Examples

Let's start by creating a channel with a capacity of 2 messages.

ch = Concurrent::Promises::Channel.new 2

We push 3 messages, then it can be observed that the last thread pushing is sleeping since the channel is full.

threads = Array.new(3) { |i| Thread.new { ch.push message: i } } #
sleep 0.01 # let the threads run
threads

When message is popped the last thread continues and finishes as well.

ch.pop
threads.map(&:join)

Same principle applies to popping as well. There are now 2 messages int he channel. Lets create 3 threads trying to pop a message, one will be blocked until new messages is pushed.

threads = Array.new(3) { |i| Thread.new { ch.pop } } #
sleep 0.01 # let the threads run
threads 
ch.push message: 3
threads.map(&:value)

Promises integration

However this channel is implemented to integrate with promises therefore all operations can be represented as futures.

ch = Concurrent::Promises::Channel.new 2
push_operations = Array.new(3) { |i| ch.push_op message: i }

We do not have to sleep here letting the futures execute as Threads. Since there is capacity for 2 messages the Promises are immediately resolved without ever allocating a Thread to execute. Push and pop operations are often more efficient. The remaining pending push operation will also never require another thread, instead it will resolve when a message is popped from the channel making a space for a new message.

ch.pop_op.value!
push_operations.map(&:value!)

pop_operations = Array.new(3) { |i| ch.pop_op }
ch.push message: 3 # (push|pop) can be freely mixed with (push_o|pop_op)
pop_operations.map(&:value)

Selecting over channels

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.

ch1    = Concurrent::Promises::Channel.new 2
ch2    = Concurrent::Promises::Channel.new 2
ch1.push 1 
ch2.push 2 

Concurrent::Promises::Channel.select([ch1, ch2])
ch1.select(ch2)

Concurrent::Promises.future { 3 + 4 }.then_channel_push(ch1)
Concurrent::Promises::Channel. 
    # or `ch1.select_op(ch2)` would be equivalent
    select_op([ch1, ch2]).
    then('got number %03d from ch%d') { |(channel, value), format| 
      format format, value, [ch1, ch2].index(channel).succ
    }.value!

try_ variants

All blocking operations (#pop, #push, #select) have non-blocking variant with try_ prefix. They always return immediately and indicate either success or failure.

ch
ch.try_push 1
ch.try_push 2
ch.try_push 3
ch.try_pop
ch.try_pop
ch.try_pop

Timeouts

All blocking operations (#pop, #push, #select) have a timeout option. Similar to try_ variants it will indicate success or timing out, when the timeout option is used.

ch
ch.push 1, 0.01
ch.push 2, 0.01
ch.push 3, 0.01
ch.pop 0.01
ch.pop 0.01
ch.pop 0.01

Backpressure

Most importantly the channel can be used to create systems with backpressure. A self adjusting system where the producers will slow down if the consumers are not keeping up.

channel = Concurrent::Promises::Channel.new 2
log     = Concurrent::Array.new

producers = Array.new 2 do |i|
  Thread.new(i) do |i|
    4.times do |j|
      log.push format "producer %d pushing %d", i, j      
      channel.push [i, j]      
    end
  end
end

consumers = Array.new 4 do |i|
  Thread.new(i) do |consumer|
    2.times do |j|
      from, message = channel.pop
      log.push format "consumer %d got %d. payload %d from producer %d", 
                      consumer, j, message, from       
      do_stuff      
    end
  end
end

# wait for all to finish
producers.map(&:join)
consumers.map(&:join)
# investigate log
log

The producers are much faster than consumers (since they do_stuff which takes some time)
but as it can be seen from the log they fill the channel and then they slow down until there is space available in the channel.

If permanent allocation of threads to the producers and consumers has to be avoided, the threads can be replaced with promises that run a thread pool.

channel = Concurrent::Promises::Channel.new 2
log     = Concurrent::Array.new

def produce(channel, log, producer, i)
  log.push format "producer %d pushing %d", producer, i      
  channel.push_op([producer, i]).then do
    i + 1 < 4 ? produce(channel, log, producer, i + 1) : :done    
  end      
end

def consume(channel, log, consumer, i)
  channel.pop_op.then(consumer, i) do |(from, message), consumer, i|
    log.push format "consumer %d got %d. payload %d from producer %d", 
                    consumer, i, message, from       
    do_stuff
    i + 1 < 2 ? consume(channel, log, consumer, i + 1) : :done       
  end
end

producers = Array.new 2 do |i|
  Concurrent::Promises.future(channel, log, i) { |*args| produce *args, 0 }.run
end

consumers = Array.new 4 do |i|
  Concurrent::Promises.future(channel, log, i) { |*args| consume *args, 0 }.run
end

# wait for all to finish
producers.map(&:value!)
consumers.map(&:value!)
# investigate log
log

Synchronization of workers by passing a value

If the capacity of the channel is zero then any push operation will succeed only when there is a matching pop operation which can take the message. The operations have to be paired to succeed.

channel = Concurrent::Promises::Channel.new 0
thread = Thread.new { channel.pop }; sleep 0.01 #
# allow the thread to go to sleep
thread
# succeeds because there is matching pop operation waiting in the thread 
channel.try_push(:v1)
# remains pending, since there is no matching operation 
push = channel.push_op(:v2)
thread.value
# the push operation resolves as a pairing pop is called
channel.pop
push