MVar

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An MVar is a mutable location that can be empty or contains a value, asynchronously blocking reads when empty and blocking writes when full.

Introduction

Use-cases:

  1. As synchronized, thread-safe mutable variables
  2. As channels, with take and put acting as “receive” and “send”
  3. As a binary semaphore, with take and put acting as “acquire” and “release”

It has two fundamental (atomic) operations:

  • put: fills the MVar if it is empty, or blocks (asynchronously) if the MVar is full, until the given value is next in line to be consumed on take
  • take: tries reading the current value, or blocks (asynchronously) until there is a value available, at which point the operation resorts to a take followed by a put

An additional but non-atomic operation is read, which tries reading the current value, or blocks (asynchronously) until there is a value available, at which point the operation resorts to a take followed by a put.

In this context “asynchronous blocking” means that we are not blocking any threads. Instead the implementation uses callbacks to notify clients when the operation has finished (notifications exposed by means of Task) and it thus works on top of Javascript as well.

Inspiration

This data type is inspired by Control.Concurrent.MVar from Haskell, introduced in the paper Concurrent Haskell, by Simon Peyton Jones, Andrew Gordon and Sigbjorn Finne, though some details of their implementation are changed (in particular, a put on a full MVar used to error, but now merely blocks).

Appropriate for building synchronization primitives and performing simple interthread communication, it’s the equivalent of a BlockingQueue(capacity = 1), except that there’s no actual thread blocking involved and it is powered by Task.

Use-case: Synchronized Mutable Variables

import monix.execution.CancelableFuture
import monix.eval.{MVar, Task}

def sum(state: MVar[Int], list: List[Int]): Task[Int] =
  list match {
    case Nil => state.take
    case x :: xs =>
      state.take.flatMap { current =>
        state.put(current + x).flatMap(_ => sum(state, xs))
      }
  }

val state = MVar(0)
val task = sum(state, (0 until 100).toList)

// Evaluate
val f: CancelableFuture[Int] = task.runAsync

This sample isn’t very useful, except to show how MVar can be used as a variable. The take and put operations are atomic. The take call will (asynchronously) block if there isn’t a value available, whereas the call to put blocks if the MVar already has a value in it waiting to be consumed.

Obviously after the call for take and before the call for put happens we could have concurrent logic that can update the same variable. While the two operations are atomic by themselves, a combination of them isn’t atomic (i.e. atomic operations don’t compose), therefore if we want this sample to be safe, then we need extra synchronization.

Use-case: Asynchronous Lock (Binary Semaphore, Mutex)

The take operation can act as “acquire” and put can act as the “release”. Let’s do it:

final class MLock {
  private[this] val mvar = MVar(())

  def acquire: Task[Unit] =
    mvar.take

  def release: Task[Unit] =
    mvar.put(())

  def greenLight[A](fa: Task[A]): Task[A] =
    for {
      _ <- acquire
      a <- fa.doOnCancel(release)
      _ <- release
    } yield a
}

And now we can apply synchronization to the previous example:

val lock = new MLock
val state = MVar(0)
val task = sum(state, (0 until 100).toList)

val atomicTask = lock.greenLight(task)

// Evaluate
val f: CancelableFuture[Int] = atomicTask.runAsync

Use-case: Producer/Consumer Channel

An obvious use-case is to model a simple producer-consumer channel.

Say that you have a producer that needs to push events. But we also need some back-pressure, so we need to wait on the consumer to consume the last event before being able to generate a new event.

// Signaling option, because we need to detect completion
type Channel[A] = MVar[Option[A]]

def producer(ch: Channel[Int], list: List[Int]): Task[Unit] =
  list match {
    case Nil =>
      ch.put(None) // we are done!
    case head :: tail =>
      // next please
      ch.put(Some(head)).flatMap(_ => producer(ch, tail))
  }

def consumer(ch: Channel[Int], sum: Long): Task[Long] =
  ch.take.flatMap {
    case Some(x) =>
      // next please
      consumer(ch, sum + x)
    case None =>
      Task.now(sum) // we are done!
  }

val channel = MVar.empty[Option[Int]]
val count = 100000

val producerTask = producer(channel, (0 until count).toList).executeWithFork
val consumerTask = consumer(channel, 0L).executeWithFork

// Ensure they run in parallel, not really necessary, just for kicks
val sumTask = Task.mapBoth(producerTask, consumerTask)((_,sum) => sum)

// Evaluate
val f: CancelableFuture[Long] = sumTask.runAsync

Running this will work as expected. Our producer pushes values into our MVar and our consumer will consume all of those values.