ConcurrentQueue

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 = SchedulerEffect.contextShift[IO](global)
// We need a `Timer` for this to work
implicit val timer = 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.

Linear Supertypes

Serializable, Serializable, AnyRef, Any

Value Members

final def !=(arg0: Any): Boolean

Definition Classes
AnyRef → Any
final def ##(): Int

Definition Classes
AnyRef → Any
final def ==(arg0: Any): Boolean

Definition Classes
AnyRef → Any
final def asInstanceOf[T0]: T0

Definition Classes
Any
def clear: F[Unit]

Removes all items from the queue.
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.
def clone(): AnyRef

Attributes
protected[java.lang]
Definition Classes
AnyRef
Annotations
@throws( ... )
def drain(minLength: Int, maxLength: Int): F[Seq[A]]

Fetches multiple elements from the queue, if available.
Fetches multiple elements from the queue, if available.
This operation back-pressures until the minLength requirement is achieved.
minLength
specifies the minimum length of the returned sequence; the operation back-pressures until this length is satisfied
maxLength
is the capacity of the used buffer, being the max length of the returned sequence
returns
a future with a sequence of length between minLength and maxLength; it can also be cancelled, interrupting the wait
final def eq(arg0: AnyRef): Boolean

Definition Classes
AnyRef
def equals(arg0: Any): Boolean

Definition Classes
AnyRef → Any
def finalize(): Unit

Attributes
protected[java.lang]
Definition Classes
AnyRef
Annotations
@throws( classOf[java.lang.Throwable] )
final def getClass(): Class[_]

Definition Classes
AnyRef → Any
def hashCode(): Int

Definition Classes
AnyRef → Any
def isEmpty: F[Boolean]

Checks if the queue is empty.
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.

Annotations
@UnsafeProtocol()
final def isInstanceOf[T0]: Boolean

Definition Classes
Any
final def ne(arg0: AnyRef): Boolean

Definition Classes
AnyRef
final def notify(): Unit

Definition Classes
AnyRef
final def notifyAll(): Unit

Definition Classes
AnyRef
def offer(a: A): F[Unit]

Pushes a value in the queue, or if the queue is full, then repeats the operation until it succeeds.
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
def offerMany(seq: Iterable[A]): F[Unit]

Pushes multiple values in the queue.
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
def poll: F[A]

Fetches a value from the queue, or if the queue is empty it awaits asynchronously until a value is made available.
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
final def synchronized[T0](arg0: ⇒ T0): T0

Definition Classes
AnyRef
def toString(): String

Definition Classes
AnyRef → Any
def tryOffer(a: A): F[Boolean]

Try pushing a value to the queue.
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.
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

Annotations
@UnsafeProtocol()
def tryPoll: F[Option[A]]

Try pulling a value out of the queue.
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

Annotations
@UnsafeProtocol()
final def wait(): Unit

Definition Classes
AnyRef
Annotations
@throws( ... )
final def wait(arg0: Long, arg1: Int): Unit

Definition Classes
AnyRef
Annotations
@throws( ... )
final def wait(arg0: Long): Unit

Definition Classes
AnyRef
Annotations
@throws( ... )

Related Docs: object ConcurrentQueue | package catnap

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

Example

Back-Pressuring and the Polling Model

Multi-threading Scenario

Value Members

final def !=(arg0: Any): Boolean

final def ##(): Int

final def ==(arg0: Any): Boolean

final def asInstanceOf[T0]: T0

def clear: F[Unit]

def clone(): AnyRef

def drain(minLength: Int, maxLength: Int): F[Seq[A]]

final def eq(arg0: AnyRef): Boolean

def equals(arg0: Any): Boolean

def finalize(): Unit

final def getClass(): Class[_]

def hashCode(): Int

def isEmpty: F[Boolean]

final def isInstanceOf[T0]: Boolean

final def ne(arg0: AnyRef): Boolean

final def notify(): Unit

final def notifyAll(): Unit

def offer(a: A): F[Unit]

def offerMany(seq: Iterable[A]): F[Unit]

def poll: F[A]

final def synchronized[T0](arg0: ⇒ T0): T0

def toString(): String

def tryOffer(a: A): F[Boolean]

def tryPoll: F[Option[A]]

final def wait(): Unit

final def wait(arg0: Long, arg1: Int): Unit

final def wait(arg0: Long): Unit

Inherited from Serializable

Inherited from Serializable

Inherited from AnyRef

Inherited from Any

Ungrouped