ZIO

A ZIO[R, E, A] value is an immutable value that lazily describes a workflow or job. The workflow requires some environment R, and may fail with an error of type E, or succeed with a value of type A.

A value of type ZIO[R, E, A] is like an effectful version of the following function type:

R => Either[E, A]

This function, which requires an R, might produce either an E, representing failure, or an A, representing success. ZIO effects are not actually functions, of course, they can model synchronous, asynchronous, concurrent, parallel, and resourceful computations.

ZIO effects use a fiber-based concurrency model, with built-in support for scheduling, fine-grained interruption, structured concurrency, and high scalability.

The ZIO[R, E, A] data type has three type parameters:

• R - Environment Type. The effect requires an environment of type R. If this type parameter is Any, it means the effect has no requirements, because we can run the effect with any value (for example, the unit value ()).
• E - Failure Type. The effect may fail with a value of type E. Some applications will use Throwable. If this type parameter is Nothing, it means the effect cannot fail, because there are no values of type Nothing.
• A - Success Type. The effect may succeed with a value of type A. If this type parameter is Unit, it means the effect produces no useful information, while if it is Nothing, it means the effect runs forever (or until failure).

In the following example, the readLine function does not require any services, it may fail with value of type IOException, or may succeed with a value of type String:

import zio._
import java.io.IOException

val readLine: ZIO[Any, IOException, String] =
  Console.readLine

ZIO values are immutable, and all ZIO functions produce new ZIO values, enabling ZIO to be reasoned about and used like any ordinary Scala immutable data structure.

ZIO values do not actually do anything; they are just values that model or describe effectful interactions.

ZIO can be interpreted by the ZIO runtime system into effectful interactions with the external world. Ideally, this occurs at a single time, in our application's main function. The App class provides this functionality automatically.

Creation

In this section we explore some of the common ways to create ZIO effects from values, from common Scala types, and from both synchronous and asynchronous side-effects. Here is the summary list of them:

Success Values

Using the ZIO.succeed method, we can create an effect that succeeds with the specified value:

import zio._

val s1 = ZIO.succeed(42)

Failure Values

Using the ZIO.fail method, we can create an effect that models failure:

import zio._

val f1 = ZIO.fail("Uh oh!")

For the ZIO data type, there is no restriction on the error type. We may use strings, exceptions, or custom data types appropriate for our application.

Many applications will model failures with classes that extend Throwable or Exception:

import zio._

val f2 = ZIO.fail(new Exception("Uh oh!"))

From Values

ZIO contains several constructors which help us to convert various data types into ZIO effects.

Option

1. ZIO.fromOption— An Option can be converted into a ZIO effect using ZIO.fromOption:

import zio._

val zoption: IO[Option[Nothing], Int] = ZIO.fromOption(Some(2))

The error type of the resulting effect is Option[Nothing], which provides no information on why the value is not there. We can change the Option[Nothing] into a more specific error type using ZIO#mapError:

import zio._

val zoption2: IO[String, Int] = zoption.mapError(_ => "It wasn't there!")

We can also readily compose it with other operators while preserving the optional nature of the result (similar to an OptionT):

import zio._

val maybeId: IO[Option[Nothing], String] = ZIO.fromOption(Some("abc123"))
def getUser(userId: String): IO[Throwable, Option[User]] = ???
def getTeam(teamId: String): IO[Throwable, Team] = ???


val result: IO[Throwable, Option[(User, Team)]] = (for {
  id   <- maybeId
  user <- getUser(id).some
  team <- getTeam(user.teamId).asSomeError
} yield (user, team)).unsome

2. ZIO.some/ZIO.none— These constructors can be used to directly create ZIO of optional values:

import zio._

val someInt: ZIO[Any, Nothing, Option[Int]]     = ZIO.some(3)
val noneInt: ZIO[Any, Nothing, Option[Nothing]] = ZIO.none

3. ZIO.getOrFail— We can lift an Option into a ZIO and if the option is not defined we can fail the ZIO with the proper error type:

• ZIO.getOrFail fails with Throwable error type.
• ZIO.getOrFailUnit fails with Unit error type.
• ZIO.getOrFailWith fails with custom error type.

import zio._

def parseInt(input: String): Option[Int] = input.toIntOption

// If the optional value is not defined it fails with Throwable error type:
val r1: ZIO[Any, Throwable, Int] =
  ZIO.getOrFail(parseInt("1.2"))

// If the optional value is not defined it fails with Unit error type:
val r2: ZIO[Any, Unit, Int] =
  ZIO.getOrFailUnit(parseInt("1.2"))

// If the optional value is not defined it fail with given error type:
val r3: ZIO[Any, NumberFormatException, Int] =
  ZIO.getOrFailWith(new NumberFormatException("invalid input"))(parseInt("1.2"))

4. ZIO.nonOrFail— It lifts an option into a ZIO value. If the option is empty it succeeds with Unit and if the option is defined it fails with a proper error type:

• ZIO.noneOrFail fails with the content of the optional value.
• ZIO.noneOrFailUnit fails with the Unit error type.
• ZIO.noneOrFailWith fails with custom error type.

import zio._

val optionalValue: Option[String] = ???

// If the optional value is empty it succeeds with Unit
// If the optional value is defined it will fail with the content of the optional value
val r1: ZIO[Any, String, Unit] =
  ZIO.noneOrFail(optionalValue)

// If the optional value is empty it succeeds with Unit
// If the optional value is defined, it will fail by applying the error function to it:
val r2: ZIO[Any, NumberFormatException, Unit] =
  ZIO.noneOrFailWith(optionalValue)(e => new NumberFormatException(e))

Either

Function	Input Type	Output Type
fromEither	Either[E, A]	IO[E, A]
left	A	UIO[Either[A, Nothing]]
right	A	UIO[Either[Nothing, A]]

An Either can be converted into a ZIO effect using ZIO.fromEither:

import zio._

val zeither = ZIO.fromEither(Right("Success!"))

The error type of the resulting effect will be whatever type the Left case has, while the success type will be whatever type the Right case has.

Try

Function	Input Type	Output Type
fromTry	scala.util.Try[A]	Task[A]

A Try value can be converted into a ZIO effect using ZIO.fromTry:

import zio._
import scala.util.Try

val ztry = ZIO.fromTry(Try(42 / 0))

The error type of the resulting effect will always be Throwable, because Try can only fail with values of type Throwable.

Future

Function	Input Type	Output Type
fromFuture	ExecutionContext => scala.concurrent.Future[A]	Task[A]
fromFutureJava	java.util.concurrent.Future[A]	RIO[Blocking, A]
fromFunctionFuture	R => scala.concurrent.Future[A]	RIO[R, A]
fromFutureInterrupt	ExecutionContext => scala.concurrent.Future[A]	Task[A]

A Future can be converted into a ZIO effect using ZIO.fromFuture:

import zio._
import scala.concurrent.Future

lazy val future = Future.successful("Hello!")

val zfuture: Task[String] =
  ZIO.fromFuture { implicit ec =>
    future.map(_ => "Goodbye!")
  }

The function passed to fromFuture is passed an ExecutionContext, which allows ZIO to manage where the Future runs (of course, we can ignore this ExecutionContext).

The error type of the resulting effect will always be Throwable, because Future can only fail with values of type Throwable.

Promise

Function	Input Type	Output Type
fromPromiseScala	scala.concurrent.Promise[A]	Task[A]

A Promise can be converted into a ZIO effect using ZIO.fromPromiseScala:

import zio._
import scala.util._

val func: String => String = s => s.toUpperCase
for {
  promise <- ZIO.succeed(scala.concurrent.Promise[String]())
  _ <- ZIO.attempt {
    Try(func("hello world from future")) match {
      case Success(value) => promise.success(value)
      case Failure(exception) => promise.failure(exception)
    }
  }.fork
  value <- ZIO.fromPromiseScala(promise)
  _ <- Console.printLine(s"Hello World in UpperCase: $value")
} yield ()

Fiber

Function	Input Type	Output Type
fromFiber	Fiber[E, A]	IO[E, A]
fromFiberZIO	IO[E, Fiber[E, A]]	IO[E, A]

A Fiber can be converted into a ZIO effect using ZIO.fromFiber:

import zio._

val io: IO[Nothing, String] = ZIO.fromFiber(Fiber.succeed("Hello from Fiber!"))

From Side-Effects

ZIO can convert both synchronous and asynchronous side-effects into ZIO effects (pure values).

These functions can be used to wrap procedural code, allowing us to seamlessly use all features of ZIO with legacy Scala and Java code, as well as third-party libraries.

Synchronous

Function	Input Type	Output Type	Note
succeed	A	UIO[A]	Imports a total synchronous effect
attempt	A	Task[A]	Imports a (partial) synchronous side-effect

A synchronous side-effect can be converted into a ZIO effect using ZIO.attempt:

import zio._
import scala.io.StdIn

val getLine: Task[String] =
  ZIO.attempt(StdIn.readLine())

The error type of the resulting effect will always be Throwable, because side-effects may throw exceptions with any value of type Throwable.

If a given side-effect is known to not throw any exceptions, then the side-effect can be converted into a ZIO effect using ZIO.succeed:

import zio._

def printLine(line: String): UIO[Unit] =
  ZIO.succeed(println(line))

val succeedTask: UIO[Long] =
  ZIO.succeed(java.lang.System.nanoTime())

We should be careful when using ZIO.succeed—when in doubt about whether or not a side-effect is total, prefer ZIO.attempt to convert the effect.

If this is too broad, the refineOrDie method of ZIO may be used to retain only certain types of exceptions, and to die on any other types of exceptions:

import zio._
import java.io.IOException

val printLine2: IO[IOException, String] =
  ZIO.attempt(scala.io.StdIn.readLine()).refineToOrDie[IOException]

Blocking Synchronous Side-Effects

Function	Input Type	Output Type
blocking	ZIO[R, E, A]	ZIO[R, E, A]
attemptBlocking	A	RIO[Blocking, A]
attemptBlockingCancelable	effect: => A, cancel: UIO[Unit]	RIO[Blocking, A]
attemptBlockingInterrupt	A	RIO[Blocking, A]
attemptBlockingIO	A	ZIO[Blocking, IOException, A]

By default, ZIO is asynchronous and all effects will be executed on a default primary thread pool which is optimized for asynchronous operations. As ZIO uses a fiber-based concurrency model, if we run Blocking I/O or CPU Work workloads on a primary thread pool, they are going to monopolize all threads of primary thread pool.

ZIO has a separate blocking thread pool specially designed for Blocking I/O and, also CPU Work workloads. We should run blocking workloads on this thread pool by using ZIO.blocking or ZIO.attemptBlocking* constructors to prevent interfering with the primary thread pool.

note

ZIO has an auto-blocking mechanism that detects blocking operations and runs them on a separate blocking thread pool. However, if you know that some code is blocking you can use the ZIO.blocking constructor to give a "hint" of this to the ZIO runtime.

The blocking operator takes a ZIO effect and returns another effect that is going to run on a blocking thread pool:

val program = ZIO.foreachPar((1 to 100).toArray)(t => ZIO.blocking(blockingTask(t)))

A blocking side-effect can be converted directly into a ZIO effect using the attemptBlocking method:

import zio._

def blockingTask(n: Int) = ZIO.attemptBlocking {
  do {
    println(s"Running blocking task number $n on dedicated blocking thread pool")
    Thread.sleep(3000)
  } while (true)
}

The resulting effect will be executed on a separate thread pool designed specifically for blocking effects.

Blocking side-effects can be interrupted by invoking Thread.interrupt using the attemptBlockingInterrupt method.

Some blocking side-effects can only be interrupted by invoking a cancellation effect. We can convert these side-effects using the attemptBlockingCancelable method:

import zio._
import java.net.ServerSocket

def accept(l: ServerSocket) =
  ZIO.attemptBlockingCancelable(l.accept())(ZIO.succeed(l.close()))

If a side-effect has already been converted into a ZIO effect, then instead of attemptBlocking, the blocking method can be used to ensure the effect will be executed on the blocking thread pool:

import zio._
import scala.io.{ Codec, Source }

def download(url: String) =
  ZIO.attempt {
    Source.fromURL(url)(Codec.UTF8).mkString
  }

def safeDownload(url: String) =
  ZIO.blocking(download(url))

Asynchronous

Function	Input Type	Output Type
async	(ZIO[R, E, A] => Unit) => Any	ZIO[R, E, A]
asyncZIO	(ZIO[R, E, A] => Unit) => ZIO[R, E, Any]	ZIO[R, E, A]
asyncMaybe	(ZIO[R, E, A] => Unit) => Option[ZIO[R, E, A]]	ZIO[R, E, A]
asyncInterrupt	(ZIO[R, E, A] => Unit) => Either[URIO[R, Any], ZIO[R, E, A]]	ZIO[R, E, A]

An asynchronous side-effect with a callback-based API can be converted into a ZIO effect using ZIO.async:

import zio._

object legacy {
  def login(
    onSuccess: User => Unit,
    onFailure: AuthError => Unit): Unit = ???
}

val login: IO[AuthError, User] =
  ZIO.async[Any, AuthError, User] { callback =>
    legacy.login(
      user => callback(ZIO.succeed(user)),
      err  => callback(ZIO.fail(err))
    )
  }

Asynchronous ZIO effects are much easier to use than callback-based APIs, and they benefit from ZIO features like interruption, resource-safety, and superior error handling.

Creating Suspended Effects

Function	Input Type	Output Type
suspend	RIO[R, A]	RIO[R, A]
suspendSucceed	ZIO[R, E, A]	ZIO[R, E, A]

A RIO[R, A] effect can be suspended using suspend function:

import zio._
import java.io.IOException

val suspendedEffect: RIO[Any, ZIO[Any, IOException, Unit]] =
  ZIO.suspend(ZIO.attempt(Console.printLine("Suspended Hello World!")))

Mapping

map

We can change an IO[E, A] to an IO[E, B] by calling the map method with a function A => B. This lets us transform values produced by actions into other values.

import zio._

val mappedValue: UIO[Int] = ZIO.succeed(21).map(_ * 2)

Tapping

Using ZIO.tap we can peek into a success value and perform any effectful operation, without changing the returning value of the original effect:

trait ZIO[-R, +E, +A] {
  def tap[R1 <: R, E1 >: E](f: A => ZIO[R1, E1, Any]): ZIO[R1, E1, A]
  def tapSome[R1 <: R, E1 >: E](f: PartialFunction[A, ZIO[R1, E1, Any]]): ZIO[R1, E1, A]
}

import zio._

import java.io.IOException

object MainApp extends ZIOAppDefault {
  def isPrime(n: Int): Boolean =
    if (n <= 1) false else (2 until n).forall(i => n % i != 0)

  val myApp: ZIO[Any, IOException, Unit] =
    for {
      ref <- Ref.make(List.empty[Int])
      prime <-
        Random
          .nextIntBetween(0, Int.MaxValue)
          .tap(random => ref.update(_ :+ random))
          .repeatUntil(isPrime)
      _ <- Console.printLine(s"found a prime number: $prime")
      tested <- ref.get
      _ <- Console.printLine(
        s"list of tested numbers: ${tested.mkString(", ")}"
      )
    } yield ()

  def run = myApp
}

Chaining

We can execute two actions in sequence with the flatMap method. The second action may depend on the value produced by the first action.

import zio._

val chainedActionsValue: UIO[List[Int]] = ZIO.succeed(List(1, 2, 3)).flatMap { list =>
  ZIO.succeed(list.map(_ + 1))
}

If the first effect fails, the callback passed to flatMap will never be invoked, and the composed effect returned by flatMap will also fail.

In any chain of effects, the first failure will short-circuit the whole chain, just like throwing an exception will prematurely exit a sequence of statements.

Because the ZIO data type supports both flatMap and map, we can use Scala's for comprehensions to build sequential effects:

import zio._

val program =
  for {
    _    <- Console.printLine("Hello! What is your name?")
    name <- Console.readLine
    _    <- Console.printLine(s"Hello, ${name}, welcome to ZIO!")
  } yield ()

For comprehensions provide a more procedural syntax for composing chains of effects.

Zipping

We can combine two effects into a single effect with the zip method. The resulting effect succeeds with a tuple that contains the success values of both effects:

import zio._

val zipped: UIO[(String, Int)] =
  ZIO.succeed("4").zip(ZIO.succeed(2))

Note that zip operates sequentially: the effect on the left side is executed before the effect on the right side.

In any zip operation, if either the left or right-hand sides fail, then the composed effect will fail, because both values are required to construct the tuple.

zipLeft and zipRight

Sometimes, when the success value of an effect is not useful (for example, it is Unit), it can be more convenient to use the zipLeft or zipRight functions, which first perform a zip, and then map over the tuple to discard one side or the other:

import zio._

val zipRight1 =
  Console.printLine("What is your name?").zipRight(Console.readLine)

The zipRight and zipLeft functions have symbolic aliases, known as *> and <*, respectively. Some developers find these operators easier to read:

import zio._

val zipRight2 =
  Console.printLine("What is your name?") *>
  Console.readLine

Parallelism

ZIO provides many operations for performing effects in parallel. These methods are all named with a Par suffix that helps us identify opportunities to parallelize our code.

For example, the ordinary ZIO#zip method zips two effects together, sequentially. But there is also a ZIO#zipPar method, which zips two effects together in parallel.

The following table summarizes some of the sequential operations and their corresponding parallel versions:

Description	Sequential	Parallel
Zip two effects into one	ZIO#zip	ZIO#zipPar
Zip two effects into one	ZIO#zipWith	ZIO#zipWithPar
Collect from many effects	ZIO.collectAll	ZIO.collectAllPar
Effectfully loop over values	ZIO.foreach	ZIO.foreachPar
Reduce many values	ZIO.reduceAll	ZIO.reduceAllPar
Merge many values	ZIO.mergeAll	ZIO.mergeAllPar

For all the parallel operations, if one effect fails, then others will be interrupted, to minimize unnecessary computation.

If the fail-fast behavior is not desired, potentially failing effects can be first converted into infallible effects using the ZIO#either or ZIO#option methods.

Racing

ZIO lets us race multiple effects in parallel, returning the first successful result:

import zio._

for {
  winner <- ZIO.succeed("Hello").race(ZIO.succeed("Goodbye"))
} yield winner

If we want the first success or failure, rather than the first success, then we can use left.either race right.either, for any effects left and right.

Timeout

ZIO lets us timeout any effect using the ZIO#timeout method, which returns a new effect that succeeds with an Option. A value of None indicates the timeout elapsed before the effect completed.

import zio._

ZIO.succeed("Hello").timeout(10.seconds)

If an effect times out, then instead of continuing to execute in the background, it will be interrupted so no resources will be wasted.

Error Management

Either

Function	Input Type	Output Type
ZIO#either	ZIO[R, E, A]	URIO[R, Either[E, A]]
ZIO.absolve	ZIO[R, E, Either[E, A]]	ZIO[R, E, A]

We can surface failures with ZIO#either, which takes a ZIO[R, E, A] and produces a ZIO[R, Nothing, Either[E, A]].

val zeither: UIO[Either[String, Int]] =
  ZIO.fail("Uh oh!").either

We can submerge failures with ZIO.absolve, which is the opposite of either and turns a ZIO[R, Nothing, Either[E, A]] into a ZIO[R, E, A]:

def sqrt(io: UIO[Double]): IO[String, Double] =
  ZIO.absolve(
    io.map(value =>
      if (value < 0.0) Left("Value must be >= 0.0")
      else Right(Math.sqrt(value))
    )
  )

Catching

Function	Input Type	Output Type
ZIO#catchAll	E => ZIO[R1, E2, A1]	ZIO[R1, E2, A1]
ZIO#catchAllCause	Cause[E] => ZIO[R1, E2, A1]	ZIO[R1, E2, A1]
ZIO#catchAllDefect	Throwable => ZIO[R1, E1, A1]	ZIO[R1, E1, A1]
ZIO#catchAllTrace	((E, Option[StackTrace])) => ZIO[R1, E2, A1]	ZIO[R1, E2, A1]
ZIO#catchSome	PartialFunction[E, ZIO[R1, E1, A1]]	ZIO[R1, E1, A1]
ZIO#catchSomeCause	PartialFunction[Cause[E], ZIO[R1, E1, A1]]	ZIO[R1, E1, A1]
ZIO#catchSomeDefect	PartialFunction[Throwable, ZIO[R1, E1, A1]]	ZIO[R1, E1, A1]
ZIO#catchSomeTrace	PartialFunction[(E, Option[StackTrace]), ZIO[R1, E1, A1]]	ZIO[R1, E1, A1]

Catching All Errors

If we want to catch and recover from all types of errors and effectfully attempt recovery, we can use the catchAll method:

val z: IO[IOException, Array[Byte]] =
  readFile("primary.json").catchAll(_ =>
    readFile("backup.json"))

In the callback passed to catchAll, we may return an effect with a different error type (or perhaps Nothing), which will be reflected in the type of effect returned by catchAll.

Catching Some Errors

If we want to catch and recover from only some types of exceptions and effectfully attempt recovery, we can use the catchSome method:

val data: IO[IOException, Array[Byte]] =
  readFile("primary.data").catchSome {
    case _ : FileNotFoundException =>
      readFile("backup.data")
  }

Unlike catchAll, catchSome cannot reduce or eliminate the error type, although it can widen the error type to a broader class of errors.

Fallback

Function	Input Type	Output Type
orElse	ZIO[R1, E2, A1]	ZIO[R1, E2, A1]
orElseEither	ZIO[R1, E2, B]	ZIO[R1, E2, Either[A, B]]
orElseFail	E1	ZIO[R, E1, A]
orElseOptional	ZIO[R1, Option[E1], A1]	ZIO[R1, Option[E1], A1]
orElseSucceed	A1	URIO[R, A1]

We can try one effect, or, if it fails, try another effect, with the orElse combinator:

val primaryOrBackupData: IO[IOException, Array[Byte]] =
  readFile("primary.data").orElse(readFile("backup.data"))

Folding

Function	Input Type	Output Type
fold	failure: E => B, success: A => B	URIO[R, B]
foldCause	failure: Cause[E] => B, success: A => B	URIO[R, B]
foldZIO	failure: E => ZIO[R1, E2, B], success: A => ZIO[R1, E2, B]	ZIO[R1, E2, B]
foldCauseZIO	failure: Cause[E] => ZIO[R1, E2, B], success: A => ZIO[R1, E2, B]	ZIO[R1, E2, B]
foldTraceZIO	failure: ((E, Option[StackTrace])) => ZIO[R1, E2, B], success: A => ZIO[R1, E2, B]	ZIO[R1, E2, B]

Scala's Option and Either data types have fold, which lets us handle both failure and success at the same time. In a similar fashion, ZIO effects also have several methods that allow us to handle both failure and success.

The first fold method, fold, lets us non-effectfully handle both failure and success by supplying a non-effectful handler for each case:

lazy val DefaultData: Array[Byte] = Array(0, 0)

val primaryOrDefaultData: UIO[Array[Byte]] =
  readFile("primary.data").fold(
    _    => DefaultData,
    data => data)

The second fold method, foldZIO, lets us effectfully handle both failure and success by supplying an effectful (but still pure) handler for each case:

val primaryOrSecondaryData: IO[IOException, Array[Byte]] =
  readFile("primary.data").foldZIO(
    _    => readFile("secondary.data"),
    data => ZIO.succeed(data))

Nearly all error handling methods are defined in terms of foldZIO, because it is both powerful and fast.

In the following example, foldZIO is used to handle both failure and success of the readUrls method:

val urls: UIO[Content] =
  readUrls("urls.json").foldZIO(
    error   => ZIO.succeed(NoContent(error)),
    success => fetchContent(success)
  )

Retrying

Function	Input Type	Output Type
retry	Schedule[R1, E, S]	ZIO[R1, E, A]
retryN	n: Int	ZIO[R, E, A]
retryOrElse	policy: Schedule[R1, E, S], orElse: (E, S) => ZIO[R1, E1, A1]	ZIO[R1, E1, A1]
retryOrElseEither	schedule: Schedule[R1, E, Out], orElse: (E, Out) => ZIO[R1, E1, B]	ZIO[R1, E1, Either[B, A]]
retryUntil	E => Boolean	ZIO[R, E, A]
retryUntilEquals	E1	ZIO[R, E1, A]
retryUntilZIO	E => URIO[R1, Boolean]	ZIO[R1, E, A]
retryWhile	E => Boolean	ZIO[R, E, A]
retryWhileEquals	E1	ZIO[R, E1, A]
retryWhileZIO	E => URIO[R1, Boolean]	ZIO[R1, E, A]

When we are building applications we want to be resilient in the face of a transient failure. This is where we need to retry to overcome these failures.

There are a number of useful methods on the ZIO data type for retrying failed effects.

The most basic of these is ZIO#retry, which takes a Schedule and returns a new effect that will retry the first effect if it fails according to the specified policy:

val retriedOpenFile: ZIO[Any, IOException, Array[Byte]] =
  readFile("primary.data").retry(Schedule.recurs(5))

The next most powerful function is ZIO#retryOrElse, which allows specification of a fallback to use if the effect does not succeed with the specified policy:

readFile("primary.data").retryOrElse(
  Schedule.recurs(5),
  (_, _:Long) => ZIO.succeed(DefaultData)
)

The final method, ZIO#retryOrElseEither, allows returning a different type for the fallback.

Resource Management

ZIO's resource management features work across synchronous, asynchronous, concurrent, and other effect types, and provide strong guarantees even in the presence of failure, interruption, or defects in the application.

Finalizing

Scala has a try / finally construct which helps us to make sure we don't leak resources because no matter what happens in the try, the finally block will be executed. So we can open files in the try block, and then we can close them in the finally block, and that gives us the guarantee that we will not leak resources.

Asynchronous Try / Finally

The problem with the try / finally construct is that it only applies to synchronous code, meaning it doesn't work for asynchronous code. ZIO gives us a method called ensuring that works with either synchronous or asynchronous actions. So we have a functional try / finally even for asynchronous regions of our code.

Like try / finally, the ensuring operation guarantees that if an effect begins executing and then terminates (for whatever reason), then the finalizer will begin executing:

import zio._

val finalizer =
  ZIO.succeed(println("Finalizing!"))

val finalized: IO[String, Unit] =
  ZIO.fail("Failed!").ensuring(finalizer)

The finalizer is not allowed to fail, which means that it must handle any errors internally.

Like try / finally, finalizers can be nested, and the failure of any inner finalizer will not affect outer finalizers. Nested finalizers will be executed in reverse order, and linearly (not in parallel).

Unlike try / finally, ensuring works across all types of effects, including asynchronous and concurrent effects.

Here is another example of ensuring that our clean-up action is called before our effect is done:

import zio._

import zio.Task
var i: Int = 0
val action: Task[String] =
  ZIO.succeed(i += 1) *>
    ZIO.fail(new Throwable("Boom!"))
val cleanupAction: UIO[Unit] = ZIO.succeed(i -= 1)
val composite = action.ensuring(cleanupAction)

caution

Finalizers offer very powerful guarantees, but they are low-level, and should generally not be used for releasing resources. For higher-level logic built on ensuring, see ZIO#acquireReleaseWith in the acquire release section.

Unstoppable Finalizers

In Scala when we nest try / finally finalizers, they cannot be stopped. If we have nested finalizers and one of them fails for some sort of catastrophic reason the ones on the outside will still be run and in the correct order.

try {
  try {
    try {
      ...
    } finally f1
  } finally f2
} finally f3

Also in ZIO like try / finally, the finalizers are unstoppable. This means if we have a buggy finalizer that is going to leak some resources, we will leak the minimum amount of resources because all other finalizers will still be run in the correct order.

val io = ???
io.ensuring(f1)
 .ensuring(f2)
 .ensuring(f3)

Acquire Release

In Scala try / finally is often used to manage resources. A common use for try / finally is safely acquiring and releasing resources, such as new socket connections or opened files:

val handle = openFile(name)

try {
  processFile(handle)
} finally closeFile(handle)

ZIO encapsulates this common pattern with ZIO#acquireRelease, which allows us to specify an acquire effect, which acquires a resource; a release effect, which releases it; and a use effect, which uses the resource. Acquire release lets us open a file and close the file and no matter what happens when we are using that resource.

The release action is guaranteed to be executed by the runtime system, even if the utilize action throws an exception or the executing fiber is interrupted.

Acquire release is a built-in primitive that let us safely acquire and release resources. It is used for a similar purpose as try / catch / finally, only acquire release work with synchronous and asynchronous actions, work seamlessly with fiber interruption, and is built on a different error model that ensures no errors are ever swallowed.

Acquire release consist of an acquire action, a utilize action (which uses the acquired resource), and a release action.

import zio._

val groupedFileData: IO[IOException, Unit] = ZIO.acquireReleaseWith(openFile("data.json"))(closeFile(_)) { file =>
  for {
    data    <- decodeData(file)
    grouped <- groupData(data)
  } yield grouped
}

Acquire releases have compositional semantics, so if an acquire release is nested inside another acquire release, and the outer resource is acquired, then the outer release will always be called, even if, for example, the inner release fails.

Let's look at a full working example on using acquire release:

import zio._
import java.io.{ File, FileInputStream }
import java.nio.charset.StandardCharsets

object Main extends ZIOAppDefault {

  // run my acquire release
  def run = myAcquireRelease

  def closeStream(is: FileInputStream) =
    ZIO.succeed(is.close())

  def convertBytes(is: FileInputStream, len: Long) =
    ZIO.attempt {
      val buffer = new Array[Byte](len.toInt)
      is.read(buffer)
      println(new String(buffer, StandardCharsets.UTF_8))
    }

  // myAcquireRelease is just a value. Won't execute anything here until interpreted
  val myAcquireRelease: Task[Unit] = for {
    file   <- ZIO.attempt(new File("/tmp/hello"))
    len    = file.length
    string <- ZIO.acquireReleaseWith(ZIO.attempt(new FileInputStream(file)))(closeStream)(convertBytes(_, len))
  } yield string
}

ZIO Aspect

There are two types of concerns in an application, core concerns, and cross-cutting concerns. Cross-cutting concerns are shared among different parts of our application. We usually find them scattered and duplicated across our application, or they are tangled up with our primary concerns. This reduces the level of modularity of our programs.

A cross-cutting concern is more about how we do something than what we are doing. For example, when we are downloading a bunch of files, creating a socket to download each one is the core concern because it is a question of what rather than the how, but the following concerns are cross-cutting ones:

• Downloading files sequentially or in parallel
• Retrying and timing out the download process
• Logging and monitoring the download process

So they don't affect the return type of our workflows, but they add some new aspects or change their behavior.

To increase the modularity of our applications, we can separate cross-cutting concerns from the main logic of our programs. ZIO supports this programming paradigm, which is called aspect-oriented programming.

The ZIO effect has a data type called ZIOAspect, which allows modifying a ZIO effect and converting it into a specialized ZIO effect. We can add a new aspect to a ZIO effect with @@ syntax like this:

import zio._

val myApp: ZIO[Any, Throwable, String] =
  ZIO.attempt("Hello!") @@ ZIOAspect.debug

As we see, the debug aspect doesn't change the return type of our effect, but it adds a new debugging aspect to our effect.

ZIOAspect is like a transformer of the ZIO effect, which takes a ZIO effect and converts it to another ZIO effect. We can think of a ZIOAspect as a function of type ZIO[R, E, A] => ZIO[R, E, A].

To compose multiple aspects, we can use @@ operator:

import zio._

def download(url: String): ZIO[Any, Throwable, Chunk[Byte]] = ZIO.succeed(???)

ZIO.foreachPar(List("zio.dev", "google.com")) { url =>
  download(url) @@
    ZIOAspect.retry(Schedule.fibonacci(1.seconds)) @@
    ZIOAspect.loggedWith[Chunk[Byte]](file => s"Downloaded $url file with size of ${file.length} bytes")
}

The order of aspect composition matters. Therefore, if we change the order, the behavior may change.