monix-catnap/monix.catnap/ConcurrentQueue

ConcurrentQueue

final class ConcurrentQueue[F[_], A] extends Serializable

A high-performance, back-pressured, generic concurrent queue implementation.

This is the pure and generic version of monix.execution.AsyncQueue.

==Example==

 import cats.implicits._
 import cats.effect._
 import monix.execution.Scheduler.global

 // For being able to do IO.start
 implicit val cs: ContextShift[IO] = SchedulerEffect.contextShift[IO](global)(IO.ioEffect)
 // We need a `Timer` for this to work
 implicit val timer: Timer[IO] = SchedulerEffect.timer[IO](global)

 def consumer(queue: ConcurrentQueue[IO, Int], index: Int): IO[Unit] =
   queue.poll.flatMap { a =>
     println(s"Worker $$index: $$a")
     consumer(queue, index)
   }

 for {
   queue     <- ConcurrentQueue[IO].bounded[Int](capacity = 32)
   consumer1 <- consumer(queue, 1).start
   consumer2 <- consumer(queue, 1).start
   // Pushing some samples
   _         <- queue.offer(1)
   _         <- queue.offer(2)
   _         <- queue.offer(3)
   // Stopping the consumer loops
   _         <- consumer1.cancel
   _         <- consumer2.cancel
 } yield ()

==Back-Pressuring and the Polling Model==

The initialized queue can be limited to a maximum buffer size, a size that could be rounded to a power of 2, so you can't rely on it to be precise. Such a bounded queue can be initialized via ConcurrentQueue.bounded. Also see BufferCapacity, the configuration parameter that can be passed in the ConcurrentQueue.withConfig builder.

On offer, when the queue is full, the implementation back-pressures until the queue has room again in its internal buffer, the task being completed when the value was pushed successfully. Similarly poll awaits the queue to have items in it. This works for both bounded and unbounded queues.

For both offer and poll, in case awaiting a result happens, the implementation does so asynchronously, without any threads being blocked.

==Multi-threading Scenario==

This queue supports a ChannelType configuration, for fine tuning depending on the needed multi-threading scenario. And this can yield better performance:

MPMC: multi-producer, multi-consumer
MPSC: multi-producer, single-consumer
SPMC: single-producer, multi-consumer
SPSC: single-producer, single-consumer

The default is MPMC, because that's the safest scenario.

 import monix.execution.ChannelType.MPSC
 import monix.execution.BufferCapacity.Bounded

 val queue = ConcurrentQueue[IO].withConfig[Int](
   capacity = Bounded(128),
   channelType = MPSC
 )

'''WARNING''': default is MPMC, however any other scenario implies a relaxation of the internal synchronization between threads.

This means that using the wrong scenario can lead to severe concurrency bugs. If you're not sure what multi-threading scenario you have, then just stick with the default MPMC.

Companion: object

trait Serializable

class Object

trait Matchable

class Any

Value members

Concrete methods

Removes all items from the queue.

Called from the consumer thread, subject to the restrictions appropriate to the implementation indicated by ChannelType.

'''WARNING:''' the clear operation should be done on the consumer side, so it must be called from the same thread(s) that call poll.

Fetches multiple elements from the queue, if available.

This operation back-pressures until the minLength requirement is achieved.

Value Params

maxLength: is the capacity of the used buffer, being the max length of the returned sequence
minLength: specifies the minimum length of the returned sequence; the operation back-pressures until this length is satisfied

Returns

a future with a sequence of length between minLength and maxLength; it can also be cancelled, interrupting the wait

@UnsafeProtocol

Checks if the queue is empty.

'''UNSAFE PROTOCOL:''' Concurrent shared state changes very frequently, therefore this function might yield nondeterministic results. Should be used carefully since some usecases might require a deeper insight into concurrent programming.

Pushes a value in the queue, or if the queue is full, then repeats the operation until it succeeds.

Returns: a task that when evaluated, will complete with a value, or wait until such a value is ready

Pushes multiple values in the queue. Back-pressures if the queue is full.

Returns: a task that will eventually complete when the push has succeeded; it can also be cancelled, interrupting the waiting

Fetches a value from the queue, or if the queue is empty it awaits asynchronously until a value is made available.

Returns: a task that when evaluated, will eventually complete after the value has been successfully pushed in the queue

@UnsafeProtocol

Try pushing a value to the queue.

The protocol is unsafe because usage of the "try*" methods imply an understanding of concurrency, or otherwise the code can be very fragile and buggy.

Value Params

a: is the value pushed in the queue

Returns

true if the operation succeeded, or false if the queue is full and cannot accept any more elements

@UnsafeProtocol

Try pulling a value out of the queue.

The protocol is unsafe because usage of the "try*" methods imply an understanding of concurrency, or otherwise the code can be very fragile and buggy.

Returns: Some(a) in case a value was successfully retrieved from the queue, or None in case the queue is empty