class Concurrent::Promise
Promises
are inspired by the JavaScript [Promises/A](wiki.commonjs.org/wiki/Promises/A) and [Promises/A+](promises-aplus.github.io/promises-spec/) specifications.
> A promise represents the eventual value returned from the single > completion of an operation.
Promises
are similar to futures and share many of the same behaviours. Promises
are far more robust, however. Promises
can be chained in a tree structure where each promise may have zero or more children. Promises
are chained using the ‘then` method. The result of a call to `then` is always another promise. Promises
are resolved asynchronously (with respect to the main thread) but in a strict order: parents are guaranteed to be resolved before their children, children before their younger siblings. The `then` method takes two parameters: an optional block to be executed upon parent resolution and an optional callable to be executed upon parent failure. The result of each promise is passed to each of its children upon resolution. When a promise is rejected all its children will be summarily rejected and will receive the reason.
Promises
have several possible states: :unscheduled, :pending, :processing, :rejected, or :fulfilled. These are also aggregated as ‘#incomplete?` and `#complete?`. When a Promise
is created it is set to :unscheduled. Once the `#execute` method is called the state becomes :pending. Once a job is pulled from the thread pool’s queue and is given to a thread for processing (often immediately upon ‘#post`) the state becomes :processing. The future will remain in this state until processing is complete. A future that is in the :unscheduled, :pending, or :processing is considered `#incomplete?`. A `#complete?` Promise
is either :rejected, indicating that an exception was thrown during processing, or :fulfilled, indicating success. If a Promise
is :fulfilled its `#value` will be updated to reflect the result of the operation. If :rejected the `reason` will be updated with a reference to the thrown exception. The predicate methods `#unscheduled?`, `#pending?`, `#rejected?`, and `#fulfilled?` can be called at any time to obtain the state of the Promise
, as can the `#state` method, which returns a symbol.
Retrieving the value of a promise is done through the ‘value` (alias: `deref`) method. Obtaining the value of a promise is a potentially blocking operation. When a promise is rejected a call to `value` will return `nil` immediately. When a promise is fulfilled a call to `value` will immediately return the current value. When a promise is pending a call to `value` will block until the promise is either rejected or fulfilled. A timeout value can be passed to `value` to limit how long the call will block. If `nil` the call will block indefinitely. If `0` the call will not block. Any other integer or float value will indicate the maximum number of seconds to block.
Promises
run on the global thread pool.
@!macro copy_options
### Examples
Start by requiring promises
“‘ruby require ’concurrent/promise’ “‘
Then create one
“‘ruby p = Concurrent::Promise.execute
do
# do something 42 end
“‘
Promises
can be chained using the ‘then` method. The `then` method accepts a block and an executor, to be executed on fulfillment, and a callable argument to be executed on rejection. The result of the each promise is passed as the block argument to chained promises.
“‘ruby p = Concurrent::Promise.new
{10}.then{|x| x * 2}.then{|result| result - 10 }.execute “`
And so on, and so on, and so on…
“‘ruby p = Concurrent::Promise.fulfill(20)
.
then{|result| result - 10 }. then{|result| result * 3 }. then(executor: different_executor){|result| result % 5 }.execute
“‘
The initial state of a newly created Promise
depends on the state of its parent:
-
if parent is unscheduled the child will be unscheduled
-
if parent is pending the child will be pending
-
if parent is fulfilled the child will be pending
-
if parent is rejected the child will be pending (but will ultimately be rejected)
Promises
are executed asynchronously from the main thread. By the time a child Promise
finishes intialization it may be in a different state than its parent (by the time a child is created its parent may have completed execution and changed state). Despite being asynchronous, however, the order of execution of Promise
objects in a chain (or tree) is strictly defined.
There are multiple ways to create and execute a new ‘Promise`. Both ways provide identical behavior:
“‘ruby # create, operate, then execute p1 = Concurrent::Promise.new
{ “Hello World!” } p1.state #=> :unscheduled p1.execute
# create and immediately execute p2 = Concurrent::Promise.new
{ “Hello World!” }.execute
# execute during creation p3 = Concurrent::Promise.execute
{ “Hello World!” } “‘
Once the ‘execute` method is called a `Promise` becomes `pending`:
“‘ruby p = Concurrent::Promise.execute
{ “Hello, world!” } p.state #=> :pending p.pending? #=> true “`
Wait a little bit, and the promise will resolve and provide a value:
“‘ruby p = Concurrent::Promise.execute
{ “Hello, world!” } sleep(0.1)
p.state #=> :fulfilled p.fulfilled? #=> true p.value #=> “Hello, world!” “‘
If an exception occurs, the promise will be rejected and will provide a reason for the rejection:
“‘ruby p = Concurrent::Promise.execute
{ raise StandardError.new(“Here comes the Boom!”) } sleep(0.1)
p.state #=> :rejected p.rejected? #=> true p.reason #=> “#<StandardError: Here comes the Boom!>” “‘
#### Rejection
When a promise is rejected all its children will be rejected and will receive the rejection ‘reason` as the rejection callable parameter:
“‘ruby p = Concurrent::Promise.execute
{ Thread.pass; raise StandardError }
c1 = p.then(-> reason { 42 }) c2 = p.then(-> reason { raise ‘Boom!’ })
c1.wait.state #=> :fulfilled c1.value #=> 45 c2.wait.state #=> :rejected c2.reason #=> Once a promise is rejected it will continue to accept children that will receive immediately rejection (they will be executed asynchronously). #### Aliases The ‘then` method is the most generic alias: it accepts a block to be executed upon parent fulfillment and a callable to be executed upon parent rejection. At least one of them should be passed. The default block is `{ |result| result }` that fulfills the child with the parent value. The default callable is `{ |reason| raise reason }` that rejects the child with the parent reason. ‘on_success { |result| … }` is the same as `then {|result| … }` ‘rescue { |reason| … }` is the same as `then(Proc.new { |reason| … } )` ‘rescue` is aliased by `catch` and `on_error`
Public Class Methods
Aggregates a collection of promises and executes the ‘then` condition if all aggregated promises succeed. Executes the `rescue` handler with a `Concurrent::PromiseExecutionError` if any of the aggregated promises fail. Upon execution will execute any of the aggregate promises that were not already executed.
@!macro promise_self_aggregate
The returned promise will not yet have been executed. Additional `#then` and `#rescue` handlers may still be provided. Once the returned promise is execute the aggregate promises will be also be executed (if they have not been executed already). The results of the aggregate promises will be checked upon completion. The necessary `#then` and `#rescue` blocks on the aggregating promise will then be executed as appropriate. If the `#rescue` handlers are executed the raises exception will be `Concurrent::PromiseExecutionError`. @param [Array] promises Zero or more promises to aggregate @return [Promise] an unscheduled (not executed) promise that aggregates the promises given as arguments
# File lib/concurrent-ruby/concurrent/promise.rb, line 464 def self.all?(*promises) aggregate(:all?, *promises) end
Aggregates a collection of promises and executes the ‘then` condition if any aggregated promises succeed. Executes the `rescue` handler with a `Concurrent::PromiseExecutionError` if any of the aggregated promises fail. Upon execution will execute any of the aggregate promises that were not already executed.
@!macro promise_self_aggregate
# File lib/concurrent-ruby/concurrent/promise.rb, line 475 def self.any?(*promises) aggregate(:any?, *promises) end
Create a new ‘Promise` object with the given block, execute it, and return the `:pending` object.
@!macro executor_and_deref_options
@!macro promise_init_options
@return [Promise] the newly created ‘Promise` in the `:pending` state
@raise [ArgumentError] if no block is given
@example
promise = Concurrent::Promise.execute{ sleep(1); 42 } promise.state #=> :pending
# File lib/concurrent-ruby/concurrent/promise.rb, line 296 def self.execute(opts = {}, &block) new(opts, &block).execute end
Create a new ‘Promise` and fulfill it immediately.
@!macro executor_and_deref_options
@!macro promise_init_options
@raise [ArgumentError] if no block is given
@return [Promise] the newly created ‘Promise`
# File lib/concurrent-ruby/concurrent/promise.rb, line 224 def self.fulfill(value, opts = {}) Promise.new(opts).tap { |p| p.send(:synchronized_set_state!, true, value, nil) } end
Initialize a new Promise
with the provided options.
@!macro executor_and_deref_options
@!macro promise_init_options
@option opts [Promise] :parent the parent `Promise` when building a chain/tree @option opts [Proc] :on_fulfill fulfillment handler @option opts [Proc] :on_reject rejection handler @option opts [object, Array] :args zero or more arguments to be passed the task block on execution
@yield The block operation to be performed asynchronously.
@raise [ArgumentError] if no block is given
@see wiki.commonjs.org/wiki/Promises/A @see promises-aplus.github.io/promises-spec/
Concurrent::IVar::new
# File lib/concurrent-ruby/concurrent/promise.rb, line 210 def initialize(opts = {}, &block) opts.delete_if { |k, v| v.nil? } super(NULL, opts.merge(__promise_body_from_block__: block), &nil) end
Create a new ‘Promise` and reject it immediately.
@!macro executor_and_deref_options
@!macro promise_init_options
@raise [ArgumentError] if no block is given
@return [Promise] the newly created ‘Promise`
# File lib/concurrent-ruby/concurrent/promise.rb, line 237 def self.reject(reason, opts = {}) Promise.new(opts).tap { |p| p.send(:synchronized_set_state!, false, nil, reason) } end
Builds a promise that produces the result of promises in an Array
and fails if any of them fails.
@overload zip(*promises)
@param [Array<Promise>] promises
@overload zip(*promises, opts)
@param [Array<Promise>] promises @param [Hash] opts the configuration options @option opts [Executor] :executor (ImmediateExecutor.new) when set use the given `Executor` instance. @option opts [Boolean] :execute (true) execute promise before returning
@return [Promise<Array>]
# File lib/concurrent-ruby/concurrent/promise.rb, line 409 def self.zip(*promises) opts = promises.last.is_a?(::Hash) ? promises.pop.dup : {} opts[:executor] ||= ImmediateExecutor.new zero = if !opts.key?(:execute) || opts.delete(:execute) fulfill([], opts) else Promise.new(opts) { [] } end promises.reduce(zero) do |p1, p2| p1.flat_map do |results| p2.then do |next_result| results << next_result end end end end
Protected Class Methods
Aggregate a collection of zero or more promises under a composite promise, execute the aggregated promises and collect them into a standard Ruby array, call the given Ruby ‘Ennnumerable` predicate (such as `any?`, `all?`, `none?`, or `one?`) on the collection checking for the success or failure of each, then executing the composite’s ‘#then` handlers if the predicate returns `true` or executing the composite’s ‘#rescue` handlers if the predicate returns false.
@!macro promise_self_aggregate
# File lib/concurrent-ruby/concurrent/promise.rb, line 505 def self.aggregate(method, *promises) composite = Promise.new do completed = promises.collect do |promise| promise.execute if promise.unscheduled? promise.wait promise end unless completed.empty? || completed.send(method){|promise| promise.fulfilled? } raise PromiseExecutionError end end composite end
Public Instance Methods
Execute an ‘:unscheduled` `Promise`. Immediately sets the state to `:pending` and passes the block to a new thread/thread pool for eventual execution. Does nothing if the `Promise` is in any state other than `:unscheduled`.
@return [Promise] a reference to ‘self`
# File lib/concurrent-ruby/concurrent/promise.rb, line 246 def execute if root? if compare_and_set_state(:pending, :unscheduled) set_pending realize(@promise_body) end else compare_and_set_state(:pending, :unscheduled) @parent.execute end self end
@!macro ivar_fail_method
@raise [Concurrent::PromiseExecutionError] if not the root promise
# File lib/concurrent-ruby/concurrent/promise.rb, line 278 def fail(reason = StandardError.new) set { raise reason } end
Yield the successful result to the block that returns a promise. If that promise is also successful the result is the result of the yielded promise. If either part fails the whole also fails.
@example
Promise.execute { 1 }.flat_map { |v| Promise.execute { v + 2 } }.value! #=> 3
@return [Promise]
# File lib/concurrent-ruby/concurrent/promise.rb, line 375 def flat_map(&block) child = Promise.new( parent: self, executor: ImmediateExecutor.new, ) on_error { |e| child.on_reject(e) } on_success do |result1| begin inner = block.call(result1) inner.execute inner.on_success { |result2| child.on_fulfill(result2) } inner.on_error { |e| child.on_reject(e) } rescue => e child.on_reject(e) end end child end
Chain onto this promise an action to be undertaken on success (fulfillment).
@yield The block to execute
@return [Promise] self
# File lib/concurrent-ruby/concurrent/promise.rb, line 349 def on_success(&block) raise ArgumentError.new('no block given') unless block_given? self.then(&block) end
Chain onto this promise an action to be undertaken on failure (rejection).
@yield The block to execute
@return [Promise] self
# File lib/concurrent-ruby/concurrent/promise.rb, line 360 def rescue(&block) self.then(block) end
@!macro ivar_set_method
@raise [Concurrent::PromiseExecutionError] if not the root promise
# File lib/concurrent-ruby/concurrent/promise.rb, line 262 def set(value = NULL, &block) raise PromiseExecutionError.new('supported only on root promise') unless root? check_for_block_or_value!(block_given?, value) synchronize do if @state != :unscheduled raise MultipleAssignmentError else @promise_body = block || Proc.new { |result| value } end end execute end
Chain a new promise off the current promise.
@return [Promise] the new promise @yield The block operation to be performed asynchronously. @overload then(rescuer, executor, &block)
@param [Proc] rescuer An optional rescue block to be executed if the promise is rejected. @param [ThreadPool] executor An optional thread pool executor to be used in the new Promise
@overload then(rescuer, executor: executor, &block)
@param [Proc] rescuer An optional rescue block to be executed if the promise is rejected. @param [ThreadPool] executor An optional thread pool executor to be used in the new Promise
# File lib/concurrent-ruby/concurrent/promise.rb, line 314 def then(*args, &block) if args.last.is_a?(::Hash) executor = args.pop[:executor] rescuer = args.first else rescuer, executor = args end executor ||= @executor raise ArgumentError.new('rescuers and block are both missing') if rescuer.nil? && !block_given? block = Proc.new { |result| result } unless block_given? child = Promise.new( parent: self, executor: executor, on_fulfill: block, on_reject: rescuer ) synchronize do child.state = :pending if @state == :pending child.on_fulfill(apply_deref_options(@value)) if @state == :fulfilled child.on_reject(@reason) if @state == :rejected @children << child end child end
Builds a promise that produces the result of self and others in an Array
and fails if any of them fails.
@overload zip(*promises)
@param [Array<Promise>] others
@overload zip(*promises, opts)
@param [Array<Promise>] others @param [Hash] opts the configuration options @option opts [Executor] :executor (ImmediateExecutor.new) when set use the given `Executor` instance. @option opts [Boolean] :execute (true) execute promise before returning
@return [Promise<Array>]
# File lib/concurrent-ruby/concurrent/promise.rb, line 440 def zip(*others) self.class.zip(self, *others) end
Protected Instance Methods
@!visibility private
# File lib/concurrent-ruby/concurrent/promise.rb, line 551 def complete(success, value, reason) children_to_notify = synchronize do set_state!(success, value, reason) @children.dup end children_to_notify.each { |child| notify_child(child) } observers.notify_and_delete_observers{ [Time.now, self.value, reason] } end
@!visibility private
# File lib/concurrent-ruby/concurrent/promise.rb, line 545 def notify_child(child) if_state(:fulfilled) { child.on_fulfill(apply_deref_options(@value)) } if_state(:rejected) { child.on_reject(@reason) } end
Concurrent::IVar#ns_initialize
# File lib/concurrent-ruby/concurrent/promise.rb, line 481 def ns_initialize(value, opts) super @executor = Options.executor_from_options(opts) || Concurrent.global_io_executor @args = get_arguments_from(opts) @parent = opts.fetch(:parent) { nil } @on_fulfill = opts.fetch(:on_fulfill) { Proc.new { |result| result } } @on_reject = opts.fetch(:on_reject) { Proc.new { |reason| raise reason } } @promise_body = opts[:__promise_body_from_block__] || Proc.new { |result| result } @state = :unscheduled @children = [] end
@!visibility private
# File lib/concurrent-ruby/concurrent/promise.rb, line 533 def on_fulfill(result) realize Proc.new { @on_fulfill.call(result) } nil end
@!visibility private
# File lib/concurrent-ruby/concurrent/promise.rb, line 539 def on_reject(reason) realize Proc.new { @on_reject.call(reason) } nil end
@!visibility private
# File lib/concurrent-ruby/concurrent/promise.rb, line 562 def realize(task) @executor.post do success, value, reason = SafeTaskExecutor.new(task, rescue_exception: true).execute(*@args) complete(success, value, reason) end end
@!visibility private
# File lib/concurrent-ruby/concurrent/promise.rb, line 520 def set_pending synchronize do @state = :pending @children.each { |c| c.set_pending } end end
@!visibility private
# File lib/concurrent-ruby/concurrent/promise.rb, line 570 def set_state!(success, value, reason) set_state(success, value, reason) event.set end
@!visibility private
# File lib/concurrent-ruby/concurrent/promise.rb, line 576 def synchronized_set_state!(success, value, reason) synchronize { set_state!(success, value, reason) } end