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Topic.scala
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Topic.scala
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/*
* Copyright (c) 2013 Functional Streams for Scala
*
* Permission is hereby granted, free of charge, to any person obtaining a copy of
* this software and associated documentation files (the "Software"), to deal in
* the Software without restriction, including without limitation the rights to
* use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
* the Software, and to permit persons to whom the Software is furnished to do so,
* subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in all
* copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
* FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
* COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
* IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
* CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/
package fs2
package concurrent
import cats.effect._
import cats.effect.implicits._
import cats.syntax.all._
import scala.collection.immutable.LongMap
/** Topic allows you to distribute `A`s published by an arbitrary
* number of publishers to an arbitrary number of subscribers.
*
* Topic has built-in back-pressure support implemented as the maximum
* number of elements (`maxQueued`) that a subscriber is allowed to enqueue.
*
* Once that bound is hit, any publishing action will semantically
* block until the lagging subscriber consumes some of its queued
* elements.
*/
abstract class Topic[F[_], A] { self =>
/** Publishes elements from source of `A` to this topic.
* [[Pipe]] equivalent of `publish1`.
* Closes the topic when the input stream terminates.
* Especially useful when the topic has a single producer.
*/
def publish: Pipe[F, A, Nothing]
/** Publishes one `A` to topic.
* No-op if the channel is closed, see [[close]] for further info.
*
* This operation does not complete until after the given element
* has been enqued on all subscribers, which means that if any
* subscriber is at its `maxQueued` limit, `publish1` will
* semantically block until that subscriber consumes an element.
*
* A semantically blocked publication can be interrupted, but there is
* no guarantee of atomicity, and it could result in the `A` being
* received by some subscribers only.
*
* Note: if `publish1` is called concurrently by multiple producers,
* different subscribers may receive messages from different producers
* in a different order.
*/
def publish1(a: A): F[Either[Topic.Closed, Unit]]
/** Subscribes for `A` values that are published to this topic.
*
* Pulling on the returned stream opens a "subscription", which allows up to
* `maxQueued` elements to be enqueued as a result of publication.
*
* If at any point, the queue backing the subscription has `maxQueued` elements in it,
* any further publications semantically block until elements are dequeued from the
* subscription queue.
*
* @param maxQueued maximum number of elements to enqueue to the subscription
* queue before blocking publishers
*/
def subscribe(maxQueued: Int): Stream[F, A]
/** Like `subscribe`, but allows an unbounded number of elements to enqueue to the subscription
* queue.
*/
def subscribeUnbounded: Stream[F, A] =
subscribe(Int.MaxValue)
/** Like `subscribe`, but represents the subscription explicitly as
* a `Resource` which returns after the subscriber is subscribed,
* but before it has started pulling elements.
*/
def subscribeAwait(maxQueued: Int): Resource[F, Stream[F, A]]
/** Like `subscribeAwait`, but allows an unbounded number of elements to enqueue to the subscription
* queue.
*/
def subscribeAwaitUnbounded: Resource[F, Stream[F, A]] =
subscribeAwait(Int.MaxValue)
/** Signal of current active subscribers.
*/
def subscribers: Stream[F, Int]
/** This method achieves graceful shutdown: when the topics gets
* closed, its subscribers will terminate naturally after consuming all
* currently enqueued elements.
*
* "Termination" here means that subscribers no longer
* wait for new elements on the topic, and not that they will be
* interrupted while performing another action: if you want to
* interrupt a subscriber, without first processing enqueued
* elements, you should use `interruptWhen` on it instead.
*
* After a call to `close`, any further calls to `publish1` or `close`
* will be no-ops.
*
* Note that `close` does not automatically unblock producers which
* might be blocked on a bound, they will only become unblocked
* if/when subscribers naturally finish to consume the respective elements.
* You can `race` the publish with `close` to interrupt them immediately.
*/
def close: F[Either[Topic.Closed, Unit]]
/** Returns true if this topic is closed */
def isClosed: F[Boolean]
/** Semantically blocks until the topic gets closed. */
def closed: F[Unit]
/** Returns an alternate view of this `Topic` where its elements are of type `B`,
* given two functions, `A => B` and `B => A`.
*/
def imap[B](f: A => B)(g: B => A): Topic[F, B] =
new Topic[F, B] {
def publish: Pipe[F, B, Nothing] = sfb => self.publish(sfb.map(g))
def publish1(b: B): F[Either[Topic.Closed, Unit]] = self.publish1(g(b))
def subscribe(maxQueued: Int): Stream[F, B] =
self.subscribe(maxQueued).map(f)
def subscribeAwait(maxQueued: Int): Resource[F, Stream[F, B]] =
self.subscribeAwait(maxQueued).map(_.map(f))
def subscribers: Stream[F, Int] = self.subscribers
def close: F[Either[Topic.Closed, Unit]] = self.close
def isClosed: F[Boolean] = self.isClosed
def closed: F[Unit] = self.closed
}
}
object Topic {
type Closed = Closed.type
object Closed
/** Constructs a Topic */
def apply[F[_], A](implicit F: Concurrent[F]): F[Topic[F, A]] =
(
F.ref(LongMap.empty[Channel[F, A]] -> 1L),
SignallingRef[F, Int](0),
F.deferred[Unit]
).mapN { case (state, subscriberCount, signalClosure) =>
new Topic[F, A] {
def foreach[B](lm: LongMap[B])(f: B => F[Unit]) =
lm.foldLeft(F.unit) { case (op, (_, b)) => op >> f(b) }
def publish1(a: A): F[Either[Topic.Closed, Unit]] =
signalClosure.tryGet.flatMap {
case Some(_) => Topic.closed.pure[F]
case None =>
state.get
.flatMap { case (subs, _) => foreach(subs)(_.send(a).void) }
.as(Topic.rightUnit)
}
def subscribeAwait(maxQueued: Int): Resource[F, Stream[F, A]] =
Resource
.eval(Channel.bounded[F, A](maxQueued))
.flatMap(subscribeAwaitImpl)
override def subscribeAwaitUnbounded: Resource[F, Stream[F, A]] =
Resource
.eval(Channel.unbounded[F, A])
.flatMap(subscribeAwaitImpl)
def subscribeAwaitImpl(chan: Channel[F, A]): Resource[F, Stream[F, A]] = {
val subscribe = state.modify { case (subs, id) =>
(subs.updated(id, chan), id + 1) -> id
} <* subscriberCount.update(_ + 1)
def unsubscribe(id: Long) =
state.modify { case (subs, nextId) =>
// _After_ we remove the bounded channel for this
// subscriber, we need to drain it to unblock to
// publish loop which might have already enqueued
// something.
def drainChannel: F[Unit] =
subs.get(id).traverse_ { chan =>
chan.close >> chan.stream.compile.drain
}
(subs - id, nextId) -> drainChannel
}.flatten >> subscriberCount.update(_ - 1)
Resource.eval(signalClosure.tryGet).flatMap {
case Some(_) => Resource.pure(Stream.empty)
case None =>
Resource
.make(subscribe)(unsubscribe)
.as(chan.stream)
}
}
def publish: Pipe[F, A, Nothing] = { in =>
(in ++ Stream.exec(close.void))
.evalMap(publish1)
.takeWhile(_.isRight)
.drain
}
def subscribe(maxQueued: Int): Stream[F, A] =
Stream.resource(subscribeAwait(maxQueued)).flatten
override def subscribeUnbounded: Stream[F, A] =
Stream.resource(subscribeAwaitUnbounded).flatten
def subscribers: Stream[F, Int] = subscriberCount.discrete
def close: F[Either[Topic.Closed, Unit]] =
signalClosure
.complete(())
.flatMap { completedNow =>
val result = if (completedNow) Topic.rightUnit else Topic.closed
state.get
.flatMap { case (subs, _) => foreach(subs)(_.close.void) }
.as(result)
}
.uncancelable
def closed: F[Unit] = signalClosure.get
def isClosed: F[Boolean] = signalClosure.tryGet.map(_.isDefined)
}
}
private final val closed: Either[Closed, Unit] = Left(Closed)
private final val rightUnit: Either[Closed, Unit] = Right(())
}