = nil
	end
end

def completed!(result)

State transition into the completed state.

Implementation

 
def completed!(result)
	@result = result
	@status = :completed
end

def failed!(exception = false)

State transition into the failed state.

Implementation

 
def failed!(exception = false)
	@result = exception
	@status = :failed
end

Discussion