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Version: ZIO 1.x


A Semaphore datatype which allows synchronization between fibers with the withPermit operation, which safely acquires and releases a permit. Semaphore is based on Ref[A] datatype.


For example a synchronization of asynchronous tasks can be done via acquiring and releasing a semaphore with given number of permits it can spend. When the acquire operation cannot be performed, due to insufficient permits value in the semaphore, such task is placed in internal suspended fibers queue and will be awaken when permits value is sufficient:

import java.util.concurrent.TimeUnit
import zio._
import zio.console._
import zio.duration.Duration

val task = for {
_ <- putStrLn("start")
_ <- ZIO.sleep(Duration(2, TimeUnit.SECONDS))
_ <- putStrLn("end")
} yield ()

val semTask = (sem: Semaphore) => for {
_ <- sem.withPermit(task)
} yield ()

val semTaskSeq = (sem: Semaphore) => (1 to 3).map(_ => semTask(sem))

val program = for {

sem <- Semaphore.make(permits = 1)

seq <- ZIO.effectTotal(semTaskSeq(sem))

_ <- ZIO.collectAllPar(seq)

} yield ()

As the binary semaphore is a special case of counting semaphore we can acquire and release any value, regarding semaphore's permits:

val semTaskN = (sem: Semaphore) => for {
_ <- sem.withPermits(5)(task)
} yield ()

The guarantee of withPermit (and its corresponding counting version withPermits) is that acquisition will be followed by equivalent release, regardless of whether the task succeeds, fails, or is interrupted.