Basics

Factory methods

Future and Event are created indirectly with constructor methods in FactoryMethods. They are not designed for inheritance but rather for composition.

Concurrent::Promises::FactoryMethods.instance_methods(false).sort
# => [:any,
#     :any_event,
#     :any_event_on,
#     :any_fulfilled_future,
#     :any_fulfilled_future_on,
#     :any_resolved_future,
#     :any_resolved_future_on,
#     :delay,
#     :delay_on,
#     :fulfilled_future,
#     :future,
#     :future_on,
#     :make_future,
#     :rejected_future,
#     :resolvable_event,
#     :resolvable_event_on,
#     :resolvable_future,
#     :resolvable_future_on,
#     :resolved_event,
#     :resolved_future,
#     :schedule,
#     :schedule_on,
#     :zip,
#     :zip_events,
#     :zip_events_on,
#     :zip_futures,
#     :zip_futures_on,
#     :zip_futures_over,
#     :zip_futures_over_on]

The module can be included or extended where needed.

Class.new do
  include Concurrent::Promises::FactoryMethods

  def a_method
    resolvable_event
  end
end.new.a_method
# => #<Concurrent::Promises::ResolvableEvent:0x000002 pending>

mod = Module.new do
  extend Concurrent::Promises::FactoryMethods
end 
mod.resolvable_event
# => #<Concurrent::Promises::ResolvableEvent:0x000003 pending>

The default executor can be changed by overriding default_executor method inherited from Concurrent::Promises::FactoryMethods.

mod = Module.new do
  extend Concurrent::Promises::FactoryMethods
  def self.default_executor
    :fast
  end
end 
mod.future { 1 }.default_executor        # => :fast
Concurrent::Promises.future { 1 }.default_executor
# => :io

The module is already extended into Concurrent::Promises for convenience.

Concurrent::Promises.resolvable_event
# => #<Concurrent::Promises::ResolvableEvent:0x000004 pending>

Asynchronous task

The most basic use-case of the framework is asynchronous processing. A task can be processed asynchronously by using a future factory method. The block will be executed on an internal thread pool.

Arguments of future are passed to the block and evaluation starts immediately.

future = Concurrent::Promises.future(0.1) do |duration|
  sleep duration
  :result
end
# => #<Concurrent::Promises::Future:0x000005 pending>
future.value                             # => :result

Asks if the future is resolved, here it will be still in the middle of the sleep call.

future.resolved?                         # => true

Retrieving the value will block until the future is resolved.

future.value                             # => :result
future.resolved?                         # => true

If the task fails, we talk about the future being rejected.

future = Concurrent::Promises.future { sleep 0.01; raise 'Boom' }
# => #<Concurrent::Promises::Future:0x000006 pending>

There is no result, the future was rejected with a reason.

future.value                             # => nil
future.reason                            # => #<RuntimeError: Boom>

It can be forced to raise the reason for rejection when retrieving the value.

begin
  future.value! 
rescue => e 
  e
end                                      # => #<RuntimeError: Boom>

Which is the same as future.value! rescue $! which will be used hereafter.

Or it can be used directly as argument for raise, since it implements exception method.

raise future rescue $!                   # => #<RuntimeError: Boom>

States

Let's define an inspection helper for methods.

def inspect_methods(*methods, of:)
  methods.reduce({}) { |h, m| h.update m => of.send(m) }
end 

Event has a pending and a resolved state.

event = Concurrent::Promises.resolvable_event 
inspect_methods(:state, :pending?, :resolved?, of: event)
# => {:state=>:pending, :pending?=>true, :resolved?=>false}

event.resolve 
inspect_methods(:state, :pending?, :resolved?, of: event)
# => {:state=>:resolved, :pending?=>false, :resolved?=>true}

Future's resolved state is further specified to be fulfilled or rejected.

future = Concurrent::Promises.resolvable_future 
inspect_methods(:state, :pending?, :resolved?, :fulfilled?, :rejected?, 
    of: future)
# => {:state=>:pending,
#     :pending?=>true,
#     :resolved?=>false,
#     :fulfilled?=>false,
#     :rejected?=>false}

future.fulfill :value 
inspect_methods(:state, :pending?, :resolved?, :fulfilled?, :rejected?,
    :result, :value, :reason, of: future)
# => {:state=>:fulfilled,
#     :pending?=>false,
#     :resolved?=>true,
#     :fulfilled?=>true,
#     :rejected?=>false,
#     :result=>[true, :value, nil],
#     :value=>:value,
#     :reason=>nil}

future = Concurrent::Promises.rejected_future StandardError.new 
inspect_methods(:state, :pending?, :resolved?, :fulfilled?, :rejected?, 
    :result, :value, :reason, of: future)
# => {:state=>:rejected,
#     :pending?=>false,
#     :resolved?=>true,
#     :fulfilled?=>false,
#     :rejected?=>true,
#     :result=>[false, nil, #<StandardError: StandardError>],
#     :value=>nil,
#     :reason=>#<StandardError: StandardError>}

Direct creation of resolved futures

When an existing value has to be wrapped in a future it does not have to go through evaluation as follows.

Concurrent::Promises.future { sleep 0.01; :value }
# => #<Concurrent::Promises::Future:0x000007 pending>

Instead, it can be created directly as already-resolved:

Concurrent::Promises.fulfilled_future(:value)
# => #<Concurrent::Promises::Future:0x000008 fulfilled with :value>
Concurrent::Promises.rejected_future(StandardError.new('Ups'))
# => #<Concurrent::Promises::Future:0x000009 rejected with #<StandardError: Ups>>
Concurrent::Promises.resolved_future(true, :value, nil)
# => #<Concurrent::Promises::Future:0x00000a fulfilled with :value>
Concurrent::Promises.resolved_future(false, nil, StandardError.new('Ups'))
# => #<Concurrent::Promises::Future:0x00000b rejected with #<StandardError: Ups>>

Chaining

A big advantage of promises is the ability to chain tasks together without blocking the current thread.

Concurrent::Promises.
    future(2) { |v| v.succ }.
    then(&:succ).
    value!                               # => 4

As future factory method takes an argument, so does the then method. Any supplied arguments are passed to the block, and the library ensures that they are visible to the block.

Concurrent::Promises.
    future('3') { |s| s.to_i }.
    then(2) { |v, arg| v + arg }.
    value                                # => 5
Concurrent::Promises.
    fulfilled_future('3').
    then(&:to_i).
    then(2, &:+).
    value                                # => 5
Concurrent::Promises.
    fulfilled_future(1).
    chain(2) { |fulfilled, value, reason, arg| value + arg }.
    value                                # => 3

Passing the arguments in (similarly as for a thread Thread.new(arg) { |arg| do_stuff arg }) is required. Both of the following bad examples may break:

arg = 1                                  # => 1
Thread.new { do_stuff arg }
# => #<Thread:0x00000c@promises.in.md:204 run>
Concurrent::Promises.future { do_stuff arg }
# => #<Concurrent::Promises::Future:0x00000d pending>

Correct:

arg = 1                                  # => 1
Thread.new(arg) { |arg| do_stuff arg }
# => #<Thread:0x00000e@promises.in.md:212 run>
Concurrent::Promises.future(arg) { |arg| do_stuff arg }
# => #<Concurrent::Promises::Future:0x00000f pending>

Branching, and zipping

Besides chaining it can also be branched.

head    = Concurrent::Promises.fulfilled_future -1 
branch1 = head.then(&:abs) 
branch2 = head.then(&:succ).then(&:succ) 

branch1.value!                           # => 1
branch2.value!                           # => 1

It can be combined back to one future by zipping (zip, &).

branch1.zip(branch2).value!              # => [1, 1]
(branch1 & branch2).
    then { |a, b| a + b }.
    value!                               # => 2
(branch1 & branch2).
    then(&:+).
    value!                               # => 2
Concurrent::Promises.
    zip(branch1, branch2, branch1).
    then { |*values| values.reduce(&:+) }.
    value!                               # => 3

Instead of zipping only the first one can be taken, if needed.

Concurrent::Promises.any(branch1, branch2).value!
# => 1
(branch1 | branch2).value!               # => 1

Blocking methods

In these examples we have used blocking methods like value extensively for their convenience, however in practice is better to avoid them and continue chaining.

If they need to be used (e.g. when integrating with threads), value! is a better option over value when rejections are not dealt with differently. Otherwise the rejections are not handled and probably silently forgotten.

Error handling

When a task in the chain fails, the rejection propagates down the chain without executing the tasks created with then.

Concurrent::Promises.
    fulfilled_future(Object.new).
    then(&:succ).
    then(&:succ).
    result
# => [false,
#     nil,
#     #<NoMethodError: undefined method `succ' for #<Object:0x000010>>]

As then chained tasks execute only on fulfilled futures, there is a rescue method which chains a task which is executed only when the future is rejected. It can be used to recover from rejection.

Using rescue to fulfill to 0 instead of the error.

Concurrent::Promises.
    fulfilled_future(Object.new).
    then(&:succ).
    then(&:succ).
    rescue { |err| 0 }.
    result                               # => [true, 0, nil]

Rescue not executed when there is no rejection.

Concurrent::Promises.
    fulfilled_future(1).
    then(&:succ).
    then(&:succ).
    rescue { |e| 0 }. 
    result                               # => [true, 3, nil]

Tasks added with chain are always evaluated.

Concurrent::Promises.
    fulfilled_future(1).
    chain { |fulfilled, value, reason| fulfilled ? value : reason }.
    value!                               # => 1
Concurrent::Promises.
    rejected_future(StandardError.new('Ups')).
    chain { |fulfilled, value, reason| fulfilled ? value : reason }.
    value!                               # => #<StandardError: Ups>

Zip is rejected if any of the zipped futures is.

rejected_zip = Concurrent::Promises.zip(
    Concurrent::Promises.fulfilled_future(1),
    Concurrent::Promises.rejected_future(StandardError.new('Ups')))
# => #<Concurrent::Promises::Future:0x000011 rejected with [nil, #<StandardError: Ups>]>
rejected_zip.result
# => [false, [1, nil], [nil, #<StandardError: Ups>]]
rejected_zip.
    rescue { |reason1, reason2| (reason1 || reason2).message }.
    value                                # => "Ups"

Delayed futures

Delayed futures will not evaluate until asked by touch or other method requiring resolution.

future = Concurrent::Promises.delay { sleep 0.01; 'lazy' }
# => #<Concurrent::Promises::Future:0x000012 pending>
sleep 0.01 
future.resolved?                         # => false
future.touch
# => #<Concurrent::Promises::Future:0x000012 pending>
sleep 0.02 
future.resolved?                         # => true

All blocking methods like wait, value call touch and trigger evaluation.

Concurrent::Promises.delay { :value }.value
# => :value

It propagates up through the chain, allowing whole or partial lazy chains.

head    = Concurrent::Promises.delay { 1 } 
branch1 = head.then(&:succ) 
branch2 = head.delay.then(&:succ) 
join    = branch1 & branch2 

sleep 0.01 

Nothing resolves.

[head, branch1, branch2, join].map(&:resolved?)
# => [false, false, false, false]

Force branch1 evaluation.

branch1.value                            # => 2
sleep 0.01 
[head, branch1, branch2, join].map(&:resolved?)
# => [true, true, false, false]

Force evaluation of both by calling value on join.

join.value                               # => [2, 2]
[head, branch1, branch2, join].map(&:resolved?)
# => [true, true, true, true]

Flatting

Sometimes it is needed to wait for an inner future. An apparent solution is to wait inside the future Concurrent::Promises.future { Concurrent::Promises.future { 1+1 }.value }.value. However, as mentioned before, value calls should be avoided to avoid blocking threads. Therefore there is a #flat method which is a correct solution in this situation and does not block any thread.

Concurrent::Promises.future { Concurrent::Promises.future { 1+1 } }.flat.value!
# => 2

A more complicated example.

Concurrent::Promises.
    future { Concurrent::Promises.future { Concurrent::Promises.future { 1 + 1 } } }.
    flat(1).
    then { |future| future.then(&:succ) }.
    flat(1).
    value!                               # => 3

Scheduling

Tasks can be planned to be executed with a time delay.

Schedule task to be executed in 0.1 seconds.

scheduled = Concurrent::Promises.schedule(0.1) { 1 }
# => #<Concurrent::Promises::Future:0x000013 pending>
scheduled.resolved?                      # => false

Value will become available after 0.1 seconds.

scheduled.value                          # => 1

It can be used in the chain as well, where the delay is counted from the moment its parent resolves. Therefore, the following future will be resolved in 0.2 seconds.

future = Concurrent::Promises.
    future { sleep 0.01; :result }.
    schedule(0.01).
    then(&:to_s).
    value!                               # => "result"

Time can be used as well.

Concurrent::Promises.schedule(Time.now + 10) { :val }
# => #<Concurrent::Promises::Future:0x000014 pending>

Resolvable Future and Event:

Sometimes it is required to resolve a future externally, in these cases resolvable_future and resolvable_event factory methods can be used. See Concurrent::Promises::ResolvableFuture and Concurrent::Promises::ResolvableEvent.

future = Concurrent::Promises.resolvable_future
# => #<Concurrent::Promises::ResolvableFuture:0x000015 pending>

The thread will be blocked until the future is resolved

thread = Thread.new { future.value } 
future.fulfill 1
# => #<Concurrent::Promises::ResolvableFuture:0x000015 fulfilled with 1>
thread.value                             # => 1

A future can be resolved only once.

future.fulfill 1 rescue $!
# => #<Concurrent::MultipleAssignmentError: Future can be resolved only once. It's [true, 1, nil], trying to set [true, 1, nil]. {:current_result=>[true, 1, nil], :new_result=>[true, 1, nil]}>
future.fulfill 2, false                  # => false

How are promises executed?

Promises use global pools to execute the tasks. Therefore each task may run on different threads which implies that users have to be careful not to depend on Thread-local variables (or they have to be set at the beginning of the task and cleaned up at the end of the task).

Since the tasks are running on may different threads of the thread pool, it's better to follow following rules:

• Use only data passed via arguments or values of parent futures, to have better control over what are futures accessing.
• The data passed in and out of futures is easier to deal with if it is immutable or at least treated as such.
• Any mutable and mutated object accessed by more than one thread or future must be thread-safe, see Concurrent::Array, Concurrent::Hash, and Concurrent::Map. (The value of a future may be consumed by many futures.)
• Futures can access outside objects, but they have to be thread-safe.

TODO: This part to be extended

Advanced

Callbacks

queue  = Queue.new                       # => #<Thread::Queue:0x000016>
future = Concurrent::Promises.delay { 1 + 1 }
# => #<Concurrent::Promises::Future:0x000017 pending>

future.on_fulfillment { queue << 1 } # evaluated asynchronously
future.on_fulfillment! { queue << 2 } # evaluated on resolving thread

queue.empty?                             # => true
future.value                             # => 2
queue.pop                                # => 2
queue.pop                                # => 1

Using executors

Factory methods, chain, and callback methods all have other versions of them which takes an executor argument.

It takes an instance of an executor, or a symbol which is a shortcut for the two global pools in concurrent-ruby. :fast for short and non-blocking tasks and :io for long-running and blocking tasks.

Concurrent::Promises.future_on(:fast) { 2 }.
    then_on(:io) { File.read __FILE__ }.
    value.size                           # => 25384

Run (simulated process)

Similar to flatting is running. When run is called on a future it will flat indefinitely as long the future fulfils into a Future value. It can be used to simulate a thread-like processing without actually occupying the thread.

count = lambda do |v|
  v += 1
  v < 5 ? Concurrent::Promises.future_on(:fast, v, &count) : v
end
# => #<Proc:0x000018@promises.in.md:521 (lambda)>
400.times.
    map { Concurrent::Promises.future_on(:fast, 0, &count).run.value! }.
    all? { |v| v == 5 }                  # => true

Therefore the above example finished fine on the the :fast thread pool even though it has much fewer threads than are simulated in the simulated process.

Interoperability

Actors

Create an actor which takes received numbers and returns the number squared.

actor = Concurrent::Actor::Utils::AdHoc.spawn :square do
  -> v { v ** 2 }
end
# => #<Concurrent::Actor::Reference:0x000019 /square (Concurrent::Actor::Utils::AdHoc)>

Send result of 1+1 to the actor, and add 2 to the result sent back from the actor.

Concurrent::Promises.
    future { 1 + 1 }.
    then_ask(actor).
    then { |v| v + 2 }.
    value!                               # => 6

So (1 + 1)**2 + 2 = 6.

The ask method returns future.

actor.ask(2).then(&:succ).value!         # => 5

Channel

There is an implementation of channel as well. Let's start by creating a channel with a capacity of 2 messages.

ch1 = Concurrent::Promises::Channel.new 2
# => #<Concurrent::Promises::Channel:0x00001a capacity taken 0 of 2>

We push 3 messages, it can be observed that the last future representing the push is not fulfilled since the capacity prevents it. When the work which fills the channel depends on the futures created by push it can be used to create backpressure – the filling work is delayed until the channel has space for more messages.

pushes = 3.times.map { |i| ch1.push_op i }
# => [#<Concurrent::Promises::Future:0x00001b fulfilled with #<Concurrent::Promises::Channel:0x00001a capacity taken 2 of 2>>,
#     #<Concurrent::Promises::Future:0x00001c fulfilled with #<Concurrent::Promises::Channel:0x00001a capacity taken 2 of 2>>,
#     #<Concurrent::Promises::ResolvableFuture:0x00001d pending>]
ch1.pop_op.value!                        # => 0
pushes
# => [#<Concurrent::Promises::Future:0x00001b fulfilled with #<Concurrent::Promises::Channel:0x00001a capacity taken 2 of 2>>,
#     #<Concurrent::Promises::Future:0x00001c fulfilled with #<Concurrent::Promises::Channel:0x00001a capacity taken 2 of 2>>,
#     #<Concurrent::Promises::ResolvableFuture:0x00001d fulfilled with #<Concurrent::Promises::Channel:0x00001a capacity taken 2 of 2>>]

A selection over channels can be created with the .select_channel factory method. It will be fulfilled with a first message available in any of the channels. It returns a pair to be able to find out which channel had the message available.

ch2    = Concurrent::Promises::Channel.new 2
# => #<Concurrent::Promises::Channel:0x00001e capacity taken 0 of 2>
result = Concurrent::Promises::Channel.select_op([ch1, ch2])
# => #<Concurrent::Promises::ResolvableFuture:0x00001f fulfilled with [#<Concurrent::Promises::Channel:0x00001a capacity taken 1 of 2>, 1]>
result.value!
# => [#<Concurrent::Promises::Channel:0x00001a capacity taken 1 of 2>, 1]

Concurrent::Promises.future { 1+1 }.then_channel_push(ch1)
# => #<Concurrent::Promises::Future:0x000020 pending>
result = (
    Concurrent::Promises.fulfilled_future('%02d') &      
        Concurrent::Promises::Channel.select_op([ch1, ch2])).
    then { |format, (channel, value)| format format, value } 
result.value!                            # => "02"

ProcessingActor

There is also a new implementation of actors based on the Channel and the ability of promises to simulate processes. The actor runs as a process but also does not occupy a thread per actor as the previously-described Concurrent::Actor implementation. This implementation is close to Erlang actors, therefore OTP can be ported for this actors (and it's planned).

The simplest actor is one which just computes without even receiving a message.

actor = Concurrent::ProcessingActor.act(an_argument = 2) do |actor, number|
  number ** 3
end
# => #<Concurrent::ProcessingActor:0x000021 termination: pending>
actor.termination.value!                 # => 8

Let's receive some messages though.

add_2_messages = Concurrent::ProcessingActor.act do |actor|
  # Receive two messages then terminate normally with the sum.
  (actor.receive & actor.receive).then do |a, b|
    a + b
  end
end
# => #<Concurrent::ProcessingActor:0x000022 termination: pending>
add_2_messages.tell_op 1
# => #<Concurrent::Promises::Future:0x000023 pending>
add_2_messages.termination.resolved?     # => false
add_2_messages.tell_op 3
# => #<Concurrent::Promises::Future:0x000024 pending>
add_2_messages.termination.value!        # => 4

Actors can also be used to apply backpressure to a producer. Let's start by defining an actor which a mailbox of size 2.

slow_counter = -> (actor, count) do
  actor.receive.then do |command, number|
    sleep 0.01
    case command
    when :add
      slow_counter.call actor, count + number
    when :done
      # terminate
      count
    end
  end
end
# => #<Proc:0x000025@promises.in.md:638 (lambda)>

actor = Concurrent::ProcessingActor.act_listening( 
    Concurrent::Promises::Channel.new(2), 
    0,
    &slow_counter)
# => #<Concurrent::ProcessingActor:0x000026 termination: pending>

Now we can create a producer which will push messages only when there is a space available in the mailbox. We use promises to free a thread during waiting on a free space in the mailbox.

produce = -> receiver, i do
  if i < 10
    receiver.
        # send a message to the actor, resolves only after the message is 
        # accepted by the actor's mailbox
        tell_op([:add, i]).
        # send incremented message when the above message is accepted 
        then(i+1, &produce)
  else
    receiver.tell_op(:done)
    # do not continue 
  end
end
# => #<Proc:0x000027@promises.in.md:662 (lambda)>

Concurrent::Promises.future(actor, 0, &produce).run.wait!
# => #<Concurrent::Promises::Future:0x000028 fulfilled with #<Concurrent::ProcessingActor:0x000026 termination: pending>>

actor.termination.value!                 # => 45

Use-cases

Simple background processing

Concurrent::Promises.future { do_stuff }
# => #<Concurrent::Promises::Future:0x000029 pending>

Parallel background processing

tasks = 4.times.map { |i| Concurrent::Promises.future(i) { |i| i*2 } }
# => [#<Concurrent::Promises::Future:0x00002a pending>,
#     #<Concurrent::Promises::Future:0x00002b pending>,
#     #<Concurrent::Promises::Future:0x00002c pending>,
#     #<Concurrent::Promises::Future:0x00002d pending>]
Concurrent::Promises.zip(*tasks).value!
# => [0, 2, 4, 6]

Actor background processing

Actors are mainly keep and isolate state, they should stay responsive not being blocked by a longer running computations. It desirable to offload the work to stateless promises.

Lets define an actor which will process jobs, while staying responsive, and tracking the number of tasks being processed.

class Computer < Concurrent::Actor::RestartingContext
  def initialize
    super()
    @jobs = {}
  end

  def on_message(msg)
    command, *args = msg
    case command
    # new job to process
    when :run
      job        = args[0]
      @jobs[job] = envelope.future
      # Process asynchronously and send message back when done.
      Concurrent::Promises.future(&job).chain(job) do |fulfilled, value, reason, job|
        self.tell [:done, job, fulfilled, value, reason]
      end
      # Do not make return value of this method to be answer of this message.
      # We are answering later in :done by resolving the future kept in @jobs.
      Concurrent::Actor::Behaviour::MESSAGE_PROCESSED
    when :done
      job, fulfilled, value, reason = *args
      future                        = @jobs.delete job
      # Answer the job's result.
      future.resolve fulfilled, value, reason
    when :status
      { running_jobs: @jobs.size }
    else
      # Continue to fail with unknown message.
      pass 
    end
  end
end                                      # => :on_message

Create the computer actor and send it 3 jobs.

computer = Concurrent::Actor.spawn Computer, :computer
# => #<Concurrent::Actor::Reference:0x00002e /computer (Computer)>
results = 3.times.map { computer.ask [:run, -> { sleep 0.01; :result }] }
# => [#<Concurrent::Promises::Future:0x00002f pending>,
#     #<Concurrent::Promises::Future:0x000030 pending>,
#     #<Concurrent::Promises::Future:0x000031 pending>]
computer.ask(:status).value!             # => {:running_jobs=>3}
results.map(&:value!)                    # => [:result, :result, :result]

Solving the Thread count limit by thread simulation

Sometimes an application requires to process a lot of tasks concurrently. If the number of concurrent tasks is high enough than it is not possible to create a Thread for each of them. A partially satisfactory solution could be to use Fibers, but that solution locks the application on MRI since other Ruby implementations are using threads for each Fiber.

This library provides a Concurrent::Promises::Future#run method on a future to simulate threads without actually accepting one all the time. The run method is similar to Concurrent::Promises::Future#flat but it will keep flattening until it's fulfilled with non future value, then the value is taken as a result of the process simulated by run.

body = lambda do |v|
  # Some computation step of the process    
  new_v = v + 1
  # Is the process finished?
  if new_v < 5
    # Continue computing with new value, does not have to be recursive.
    # It just has to return a future.
    Concurrent::Promises.future(new_v, &body)
  else
    # The process is finished, fulfill the final value with `new_v`.
    new_v
  end
end
# => #<Proc:0x000032@promises.in.md:765 (lambda)>
Concurrent::Promises.future(0, &body).run.value! # => 5

This solution works well an any Ruby implementation.

TODO: More examples to be added.

Throttling concurrency

By creating an actor managing the resource we can control how many threads is accessing the resource. In this case one at the time.

data      = Array.new(10) { |i| '*' * i }
# => ["",
#     "*",
#     "**",
#     "***",
#     "****",
#     "*****",
#     "******",
#     "*******",
#     "********",
#     "*********"]
DB = Concurrent::Actor::Utils::AdHoc.spawn :db, data do |data|
  lambda do |message|
    # pretending that this queries a DB
    data[message]
  end
end
# => #<Concurrent::Actor::Reference:0x000033 /db (Concurrent::Actor::Utils::AdHoc)>

concurrent_jobs = 11.times.map do |v|
  DB.
      # ask the DB with the `v`, only one at the time, rest is parallel
      ask(v).
      # get size of the string, rejects for 11
      then(&:size).
      # translate error to a value (message of the exception)
      rescue { |reason| reason.message } 
end 

Concurrent::Promises.zip(*concurrent_jobs).value!
# => [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "undefined method `size' for nil:NilClass"]

Often there is more then one DB connections, then the pool can be used.

pool_size = 5                            # => 5

DB_POOL = Concurrent::Actor::Utils::Pool.spawn!('DB-pool', pool_size) do |index|
  # DB connection constructor
  Concurrent::Actor::Utils::AdHoc.spawn(
      name: "connection-#{index}", 
      args: [data]) do |data|
    lambda do |message|
      # pretending that this queries a DB
      data[message]
    end
  end
end
# => #<Concurrent::Actor::Reference:0x000034 /DB-pool (Concurrent::Actor::Utils::Pool)>

concurrent_jobs = 11.times.map do |v|
  DB_POOL.
      # ask the DB with the `v`, only one at the time, rest is parallel
      ask(v).
      # get size of the string, rejects for 11
      then(&:size).
      # translate error to a value (message of the exception)
      rescue { |reason| reason.message } 
end 

Concurrent::Promises.zip(*concurrent_jobs).value!
# => [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "undefined method `size' for nil:NilClass"]

In other cases the DB adapter maintains its internal connection pool and we just need to limit concurrent access to the DB's API to avoid the calls being blocked.

Lets pretend that the #[] method on DB_INTERNAL_POOL is using the internal pool of size 3. We create throttle with the same size

DB_INTERNAL_POOL = Concurrent::Array.new data 
# => ["",
#     "*",
#     "**",
#     "***",
#     "****",
#     "*****",
#     "******",
#     "*******",
#     "********",
#     "*********"]

max_tree = Concurrent::Throttle.new 3
# => #<Concurrent::Throttle:0x000035 capacity available 3 of 3>

futures = 11.times.map do |i|
  max_tree.
      # throttled tasks, at most 3 simultaneous calls of [] on the database
      future { DB_INTERNAL_POOL[i] }.
      # un-throttled tasks, unlimited concurrency
      then { |starts| starts.size }.
      rescue { |reason| reason.message }
end 

futures.map(&:value!)
# => [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "undefined method `size' for nil:NilClass"]

Long stream of tasks, applying backpressure

Let's assume that we are querying an API for data and the queries can be faster than we are able to process them. This example shows how to use channel as a buffer and how to apply backpressure to slow down the queries.

require 'json' 

channel              = Concurrent::Promises::Channel.new 6
# => #<Concurrent::Promises::Channel:0x000036 capacity taken 0 of 6>
cancellation, origin = Concurrent::Cancellation.new
# => #<Concurrent::Cancellation:0x000037 pending>

def query_random_text(cancellation, channel)
  Concurrent::Promises.future do
    # for simplicity the query is omitted
    # url = 'some api'
    # Net::HTTP.get(URI(url))
    sleep 0.01
    { 'message' => 
        'Lorem ipsum rhoncus scelerisque vulputate diam inceptos' 
    }.to_json
  end.then_flat_event(cancellation) do |value, cancellation|
    # The push to channel is fulfilled only after the message is successfully
    # published to the channel, therefore it will not continue querying until 
    # current message is pushed.
    cancellation.origin | channel.push_op(value) 
    # It could wait on the push indefinitely if the token is not checked
    # here with `or` (the pipe).        
  end.then(cancellation) do |cancellation|
    # query again after the message is pushed to buffer
    query_random_text(cancellation, channel) unless cancellation.canceled?
  end
end                                      # => :query_random_text

words          = []                      # => []
words_throttle = Concurrent::Throttle.new 1
# => #<Concurrent::Throttle:0x000038 capacity available 1 of 1>

def count_words_in_random_text(cancellation, channel, words, words_throttle)
  channel.pop_op.then do |response|
    string = JSON.load(response)['message']
    # processing is slower than querying
    sleep 0.02
    words_count = string.scan(/\w+/).size
  end.then_on(words_throttle.on(:io), words) do |words_count, words|
    # safe since throttled to only 1 task at a time
    words << words_count
  end.then_on(:io, cancellation) do |_, cancellation|
    # count words in next message
    unless cancellation.canceled?
      count_words_in_random_text(cancellation, channel, words, words_throttle)
    end
  end
end                                      # => :count_words_in_random_text

query_processes = 3.times.map do
  Concurrent::Promises.future(cancellation, channel, &method(:query_random_text)).run
end
# => [#<Concurrent::Promises::Future:0x000039 pending>,
#     #<Concurrent::Promises::Future:0x00003a pending>,
#     #<Concurrent::Promises::Future:0x00003b pending>]

word_counter_processes = 2.times.map do
  Concurrent::Promises.future(cancellation, channel, words, words_throttle, 
      &method(:count_words_in_random_text)).run
end
# => [#<Concurrent::Promises::Future:0x00003c pending>,
#     #<Concurrent::Promises::Future:0x00003d pending>]

sleep 0.05 

Let it run for a while, then cancel it, and ensure that the runs were all fulfilled (therefore ended) after the cancellation. Finally, print the result.

origin.resolve
# => #<Concurrent::Promises::ResolvableEvent:0x00003e resolved>
query_processes.map(&:wait!) 
# => [#<Concurrent::Promises::Future:0x000039 fulfilled with nil>,
#     #<Concurrent::Promises::Future:0x00003a fulfilled with nil>,
#     #<Concurrent::Promises::Future:0x00003b fulfilled with nil>]
word_counter_processes.map(&:wait!)
# => [#<Concurrent::Promises::Future:0x00003c fulfilled with nil>,
#     #<Concurrent::Promises::Future:0x00003d fulfilled with nil>]
words                                    # => [7, 7, 7, 7]

Compared to using threads directly, this is highly configurable and composable solution.

Periodic task

A periodically executed task can be creating by combining schedule, run and Cancellation.

repeating_scheduled_task = -> interval, cancellation, task do
  Concurrent::Promises.
      # Schedule the task.
      schedule(interval, cancellation, &task).
      # If successful schedule again. 
      # Alternatively use chain to schedule always.
      then { repeating_scheduled_task.call(interval, cancellation, task) }
end
# => #<Proc:0x00003f@promises.in.md:951 (lambda)>

cancellation, origin = Concurrent::Cancellation.new
# => #<Concurrent::Cancellation:0x000040 pending>

task = -> cancellation do
  5.times do
    cancellation.check!
    do_stuff
  end
end
# => #<Proc:0x000041@promises.in.md:962 (lambda)>

result = Concurrent::Promises.future(0.1, cancellation, task, &repeating_scheduled_task).run
# => #<Concurrent::Promises::Future:0x000042 pending>
sleep 0.03 
origin.resolve
# => #<Concurrent::Promises::ResolvableEvent:0x000043 resolved>
result.result
# => [false,
#     nil,
#     #<Concurrent::CancelledOperationError: Concurrent::CancelledOperationError>]