Class: Concurrent::IVar

Inherits:
Synchronization::LockableObject
  • Object
show all
Includes:
Concern::Obligation, Concern::Observable
Defined in:
lib/concurrent-ruby/concurrent/ivar.rb

Overview

An IVar is like a future that you can assign. As a future is a value that is being computed that you can wait on, an IVar is a value that is waiting to be assigned, that you can wait on. IVars are single assignment and deterministic.

Then, express futures as an asynchronous computation that assigns an IVar. The IVar becomes the primitive on which [futures](Future) and [dataflow](Dataflow) are built.

An IVar is a single-element container that is normally created empty, and can only be set once. The I in IVar stands for immutable. Reading an IVar normally blocks until it is set. It is safe to set and read an IVar from different threads.

If you want to have some parallel task set the value in an IVar, you want a Future. If you want to create a graph of parallel tasks all executed when the values they depend on are ready you want dataflow. IVar is generally a low-level primitive.

## Examples

Create, set and get an IVar

“‘ruby ivar = Concurrent::IVar.new ivar.set 14 ivar.value #=> 14 ivar.set 2 # would now be an error “`

## See Also

  1. For the theory: Arvind, R. Nikhil, and K. Pingali. [I-Structures: Data structures for parallel computing](dl.acm.org/citation.cfm?id=69562). In Proceedings of Workshop on Graph Reduction, 1986.

  2. For recent application: [DataDrivenFuture in Habanero Java from Rice](www.cs.rice.edu/~vs3/hjlib/doc/edu/rice/hj/api/HjDataDrivenFuture.html).

Direct Known Subclasses

Future, Promise, ScheduledTask

Instance Method Summary collapse

Methods included from Concern::Observable

#count_observers, #delete_observer, #delete_observers, #with_observer

Methods included from Concern::Obligation

#complete?, #exception, #fulfilled?, #incomplete?, #pending?, #reason, #rejected?, #state, #unscheduled?, #value, #value!, #wait, #wait!

Methods included from Concern::Dereferenceable

#value

Constructor Details

#initialize(value = NULL, opts = {}, &block) ⇒ IVar

Create a new IVar in the :pending state with the (optional) initial value.

Options Hash (opts):

  • :dup_on_deref (String) — default: false —

    call #dup before returning the data

  • :freeze_on_deref (String) — default: false —

    call #freeze before returning the data

  • :copy_on_deref (String) — default: nil —

    call the given Proc passing the internal value and returning the value returned from the proc



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# File 'lib/concurrent-ruby/concurrent/ivar.rb', line 61

def initialize(value = NULL, opts = {}, &block)
  if value != NULL && block_given?
    raise ArgumentError.new('provide only a value or a block')
  end
  super(&nil)
  synchronize { ns_initialize(value, opts, &block) }
end

Instance Method Details

#add_observer(observer = nil, func = :update, &block) ⇒ Object

Add an observer on this object that will receive notification on update.

Upon completion the IVar will notify all observers in a thread-safe way. The func method of the observer will be called with three arguments: the Time at which the Future completed the asynchronous operation, the final value (or nil on rejection), and the final reason (or nil on fulfillment).

Raises:

  • (ArgumentError) 


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# File 'lib/concurrent-ruby/concurrent/ivar.rb', line 80

def add_observer(observer = nil, func = :update, &block)
  raise ArgumentError.new('cannot provide both an observer and a block') if observer && block
  direct_notification = false

  if block
    observer = block
    func = :call
  end

  synchronize do
    if event.set?
      direct_notification = true
    else
      observers.add_observer(observer, func)
    end
  end

  observer.send(func, Time.now, self.value, reason) if direct_notification
  observer
end

#fail(reason = StandardError.new) ⇒ IVar

Set the IVar to failed due to some error and wake or notify all threads waiting on it.

Raises:



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# File 'lib/concurrent-ruby/concurrent/ivar.rb', line 134

def fail(reason = StandardError.new)
  complete(false, nil, reason)
end

#set(value = NULL) { ... } ⇒ IVar

Set the IVar to a value and wake or notify all threads waiting on it.

Yields:

  • A block operation to use for setting the value

Raises:



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# File 'lib/concurrent-ruby/concurrent/ivar.rb', line 112

def set(value = NULL)
  check_for_block_or_value!(block_given?, value)
  raise MultipleAssignmentError unless compare_and_set_state(:processing, :pending)

  begin
    value = yield if block_given?
    complete_without_notification(true, value, nil)
  rescue => ex
    complete_without_notification(false, nil, ex)
  end

  notify_observers(self.value, reason)
  self
end

#try_set(value = NULL) { ... } ⇒ Boolean

Attempt to set the IVar with the given value or block. Return a boolean indicating the success or failure of the set operation.

Yields:

  • A block operation to use for setting the value

Raises:



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# File 'lib/concurrent-ruby/concurrent/ivar.rb', line 144

def try_set(value = NULL, &block)
  set(value, &block)
  true
rescue MultipleAssignmentError
  false
end