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Backpressure.scala
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Backpressure.scala
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/*
* Copyright 2020-2022 Typelevel
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package cats.effect.std
import cats.effect.kernel._
import cats.effect.kernel.implicits._
import cats.syntax.all._
/**
* Utility to apply backpressure semantics to the execution of an Effect. Backpressure instances
* will apply a [[Backpressure.Strategy]] to the execution where each strategy works as follows:
*
* [[Backpressure.Strategy.Lossy]] will mean that effects will not be run in the presence of
* backpressure, meaning the result will be None
*
* [[Backpressure.Strategy.Lossless]] will mean that effects will run in the presence of
* backpressure, meaning the effect will semantically block until backpressure is alleviated
*/
trait Backpressure[F[_]] {
/**
* Applies rate limiting to an effect based on backpressure semantics
*
* @param f
* the effect that backpressure is applied to
* @return
* an Option where Option denotes if the effect was run or not according to backpressure
* semantics
*/
def metered[A](f: F[A]): F[Option[A]]
}
object Backpressure {
/**
* Creates an instance of Backpressure that can be used to rate limit effects
* @param strategy
* strategy to apply for this backpressure instance
* @param bound
* depth of the queue that the backpressure instance should manage
* @return
* a [[Backpressure]] instance
*/
def apply[F[_]](
strategy: Strategy,
bound: Int
)(implicit GC: GenConcurrent[F, _]): F[Backpressure[F]] = {
require(bound > 0)
val notAcquired: F[None.type] = GC.pure(None)
Semaphore[F](bound.toLong).map(sem =>
strategy match {
case Strategy.Lossy =>
new Backpressure[F] {
override def metered[A](f: F[A]): F[Option[A]] =
sem
.tryAcquire
.bracket {
case true => f.map(_.some)
case false => notAcquired.widen[Option[A]]
} {
case true => sem.release
case false => GC.unit
}
}
case Strategy.Lossless =>
new Backpressure[F] {
override def metered[A](f: F[A]): F[Option[A]] =
sem.acquire.bracket(_ => f.map(_.some))(_ => sem.release)
}
})
}
sealed trait Strategy
object Strategy {
case object Lossy extends Strategy
case object Lossless extends Strategy
}
}