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Version: ZIO 2.x



A ZStream[R, E, O] is a description of a program that, when evaluated, may emit zero or more values of type O, may fail with errors of type E, and uses an environment of type R.

One way to think of ZStream is as a ZIO program that could emit multiple values. As we know, a ZIO[R, E, A] data type, is a functional effect which is a description of a program that needs an environment of type R, it may end with an error of type E, and in case of success, it returns a value of type A. The important note about ZIO effects is that in the case of success they always end with exactly one value. There is no optionality here, no multiple infinite values, we always get exact value:

val failedEffect: ZIO[Any, String, Nothing]       ="fail!")val oneIntValue : ZIO[Any, Nothing, Int]          = ZIO.succeed(3)val oneListValue: ZIO[Any, Nothing, List[Int]]    = ZIO.succeed(List(1, 2, 3))val oneOption   : ZIO[Any, Nothing , Option[Int]] = ZIO.succeed(None)

A functional stream is pretty similar, it is a description of a program that requires an environment of type R and it may signal with errors of type E and it yields O, but the difference is that it will yield zero or more values.

So a ZStream represents one of the following cases in terms of its elements:

  • An Empty Stream โ€” It might end up empty; which represent an empty stream, e.g. ZStream.empty.
  • One Element Stream โ€” It can represent a stream with just one value, e.g. ZStream.succeed(3).
  • Multiple Finite Element Stream โ€” It can represent a stream of finite values, e.g. ZStream.range(1, 10)
  • Multiple Infinite Element Stream โ€” It can even represent a stream that never ends as an infinite stream, e.g. ZStream.iterate(1)(_ + 1).
import emptyStream         : ZStream[Any, Nothing, Nothing]   = ZStream.emptyval oneIntValueStream   : ZStream[Any, Nothing, Int]       = ZStream.succeed(4)val oneListValueStream  : ZStream[Any, Nothing, List[Int]] = ZStream.succeed(List(1, 2, 3))val finiteIntStream     : ZStream[Any, Nothing, Int]       = ZStream.range(1, 10)val infiniteIntStream   : ZStream[Any, Nothing, Int]       = ZStream.iterate(1)(_ + 1)

Another example of a stream is when we're pulling a Kafka topic or reading from a socket. There is no inherent definition of an end there. Stream elements arrive at some point, or even they might never arrive at any point.

Stream Types#

Based on type parameters of ZStream, there are 4 types of streams:

  1. ZStream[Any, Nothing, O] โ€” A stream that emits O values and cannot fail.
  2. ZStream[Any, Throwable, O] โ€” A stream that emits O values and can fail with Throwable.
  3. ZStream[Any, Nothing, Nothing] โ€” A stream that emits no elements.
  4. ZStream[R, E, O] โ€” A stream that requires access to the R service, can fail with error of type E and emits O values.


Every time we are working with streams, we are always working with chunks. There are no streams with individual elements, these streams have always chunks in their underlying implementation. So every time we evaluate a stream, when we pull an element out of a stream, we are actually pulling out a chunk of elements.

So why streams are designed in this way? This is because of the efficiency and performance issues. Every I/O operation in the programming world works with batches. We never work with a single element. For example, whenever we are reading or writing from/to a file descriptor, or a socket we are reading or writing multiple elements at a time. This is also true when we are working with an HTTP server or even JDBC drivers. We always read and write multiple bytes to be more performant.

So let's talk a bit about Chunk. Chunk is a ZIOs immutable array-backed collection. It is initially written for ZIO stream, but later it has been evolved into a very attractive general collection type which is also useful for other purposes. It is an immutable array-backed collection. Most importantly it tries to keep primitives unboxed. This is super important for the efficient processing of files and sockets. They are also very useful and efficient for encoding and decoding and writing transducers. To learn more about this data type, we have introduced that at the Chunk section.

Creating a Stream#

There are several ways to create ZIO Stream. In this section, we are going to enumerate some of the important ways of creating ZStream.

Common Constructors#

ZStream.apply โ€” Creates a pure stream from a variable list of values:

val stream: ZStream[Any, Nothing, Int] = ZStream(1, 2, 3)

ZStream.unit โ€” A stream that contains a single Unit value:

val unit: ZStream[Any, Nothing, Unit] = ZStream.unit

ZStream.never โ€” A stream that produces no value or fails with an error:

val never: ZStream[Any, Nothing, Nothing] = ZStream.never

ZStream.repeat โ€” Takes an initial value and applies the given function to the initial value iteratively. The initial value is the first value produced by the stream, followed by f(init), f(f(init)), ...

val nats: ZStream[Any, Nothing, Int] =   ZStream.iterate(1)(_ + 1) // 1, 2, 3, ...

ZStream.range โ€” A stream from a range of integers [min, max):

val range: ZStream[Any, Nothing, Int] = ZStream.range(1, 5) // 1, 2, 3, 4

ZStream.environment[R] โ€” Create a stream that extract the request service from the environment:

val clockStream: ZStream[Has[Clock], Nothing, Has[Clock]] = ZStream.environment[Has[Clock]]

ZStream.managed โ€” Creates a single-valued stream from a managed resource:

val managedStream: ZStream[Any, Throwable, BufferedReader] =  ZStream.managed(    ZManaged.fromAutoCloseable(      ZIO.attemptBlocking(        Files.newBufferedReader(java.nio.file.Paths.get("file.txt"))      )    )  )

From Success and Failure#

Similar to ZIO data type, we can create a ZStream using fail and succeed methods:

val s1: ZStream[Any, String, Nothing] ="Uh oh!")val s2: ZStream[Any, Nothing, Int]    = ZStream.succeed(5)

From Chunks#

We can create a stream from a Chunk:

val s1 = ZStream.fromChunk(Chunk(1, 2, 3))// s1: ZStream[Any, Nothing, Int] =$$anon$1@27037d90

Or from multiple Chunks:

val s2 = ZStream.fromChunks(Chunk(1, 2, 3), Chunk(4, 5, 6))// s2: ZStream[Any, Nothing, Int] =$$anon$1@7f7bf5c6

From Effect#

ZStream.fromZIO โ€” We can create a stream from an effect by using ZStream.fromZIO constructor. For example, the following stream is a stream that reads a line from a user:

val readline: ZStream[Has[Console], IOException, String] =   ZStream.fromZIO(Console.readLine)

A stream that produces one random number:

val randomInt: ZStream[Has[Random], Nothing, Int] =   ZStream.fromZIO(Random.nextInt)

ZStream.fromZIOOption โ€” In some cases, depending on the result of the effect, we should decide to emit an element or return an empty stream. In these cases, we can use fromZIOOption constructor:

object ZStream {  def fromZIOOption[R, E, A](fa: ZIO[R, Option[E], A]): ZStream[R, E, A] = ???}

Let's see an example of using this constructor. In this example, we read a string from user input, and then decide to emit that or not; If the user enters an EOF string, we emit an empty stream, otherwise we emit the user input:

val userInput: ZStream[Has[Console], IOException, String] =   ZStream.fromZIOOption(    Console.readLine.mapError(Option(_)).flatMap {      case "EOF" =>[Option[IOException]](None)      case o     => ZIO.succeed(o)    }  ) 

From Asynchronous Callback#

Assume we have an asynchronous function that is based on callbacks. We would like to register a callbacks on that function and get back a stream of the results emitted by those callbacks. We have ZStream.async which can adapt functions that call their callbacks multiple times and emit the results over a stream:

// Asynchronous Callback-based APIdef registerCallback(    name: String,    onEvent: Int => Unit,    onError: Throwable => Unit): Unit = ???
// Lifting an Asynchronous API to ZStreamval stream = ZStream.async[Any, Throwable, Int] { cb =>  registerCallback(    "foo",    event => cb(ZIO.succeed(Chunk(event))),    error => cb(  )}

The error type of the register function is optional, so by setting the error to the None we can use it to signal the end of the stream.

From Iterators#

Iterators are data structures that allow us to iterate over a sequence of elements. Similarly, we can think of ZIO Streams as effectual Iterators; every ZStream represents a collection of one or more, but effectful values.

ZStream.fromIteratorSucceed โ€” We can convert an iterator that does not throw exception to ZStream by using ZStream.fromIteratorSucceed:

val s1: ZStream[Any, Throwable, Int] = ZStream.fromIterator(Iterator(1, 2, 3))val s2: ZStream[Any, Throwable, Int] = ZStream.fromIterator(Iterator.range(1, 4))val s3: ZStream[Any, Throwable, Int] = ZStream.fromIterator(Iterator.continually(0))

Also, there is another constructor called ZStream.fromIterator that creates a stream from an iterator which may throw an exception.

ZStream.fromIteratorZIO โ€” If we have an effectful Iterator that may throw Exception, we can use fromIteratorZIO to convert that to the ZIO Stream:

import lines: ZStream[Any, Throwable, String] =   ZStream.fromIteratorZIO(Task(Source.fromFile("file.txt").getLines()))

Using this method is not good for resourceful effects like above, so it's better to rewrite that using ZStream.fromIteratorManaged function.

ZStream.fromIteratorManaged โ€” Using this constructor we can convert a managed iterator to ZIO Stream:

val lines: ZStream[Any, Throwable, String] =   ZStream.fromIteratorManaged(    ZManaged.fromAutoCloseable(      Task("file.txt"))    ).map(_.getLines())  )

ZStream.fromJavaIterator โ€” It is the Java version of these constructors which create a stream from Java iterator that may throw an exception. We can convert any Java collection to an iterator and then lift them to the ZIO Stream.

For example, to convert the Java Stream to the ZIO Stream, ZStream has a fromJavaStream constructor which convert the Java Stream to the Java Iterator and then convert that to the ZIO Stream using ZStream.fromJavaIterator constructor:

def fromJavaStream[A](stream: =>[A]): ZStream[Any, Throwable, A] =  ZStream.fromJavaIterator(stream.iterator())

Similarly, ZStream has ZStream.fromJavaIteratorSucceed, ZStream.fromJavaIteratorZIO and ZStream.fromJavaIteratorManaged constructors.

From Iterables#

ZStream.fromIterable โ€” We can create a stream from Iterable collection of values:

val list = ZStream.fromIterable(List(1, 2, 3))

ZStream.fromIterableZIO โ€” If we have an effect producing a value of type Iterable we can use fromIterableZIO constructor to create a stream of that effect.

Assume we have a database that returns a list of users using Task:

trait Database {  def getUsers: Task[List[User]]}
object Database {  def getUsers: ZIO[Has[Database], Throwable, List[User]] =     ZIO.serviceWith[Database](_.getUsers)}

As this operation is effectful, we can use ZStream.fromIterableZIO to convert the result to the ZStream:

val users: ZStream[Has[Database], Throwable, User] =   ZStream.fromIterableZIO(Database.getUsers)

From Repetition#

ZStream.repeat โ€” Repeats the provided value infinitely:

val repeatZero: ZStream[Any, Nothing, Int] = ZStream.repeat(0)

ZStream.repeatWith โ€” This is another variant of repeat, which repeats according to the provided schedule. For example, the following stream produce zero value every second:

import zio._import zio.Clock._import zio.Duration._import zio.Random._import zio.Scheduleval repeatZeroEverySecond: ZStream[Has[Clock], Nothing, Int] =   ZStream.repeatWithSchedule(0, Schedule.spaced(1.seconds))

ZStream.repeatZIO โ€” Assume we have an effectful API, and we need to call that API and create a stream from the result of that. We can create a stream from that effect that repeats forever.

Let's see an example of creating a stream of random numbers:

val randomInts: ZStream[Has[Random], Nothing, Int] =  ZStream.repeatZIO(Random.nextInt)

ZStream.repeatZIOOption โ€” We can repeatedly evaluate the given effect and terminate the stream based on some conditions.

Let's create a stream repeatedly from user inputs until user enter "EOF" string:

val userInputs: ZStream[Has[Console], IOException, String] =   ZStream.repeatZIOOption(    Console.readLine.mapError(Option(_)).flatMap {      case "EOF" =>[Option[IOException]](None)      case o     => ZIO.succeed(o)    }  )

Here is another interesting example of using repeatZIOOption; In this example, we are draining an Iterator to create a stream of that iterator:

def drainIterator[A](it: Iterator[A]): ZStream[Any, Throwable, A] =  ZStream.repeatZIOOption {    ZIO(it.hasNext).mapError(Some(_)).flatMap { hasNext =>      if (hasNext) ZIO(      else    }  }

ZStream.tick โ€” A stream that emits Unit values spaced by the specified duration:

val stream: ZStream[Has[Clock], Nothing, Unit] =   ZStream.tick(1.seconds)

There are some other variant of repetition API like repeatZIOWith, repeatZIOOption, repeatZIOChunk and repeatZIOChunkOption.

From Unfolding/Pagination#

In functional programming, unfold is dual to fold.

With fold we can process a data structure and build a return value. For example, we can process a List[Int] and return the sum of all its elements.

The unfold represents an operation that takes an initial value and generates a recursive data structure, one-piece element at a time by using a given state function. For example, we can create a natural number by using one as the initial element and the inc function as the state function.


ZStream.unfold โ€” ZStream has unfold function, which is defined as follows:

object ZStream {  def unfold[S, A](s: S)(f: S => Option[(A, S)]): ZStream[Any, Nothing, A] = ???}
  • s โ€” An initial state value
  • f โ€” A state function f that will be applied to the initial state s. If the result of this application is None the stream will end, otherwise the result is Some, so the next element in the stream would be A and the current state of transformation changed to the new S, this new state is the basis of the next unfold process.

For example, we can a stream of natural numbers using ZStream.unfold:

val nats: ZStream[Any, Nothing, Int] = ZStream.unfold(1)(n => Some((n, n + 1)))

We can write countdown function using unfold:

def countdown(n: Int) = ZStream.unfold(n) {  case 0 => None  case s => Some((s, s - 1))}

Running this function with an input value of 3 returns a ZStream which contains 3, 2, 1 values.

ZStream.unfoldZIO โ€” unfoldZIO is an effectful version of unfold. It helps us to perform effectful state transformation when doing unfold operation.

Let's write a stream of lines of input from a user until the user enters the exit command:

val inputs: ZStream[Has[Console], IOException, String] = ZStream.unfoldZIO(()) { _ => {    case "exit"  => None    case i => Some((i, ()))  } }   

ZStream.unfoldChunk, and ZStream.unfoldChunkM are other variants of unfold operations but for Chunk data type.


ZStream.paginate โ€” This is similar to unfold, but allows the emission of values to end one step further. For example the following stream emits 0, 1, 2, 3 elements:

val stream = ZStream.paginate(0) { s =>  s -> (if (s < 3) Some(s + 1) else None)}

Similar to unfold API, ZStream has various other forms as well as ZStream.paginateM, ZStream.paginateChunk and ZStream.paginateChunkM.

Unfolding vs. Pagination#

One might ask what is the difference between unfold and paginate combinators? When we should prefer one over another? So, let's find the answer to this question by doing another example.

Assume we have a paginated API that returns an enormous amount of data in a paginated fashion. When we call that API, it returns a data type ResultPage which contains the first-page result and, it also contains a flag indicating whether that result is the last one, or we have more data on the next page:

case class PageResult(results: Chunk[RowData], isLast: Boolean)
def listPaginated(pageNumber: Int): ZIO[Has[Console], Throwable, PageResult] = ???

We want to convert this API to a stream of RowData events. For the first attempt, we might think we can do it by using unfold operation as below:

val firstAttempt: ZStream[Has[Console], Throwable, RowData] =   ZStream.unfoldChunkZIO(0) { pageNumber =>    for {      page <- listPaginated(pageNumber)    } yield      if (page.isLast) None      else Some((page.results, pageNumber + 1))  }

But it doesn't work properly; it doesn't include the last page result. So let's do a trick and to perform another API call to include the last page results:

val secondAttempt: ZStream[Has[Console], Throwable, RowData] =   ZStream.unfoldChunkZIO(Option[Int](0)) {    case None => ZIO.none // We already hit the last page    case Some(pageNumber) => // We did not hit the last page yet     for {        page <- listPaginated(pageNumber)      } yield Some(page.results, if (page.isLast) None else Some(pageNumber + 1))  }

This works and contains all the results of returned pages. It works but as we saw, unfold is not friendliness to retrieve data from paginated APIs.

We need to do some hacks and extra works to include results from the last page. This is where ZStream.paginate operation comes to play, it helps us to convert a paginated API to ZIO stream in a more ergonomic way. Let's rewrite this solution by using paginate:

val finalAttempt: ZStream[Has[Console], Throwable, RowData] =   ZStream.paginateChunkZIO(0) { pageNumber =>    for {      page <- listPaginated(pageNumber)    } yield page.results -> (if (!page.isLast) Some(pageNumber + 1) else None)  }

From Wrapped Streams#

Sometimes we have an effect that contains a ZStream, we can unwrap the embedded stream and produce a stream from those effects. If the stream is wrapped with the ZIO effect, we use unwrap, and if it is wrapped with ZManaged we use unwrapManaged:

val wrappedWithZIO: UIO[ZStream[Any, Nothing, Int]] =   ZIO.succeed(ZStream(1, 2, 3))val s1: ZStream[Any, Nothing, Int] =   ZStream.unwrap(wrappedWithZIO)
val wrappedWithZManaged = ZManaged.succeed(ZStream(1, 2, 3))val s2: ZStream[Any, Nothing, Int] =   ZStream.unwrapManaged(wrappedWithZManaged)

From Java IO#

ZStream.fromFile โ€” Create ZIO Stream from a file:

import java.nio.file.Pathsval file: ZStream[Any, Throwable, Byte] =   ZStream.fromFile(Paths.get("file.txt"))

ZStream.fromInputStream โ€” Creates a stream from a

val stream: ZStream[Any, IOException, Byte] =   ZStream.fromInputStream(new FileInputStream("file.txt"))

Note that the InputStream will not be explicitly closed after it is exhausted. Use ZStream.fromInputStreamZIO, or ZStream.fromInputStreamManaged instead.

ZStream.fromInputStreamZIO โ€” Creates a stream from a Ensures that the InputStream is closed after it is exhausted:

val stream: ZStream[Any, IOException, Byte] =   ZStream.fromInputStreamZIO(    ZIO.attempt(new FileInputStream("file.txt"))      .refineToOrDie[IOException]  )

ZStream.fromInputStreamManaged โ€” Creates a stream from a managed value:

val managed: ZManaged[Any, IOException, FileInputStream] =  ZManaged.fromAutoCloseable(    ZIO.attempt(new FileInputStream("file.txt"))  ).refineToOrDie[IOException]
val stream: ZStream[Any, IOException, Byte] =   ZStream.fromInputStreamManaged(managed)

ZStream.fromResource โ€” Create a stream from resource file:

val stream: ZStream[Any, IOException, Byte] =  ZStream.fromResource("file.txt")

ZStream.fromReader โ€” Creates a stream from a

val stream: ZStream[Any, IOException, Char] =    ZStream.fromReader(new FileReader("file.txt"))

ZIO Stream also has ZStream.fromReaderZIO and ZStream.fromReaderManaged variants.

From Java Stream#

We can use ZStream.fromJavaStreamTotal to convert a Java Stream to ZIO Stream:

val stream: ZStream[Any, Throwable, Int] =   ZStream.fromJavaStream(, 2, 3))

ZIO Stream also has ZStream.fromJavaStream, ZStream.fromJavaStreamZIO and ZStream.fromJavaStreamManaged variants.

From Queue and Hub#

Queue and Hub are two asynchronous messaging data types in ZIO that can be converted into the ZIO Stream:

object ZStream {  def fromQueue[R, E, O](    queue: ZQueue[Nothing, R, Any, E, Nothing, O],    maxChunkSize: Int = DefaultChunkSize  ): ZStream[R, E, O] = ???
  def fromHub[R, E, A](    hub: ZHub[Nothing, R, Any, E, Nothing, A]  ): ZStream[R, E, A] = ???}

If they contain Chunk of elements, we can use ZStream.fromChunk... constructors to create a stream from those elements (e.g. ZStream.fromChunkQueue):

for {  promise <- Promise.make[Nothing, Unit]  hub     <- ZHub.unbounded[Chunk[Int]]  managed = ZStream.fromChunkHubManaged(hub).tapZIO(_ => promise.succeed(()))  stream  = ZStream.unwrapManaged(managed)  fiber   <- stream.foreach(printLine(_)).fork  _       <- promise.await  _       <- hub.publish(Chunk(1, 2, 3))  _       <- fiber.join} yield ()

Also, If we need to shutdown a Queue or Hub, once the stream is closed, we should use ZStream.from..Shutdown constructors (e.g. ZStream.fromQueueWithShutdown).

Also, we can lift a TQueue to the ZIO Stream:

for {  q <- STM.atomically(TQueue.unbounded[Int])  stream = ZStream.fromTQueue(q)  fiber <- stream.foreach(printLine(_)).fork  _     <- STM.atomically(q.offer(1))  _     <- STM.atomically(q.offer(2))  _     <- fiber.join} yield ()

From Schedule#

We can create a stream from a Schedule that does not require any further input. The stream will emit an element for each value output from the schedule, continuing for as long as the schedule continues:

val stream: ZStream[Has[Clock], Nothing, Long] =  ZStream.fromSchedule(Schedule.spaced(1.second) >>> Schedule.recurs(10))

Resourceful Streams#

Most of the constructors of ZStream have a special variant to lift a Managed resource to a Stream (e.g. ZStream.fromReaderManaged). By using these constructors, we are creating streams that are resource-safe. Before creating a stream, they acquire the resource, and after usage; they close the stream.

ZIO Stream also has acquireRelease and finalizer constructors which are similar to ZManaged. They allow us to clean up or finalizing before the stream ends:

Acquire Release#

We can provide acquire and release actions to ZStream.acquireReleaseWith to create a resourceful stream:

object ZStream {  def acquireReleaseWith[R, E, A](    acquire: ZIO[R, E, A]  )(    release: A => URIO[R, Any]  ): ZStream[R, E, A] = ???

Let's see an example of using an acquire release when reading a file. In this example, by providing acquire and release actions to ZStream.acquireReleaseWith, it gives us a managed stream of BufferedSource. As this stream is managed, we can convert that BufferedSource to a stream of its lines and then run it, without worrying about resource leakage:

import zio.Console._
val lines: ZStream[Has[Console], Throwable, String] =  ZStream    .acquireReleaseWith(      ZIO.attempt(Source.fromFile("file.txt")) <* printLine("The file was opened.")    )(x => URIO.succeed(x.close()) <* printLine("The file was closed.").orDie)    .flatMap { is =>      ZStream.fromIterator(is.getLines())    }


We can also create a stream that never fails and define a finalizer for it, so that finalizer will be executed before that stream ends.

object ZStream {  def finalizer[R](    finalizer: URIO[R, Any]  ): ZStream[R, Nothing, Any] = ???}

It is useful when need to add a finalizer to an existing stream. Assume we need to clean up the temporary directory after our streaming application ends:

import zio.Console._
def application: ZStream[Has[Console], IOException, Unit] = ZStream.fromZIO(printLine("Application Logic."))def deleteDir(dir: Path): ZIO[Has[Console], IOException, Unit] = printLine("Deleting file.")
val myApp: ZStream[Has[Console], IOException, Any] =  application ++ ZStream.finalizer(    (deleteDir(Paths.get("tmp")) *>      printLine("Temporary directory was deleted.")).orDie  )


We might want to run some code before or after the execution of the stream's finalization. To do so, we can use ZStream#ensuringFirst and ZStream#ensuring operators:

ZStream  .finalizer(Console.printLine("Finalizing the stream").orDie)  .ensuringFirst(    printLine("Doing some works before stream's finalization").orDie  )  .ensuring(    printLine("Doing some other works after stream's finalization").orDie  )  // Output:// Doing some works before stream's finalization// Finalizing the stream// Doing some other works after stream's finalization



Tapping is an operation of running an effect on each emission of the ZIO Stream. We can think of ZStream#tap as an operation that allows us to observe each element of the stream, do some effectful operation and discard the result of this observation. The tap operation does not change elements of the stream, it does not affect the return type of the stream.

For example, we can print each element of a stream by using the tap operation:

val stream: ZStream[Has[Console], IOException, Int] =  ZStream(1, 2, 3)    .tap(x => printLine(s"before mapping: $x"))    .map(_ * 2)    .tap(x => printLine(s"after mapping: $x"))

Taking Elements#

We can take a certain number of elements from a stream:

val stream = ZStream.iterate(0)(_ + 1)val s1 = stream.take(5)// Output: 0, 1, 2, 3, 4
val s2 = stream.takeWhile(_ < 5)// Output: 0, 1, 2, 3, 4
val s3 = stream.takeUntil(_ == 5)// Output: 0, 1, 2, 3, 4, 5
val s4 = s3.takeRight(3)// Output: 3, 4, 5


map โ€” Applies a given function to all element of this stream to produce another stream:

val intStream: UStream[Int] = Stream.fromIterable(0 to 100)val stringStream: UStream[String] =

If our transformation is effectful, we can use ZStream#mapZIO instead.

mapZIOPar โ€” It is similar to mapZIO, but will evaluate effects in parallel. It will emit the results downstream in the original order. The n argument specifies the number of concurrent running effects.

Let's write a simple page downloader, which download URLs concurrently:

def fetchUrl(url: URL): Task[String] = Task.succeed(???)def getUrls: Task[List[URL]] = Task.succeed(???)
val pages = ZStream.fromIterableZIO(getUrls).mapZIOPar(8)(fetchUrl)  

mapChunk โ€” Each stream is backed by some Chunks. By using mapChunk we can batch the underlying stream and map every Chunk at once:

val chunked =   ZStream    .fromChunks(Chunk(1, 2, 3), Chunk(4, 5), Chunk(6, 7, 8, 9))
val stream = chunked.mapChunks(x => x.tail)
// Input:  1, 2, 3, 4, 5, 6, 7, 8, 9// Output:    2, 3,    5,    7, 8, 9

If our transformation is effectful we can use mapChunkM combinator.

mapAccum โ€” It is similar to a map, but it transforms elements statefully. mapAccum allows us to map and accumulate in the same operation.

abstract class ZStream[-R, +E, +O] {  def mapAccum[S, O1](s: S)(f: (S, O) => (S, O1)): ZStream[R, E, O1]}

Let's write a transformation, which calculate running total of input stream:

def runningTotal(stream: UStream[Int]): UStream[Int] =  stream.mapAccum(0)((acc, next) => (acc + next, acc + next))
// input:  0, 1, 2, 3,  4,  5// output: 0, 1, 3, 6, 10, 15

mapConcat โ€” It is similar to map, but maps each element to zero or more elements with the type of Iterable and then flattens the whole stream:

val numbers: UStream[Int] =   ZStream("1-2-3", "4-5", "6")    .mapConcat(_.split("-"))    .map(_.toInt)
// Input:  "1-2-3", "4-5", "6"// Output: 1, 2, 3, 4, 5, 6

The effectful version of mapConcat is mapConcatM.

ZStream also has chunked versions of that which are mapConcatChunk and mapConcatChunkM.

as โ€” The ZStream#as method maps the success values of this stream to the specified constant value.

For example, we can map all element to the unit value:

val unitStream: ZStream[Any, Nothing, Unit] =   ZStream.range(1, 5).as(())


The ZStream#filter allows us to filter emitted elements:

val s1 = ZStream.range(1, 11).filter(_ % 2 == 0)// Output: 2, 4, 6, 8, 10
// The `ZStream#withFilter` operator enables us to write filter in for-comprehension styleval s2 = for {  i <- ZStream.range(1, 11).take(10)  if i % 2 == 0} yield i// Output: 2, 4, 6, 8, 10
val s3 = ZStream.range(1, 11).filterNot(_ % 2 == 0)// Output: 1, 3, 5, 7, 9


Scans are like folds, but with a history. Like folds, they take a binary operator with an initial value. A fold combines elements of a stream and emits every intermediary result as an output of the stream:

val scan = ZStream(1, 2, 3, 4, 5).scan(0)(_ + _)// Output: 0, 1, 3, 6, 10// Iterations://        =>  0 (initial value)//  0 + 1 =>  1//  1 + 2 =>  3//  3 + 3 =>  6//  6 + 4 => 10// 10 + 5 => 15
val fold = ZStream(1, 2, 3, 4, 5).fold(0)(_ + _)// Output: 10 (ZIO effect containing 10)


Assume we have an effectful stream, which contains a sequence of effects; sometimes we might want to execute its effect without emitting any element, in these situations to discard the results we should use the ZStream#drain method. It removes all output values from the stream:

val s1: ZStream[Any, Nothing, Nothing] = ZStream(1, 2, 3, 4, 5).drain// Emitted Elements: <empty stream, it doesn't emit any element>
val s2: ZStream[Has[Console] with Has[Random], IOException, Int] =  ZStream    .repeatZIO {      for {        nextInt <- Random.nextInt        number = Math.abs(nextInt % 10)        _ <- Console.printLine(s"random number: $number")      } yield (number)    }    .take(3)// Emitted Elements: 1, 4, 7// Result of Stream Effect on the Console:// random number: 1// random number: 4// random number: 7
val s3: ZStream[Has[Console] with Has[Random], IOException, Nothing] = s2.drain// Emitted Elements: <empty stream, it doesn't emit any element>// Result of Stream Effect on the Console:// random number: 4// random number: 8// random number: 2

The ZStream#drain often used with ZStream#merge to run one side of the merge for its effect while getting outputs from the opposite side of the merge:

val logging = ZStream.fromZIO(  printLine("Starting to merge with the next stream"))val stream = ZStream(1, 2, 3) ++ logging.drain ++ ZStream(4, 5, 6)
// Emitted Elements: 1, 2, 3, 4, 5, 6// Result of Stream Effect on the Console:// Starting to merge with the next stream

Note that if we do not drain the logging stream, the emitted elements would be contained unit value:

val stream = ZStream(1, 2, 3) ++ logging ++ ZStream(4, 5, 6)
// Emitted Elements: 1, 2, 3, (), 4, 5, 6// Result of Stream Effect on the Console:// Starting to merge with the next stream


The ZStream#changes emits elements that are not equal to the previous element:

val changes = ZStream(1, 1, 1, 2, 2, 3, 4).changes// Output: 1, 2, 3, 4

The ZStream#changes operator, uses natural equality to determine whether two elements are equal. If we prefer the specialized equality checking, we can provide a function of type (O, O) => Boolean to the ZStream#changesWith operator.

Assume we have a stream of events with a composite key of partition and offset attributes, and we know that the offset is monotonic in each partition. So, we can use the changesWith operator to create a stream of unique elements:

case class Event(partition: Long, offset: Long, metadata: String) val events: ZStream[Any, Nothing, Event] = ZStream.fromIterable(???)
val uniques = events.changesWith((e1, e2) => (e1.partition == e2.partition && e1.offset == e2.offset))


We can perform filter and map operations in a single step using the ZStream#collect operation:

val source1 = ZStream(1, 2, 3, 4, 0, 5, 6, 7, 8)  val s1 = source1.collect { case x if x < 6 => x * 2 }// Output: 2, 4, 6, 8, 0, 10
val s2 = source1.collectWhile { case x if x != 0 => x * 2 }// Output: 2, 4, 6, 8
val source2 = ZStream(Left(1), Right(2), Right(3), Left(4), Right(5))
val s3 = source2.collectLeft// Output: 1, 4
val s4 = source2.collectWhileLeft// Output: 1
val s5 = source2.collectRight// Output: 2, 3, 5
val s6 = source2.drop(1).collectWhileRight// Output: 2, 3
val s7 = Output: 2, 3, 5
val s8 = Output: empty stream

We can also do effectful collect using ZStream#collectZIO and ZStream#collectWhileZIO.

ZIO stream has ZStream#collectSuccess which helps us to perform effectful operations and just collect the success values:

val urls = ZStream(  "",  "",  "",  "")
def fetch(url: String): ZIO[Any, Throwable, String] =   ZIO.attemptBlocking(???)
val pages = urls  .mapZIO(url => fetch(url).exit)  .collectSuccess


We can zip two stream by using or ZStream#zipWith operator:

val s1: UStream[(Int, String)] =  ZStream(1, 2, 3, 4, 5, 6).zipWith(ZStream("a", "b", "c"))((a, b) => (a, b))
val s2: UStream[(Int, String)] =   ZStream(1, 2, 3, 4, 5, 6).zip(ZStream("a", "b", "c"))  // Output: (1, "a"), (2, "b"), (3, "c")

The new stream will end when one of the streams ends.

In case of ending one stream before another, we might need to zip with default values; the ZStream#zipAll or ZStream#zipAllWith takes default values of both sides to perform such mechanism for us:

val s1 = ZStream(1, 2, 3)  .zipAll(ZStream("a", "b", "c", "d", "e"))(0, "x")val s2 = ZStream(1, 2, 3).zipAllWith(  ZStream("a", "b", "c", "d", "e"))(_ => 0, _ => "x")((a, b) => (a, b))
// Output: (1, a), (2, b), (3, c), (0, d), (0, e)

ZIO Stream also has a ZStream#zipAllWithExec function, which takes ExecutionStrategy as an argument. The execution strategy will be used to determine whether to pull from the streams sequentially or in parallel:

def zipAllWithExec[R1 <: R, E1 >: E, O2, O3](  that: ZStream[R1, E1, O2])(exec: ExecutionStrategy)(  left: O => O3, right: O2 => O3)(both: (O, O2) => O3): ZStream[R1, E1, O3] = ???

Sometimes we want to zip stream, but we do not want to zip two elements one by one. For example, we may have two streams with two different speeds, we do not want to wait for the slower one when zipping elements, assume need to zip elements with the latest element of the slower stream. The ZStream#zipWithLates do this for us. It zips two streams so that when a value is emitted by either of the two streams; it is combined with the latest value from the other stream to produce a result:

val s1 = ZStream(1, 2, 3)  .schedule(Schedule.spaced(1.second))
val s2 = ZStream("a", "b", "c", "d")  .schedule(Schedule.spaced(500.milliseconds))  .rechunk(3)
s1.zipWithLatest(s2)((a, b) => (a, b))
// Output: (1, a), (1, b), (1, c), (1, d), (2, d), (3, d)

ZIO Stream also has three useful operators for zipping element of a stream with their previous/next elements and also both of them:

val stream: UStream[Int] = ZStream.fromIterable(1 to 5)
val s1: UStream[(Option[Int], Int)]              = stream.zipWithPreviousval s2: UStream[(Int, Option[Int])]              = stream.zipWithNextval s3: UStream[(Option[Int], Int, Option[Int])] = stream.zipWithPreviousAndNext

By using ZStream#zipWithIndex we can index elements of a stream:

val indexedStream: ZStream[Any, Nothing, (String, Long)] =   ZStream("Mary", "James", "Robert", "Patricia").zipWithIndex
// Output: ("Mary", 0L), ("James", 1L), ("Robert", 2L), ("Patricia", 3L)

Cross Product#

ZIO stream has ZStram#cross and its variants to compute Cartesian Product of two streams:

val first = ZStream(1, 2, 3)val second = ZStream("a", "b")
val s1 = first cross secondval s2 = first <*> secondval s3 = first.crossWith(second)((a, b) => (a, b))// Output: (1,a), (1,b), (2,a), (2,b), (3,a), (3,b)
val s4 = first crossLeft second val s5 = first <* second// Keep only elements from the left stream// Output: 1, 1, 2, 2, 3, 3 
val s6 = first crossRight secondval s7 = first *> second// Keep only elements from the right stream// Output: a, b, a, b, a, b

Note that the right-hand side stream would be run multiple times, for every element in the left stream.

ZIO stream also has ZStream.crossN which takes streams up to four one.



ZStream#partition function splits the stream into tuple of streams based on the predicate. The first stream contains all element evaluated to true, and the second one contains all element evaluated to false.

The faster stream may advance by up to buffer elements further than the slower one. Two streams are wrapped by ZManaged type.

In the example below, left stream consists of even numbers only:

val partitionResult: ZManaged[Any, Nothing, (ZStream[Any, Nothing, Int], ZStream[Any, Nothing, Int])] =  Stream    .fromIterable(0 to 100)    .partition(_ % 2 == 0, buffer = 50)


If we need to partition a stream using an effectful predicate we can use ZStream.partitionEither.

abstract class ZStream[-R, +E, +O] {  final def partitionEither[R1 <: R, E1 >: E, O2, O3](    p: O => ZIO[R1, E1, Either[O2, O3]],    buffer: Int = 16  ): ZManaged[R1, E1, (ZStream[Any, E1, O2], ZStream[Any, E1, O3])]}

Here is a simple example of using this function:

val partitioned: ZManaged[Any, Nothing, (ZStream[Any, Nothing, Int], ZStream[Any, Nothing, Int])] =  ZStream    .fromIterable(1 to 10)    .partitionEither(x => ZIO.succeed(if (x < 5) Left(x) else Right(x)))



To partition the stream by function result we can use groupBy by providing a function of type O => K which determines by which keys the stream should be partitioned.

abstract class ZStream[-R, +E, +O] {  final def groupByKey[K](    f: O => K,    buffer: Int = 16  ): ZStream.GroupBy[R, E, K, O]}

In the example below, exam results are grouped into buckets and counted:

import zio._import
  case class Exam(person: String, score: Int)
  val examResults = Seq(    Exam("Alex", 64),    Exam("Michael", 97),    Exam("Bill", 77),    Exam("John", 78),    Exam("Bobby", 71)  )
  val groupByKeyResult: ZStream[Any, Nothing, (Int, Int)] =    Stream      .fromIterable(examResults)      .groupByKey(exam => exam.score / 10 * 10) {        case (k, s) => ZStream.fromZIO( => k -> l.size))      }


groupByKey partition the stream by a simple function of type O => K; It is not an effectful function. In some cases we need to partition the stream by using an effectful function of type O => ZIO[R1, E1, (K, V)]; So we can use groupBy which is the powerful version of groupByKey function.


It takes an effectful function of type O => ZIO[R1, E1, (K, V)]; ZIO Stream uses this function to partition the stream and gives us a new data type called ZStream.GroupBy which represent a grouped stream. GroupBy has an apply method, that takes a function of type (K, ZStream[Any, E, V]) => ZStream[R1, E1, A]; ZIO Runtime runs this function across all groups and then merges them in a non-deterministic fashion as a result.

abstract class ZStream[-R, +E, +O] {  final def groupBy[R1 <: R, E1 >: E, K, V](    f: O => ZIO[R1, E1, (K, V)],    buffer: Int = 16  ): ZStream.GroupBy[R1, E1, K, V]}

In the example below, we are going groupBy given names by their first character and then count the number of names in each group:

val counted: UStream[(Char, Long)] =  ZStream("Mary", "James", "Robert", "Patricia", "John", "Jennifer", "Rebecca", "Peter")    .groupBy(x => ZIO.succeed((x.head, x))) { case (char, stream) =>      ZStream.fromZIO( => char -> count))    }// Input:  Mary, James, Robert, Patricia, John, Jennifer, Rebecca, Peter// Output: (P, 2), (R, 2), (M, 1), (J, 3)

Let's change the above example a bit into an example of classifying students. The teacher assigns the student to a specific class based on the student's talent. Note that the partitioning operation is an effectful:

val classifyStudents: ZStream[Has[Console], IOException, (String, Seq[String])] =  ZStream.fromZIO(    printLine("Please assign each student to one of the A, B, or C classrooms.")  ) *> ZStream("Mary", "James", "Robert", "Patricia", "John", "Jennifer", "Rebecca", "Peter")    .groupBy(student =>      printLine(s"What is the classroom of $student? ") *> => (classroom, student))    ) { case (classroom, students) =>      ZStream.fromZIO(        students          .fold(Seq.empty[String])((s, e) => s :+ e)          .map(students => classroom -> students)      )    }
// Input: // Please assign each student to one of the A, B, or C classrooms.// What is the classroom of Mary? A// What is the classroom of James? B// What is the classroom of Robert? A// What is the classroom of Patricia? C// What is the classroom of John? B// What is the classroom of Jennifer? A// What is the classroom of Rebecca? C// What is the classroom of Peter? A//// Output: // (B,List(James, John))// (A,List(Mary, Robert, Jennifer, Peter))// (C,List(Patricia, Rebecca))



To partition the stream results with the specified chunk size, we can use the grouped function.

val groupedResult: ZStream[Any, Nothing, Chunk[Int]] =  Stream.fromIterable(0 to 8).grouped(3)
// Input:  0, 1, 2, 3, 4, 5, 6, 7, 8// Output: Chunk(0, 1, 2), Chunk(3, 4, 5), Chunk(6, 7, 8)


It allows grouping events by time or chunk size, whichever is satisfied first. In the example below every chunk consists of 30 elements and is produced every 3 seconds.

import zio._import zio.Duration._import
val groupedWithinResult: ZStream[Has[Clock], Nothing, Chunk[Int]] =  Stream.fromIterable(0 to 10)    .repeat(Schedule.spaced(1.seconds))    .groupedWithin(30, 10.seconds)


We can concatenate two streams by using ZStream#++ or ZStream#concat operator which returns a stream that emits the elements from the left-hand stream and then emits the elements from the right stream:

val a = ZStream(1, 2, 3)val b = ZStream(4, 5)val c1 = a ++ bval c2 = a concat b

Also, we can use ZStream.concatAll constructor to concatenate given streams together:

val c3 = ZStream.concatAll(Chunk(a, b))

There is also the ZStream#flatMap combinator which create a stream which elements are generated by applying a function of type O => ZStream[R1, E1, O2] to each output of the source stream and concatenated all of the results:

val stream = ZStream(1, 2, 3).flatMap(x => ZStream.repeat(x).take(4))// Input:  1, 2, 3// Output: 1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3

Assume we have an API that takes an author name and returns all its book:

def getAuthorBooks(author: String): ZStream[Any, Throwable, Book] = ZStream(???)

If we have a stream of author's names, we can use ZStream#flatMap to concatenate the results of all API calls:

val authors: ZStream[Any, Throwable, String] =   ZStream("Mary", "James", "Robert", "Patricia", "John")val allBooks: ZStream[Any, Throwable, Book]  =   authors.flatMap(getAuthorBooks _)

If we need to do the flatMap concurrently, we can use ZStream#flatMapPar, and also if the order of concatenation is not important for us, we can use the ZStream#flatMapParSwitch operator.


Sometimes we need to interleave the emission of two streams and create another stream. In these cases, we can't use the ZStream.concat operation because the concat operation waits for the first stream to finish and then consumes the second stream. So we need a non-deterministic way of picking elements from different sources. ZIO Stream's merge operations, do this for use. Let's discuss some variant of this operation:


The ZSstream#merge picks elements randomly from specified streams:

val s1 = ZStream(1, 2, 3).rechunk(1)val s2 = ZStream(4, 5, 6).rechunk(1)
val merged = s1 merge s2// As the merge operation is not deterministic, it may output the following stream of numbers:// Output: 4, 1, 2, 5, 6, 3

Merge operation always try to pull one chunk from each stream, if we chunk our streams equal or over 3 elements in the last example, we encounter a new stream containing one of the 1, 2, 3, 4, 5, 6 or 4, 5, 6, 1, 2, 3 elements.

Termination Strategy#

When we merge two streams, we should think about the termination strategy of this operation. Each stream has a specific lifetime. One stream may emit all its elements and finish its job, another stream may end after one hour of emission, one another may have a long-running lifetime and never end. So when we merge two streams with different lifetimes, what is the termination strategy of the resulting stream?

By default, when we merge two streams using ZStream#merge operation, the newly produced stream will terminate when both specified streams terminate. We can also define the termination strategy corresponding to our requirement. ZIO Stream supports four different termination strategies:

  • Left โ€” The resulting stream will terminate when the left-hand side stream terminates.
  • Right โ€” The resulting stream will terminate when the right-hand side stream finishes.
  • Both โ€” The resulting stream will terminate when both streams finish.
  • Either โ€” The resulting stream will terminate when one of the streams finishes.

Here is an example of specifying termination strategy when merging two streams:

import s1 = ZStream.iterate(1)(_+1).take(5).rechunk(1)val s2 = ZStream.repeat(0).rechunk(1)
val merged = s1.merge(s2, TerminationStrategy.Left)

We can also use ZStream#mergeTerminateLeft, ZStream#mergeTerminateRight or ZStream#mergeTerminateEither operations instead of specifying manually the termination strategy.


Usually, micro-services or long-running applications are composed of multiple components that need to run infinitely in the background and if something happens to them, or they terminate abruptly we should crash the entire application.

So our main fiber should perform these three things:

  • Launch and wait โ€” It should launch all of those background components and wait infinitely. It should not exit prematurely, because then our application won't be running.
  • Interrupt everything โ€” It should interrupt all those components whenever we receive a termination signal from the operating system.
  • Watch all fibers โ€” It should watch all those fibers (background components), and quickly exit if something goes wrong.

So how should we do that with our main fiber? Let's try to create a long-running application:

val main =   kafkaConsumer.runDrain.fork *>  httpServer.fork *>  scheduledJobRunner.fork *>  ZIO.never

We can launch the Kafka consumer, the HTTP server, and our job runner and fork them, and then wait using ZIO.never. This will indeed wait, but if something happens to any of them and if they crash, nothing happens. So our application just hangs and remains up without anything working in the background. So this approach does not work properly.

So another idea is to watch background components. The ZIO#forkManaged enables us to race all forked fibers in a ZManaged context. By using ZIO.raceAll as soon as one of those fibers terminates with either success or failure, it will interrupt all the rest components as the part of the release action of ZManaged:

val managedApp = for {  kafka <- kafkaConsumer.runDrain.forkManaged  http  <- httpServer.forkManaged  jobs  <- scheduledJobRunner.forkManaged} yield ZIO.raceAll(kafka.await, List(http.await, jobs.await))
val mainApp = managedApp.use(identity).exitCode

This solution is very nice and elegant, but we can do it in a more declarative fashion with ZIO streams:

val managedApp =  for {  //_ <- other resources    _ <- ZStream      .mergeAllUnbounded(16)(        kafkaConsumer.drain,        ZStream.fromZIO(httpServer),        ZStream.fromZIO(scheduledJobRunner)      )      .runDrain      .toManaged  } yield ()
val myApp = managedApp.useDiscard(ZIO.unit).exitCode

Using ZStream.mergeAll we can combine all these streaming components concurrently into one application.


Sometimes we need to merge two streams and after that, unify them and convert them to new element types. We can do this by using the ZStream#mergeWith operation:

val s1 = ZStream("1", "2", "3")val s2 = ZStream(4.1, 5.3, 6.2)
val merged = s1.mergeWith(s2)(_.toInt, _.toInt)


When we merge two streams, the ZIO Stream picks elements from two streams randomly. But how to merge two streams deterministically? The answer is the ZStream#interleave operation.

The ZStream#interleave operator pulls an element from each stream, one by one, and then returns an interleaved stream. When one stream is exhausted, all remaining values in the other stream will be pulled:

val s1 = ZStream(1, 2, 3)val s2 = ZStream(4, 5, 6, 7, 8)
val interleaved = s1 interleave s2
// Output: 1, 4, 2, 5, 3, 6, 7, 8

ZIO Stream also has the interleaveWith operator, which is a more powerful version of interleave. By using ZStream#interleaveWith, we can specify the logic of interleaving:

val s1 = ZStream(1, 3, 5, 7, 9)val s2 = ZStream(2, 4, 6, 8, 10)
val interleaved = s1.interleaveWith(s2)(ZStream(true, false, false).forever)// Output: 1, 2, 4, 3, 6, 8, 5, 10, 7, 9

ZStream#interleaveWith uses a stream of boolean to decide which stream to choose. If it reaches a true value, it will pick a value from the left-hand side stream, otherwise, it will pick from the right-hand side.


We can intersperse any stream by using ZStream#intersperse operator:

val s1 = ZStream(1, 2, 3, 4, 5).intersperse(0)// Output: 1, 0, 2, 0, 3, 0, 4, 0, 5
val s2 = ZStream("a", "b", "c", "d").intersperse("[", "-", "]")// Output: [, -, a, -, b, -, c, -, d]


We can broadcast a stream by using ZStream#broadcast, it returns a managed list of streams that have the same elements as the source stream. The broadcast operation emits each element to the inputs of returning streams. The upstream stream can emit events as much as maximumLag, then it decreases its speed by the slowest downstream stream.

In the following example, we are broadcasting stream of random numbers to the two downstream streams. One of them is responsible to compute the maximum number, and the other one does some logging job with additional delay. The upstream stream decreases its speed by the logging stream:

val stream: ZIO[Has[Console] with Has[Random] with Has[Clock], IOException, Unit] =  ZStream    .fromIterable(1 to 20)    .mapZIO(_ => Random.nextInt)    .map(Math.abs)    .map(_ % 100)    .tap(e => printLine(s"Emit $e element before broadcasting"))    .broadcast(2, 5)    .use {      case s1 :: s2 :: Nil =>        for {          out1 <- s1.fold(0)((acc, e) => Math.max(acc, e))                    .flatMap(x => printLine(s"Maximum: $x"))                    .fork          out2 <- s2.schedule(Schedule.spaced(1.second))                    .foreach(x => printLine(s"Logging to the Console: $x"))                    .fork          _    <- out1.join.zipPar(out2.join)        } yield ()
      case _ => ZIO.dieMessage("unhandled case")    }


The ZStream#distributedWith operator is a more powerful version of ZStream#broadcast. It takes a decide function, and based on that decide how to distribute incoming elements into the downstream streams:

abstract class ZStream[-R, +E, +O] {  final def distributedWith[E1 >: E](    n: Int,    maximumLag: Int,    decide: O => UIO[Int => Boolean]  ): ZManaged[R, Nothing, List[Dequeue[Exit[Option[E1], O]]]] = ???}

In the example below, we are partitioning incoming elements into three streams using ZStream#distributedWith operator:

val partitioned: ZManaged[Has[Clock], Nothing, (UStream[Int], UStream[Int], UStream[Int])] =  ZStream    .iterate(1)(_ + 1)    .fixed(1.seconds)    .distributedWith(3, 10, x => ZIO.succeed(q => x % 3 == q))    .flatMap {       case q1 :: q2 :: q3 :: Nil =>        ZManaged.succeed(          ZStream.fromQueue(q1).flattenExitOption,          ZStream.fromQueue(q2).flattenExitOption,          ZStream.fromQueue(q3).flattenExitOption        )      case _ => ZManaged.dieMessage("Impossible!")    }


Since the ZIO streams are pull-based, it means the consumers do not need to message the upstream to slow down. Whenever a downstream stream pulls a new element, the upstream produces a new element. So, the upstream stream is as fast as the slowest downstream stream. Sometimes we need to run producer and consumer independently, in such a situation we can use an asynchronous non-blocking queue for communication between faster producer and slower consumer; the queue can buffer elements between two streams. ZIO stream also has a built-in ZStream#buffer operator which does the same thing for us.

The ZStream#buffer allows a faster producer to progress independently of a slower consumer by buffering up to capacity chunks in a queue.

In the following example, we are going to buffer a stream. We print each element to the console as they are emitting before and after the buffering:

ZStream  .fromIterable(1 to 10)  .rechunk(1)  .tap(x => Console.printLine(s"before buffering: $x"))  .buffer(4)  .tap(x => Console.printLine(s"after buffering: $x"))  .schedule(Schedule.spaced(5.second))  

We spaced 5 seconds between each emission to show the lag between producing and consuming messages.

Based on the type of underlying queue we can use one the buffering operators:

  • Bounded Queue โ€” ZStream#buffer(capacity: Int)
  • Unbounded Queue โ€” ZStream#bufferUnbounded
  • Sliding Queue โ€” ZStream#bufferDropping(capacity: Int)
  • Dropping Queue ZStream#bufferSliding(capacity: Int)


The ZStream#debounce method debounces the stream with a minimum period of d between each element:

val stream = (  ZStream(1, 2, 3) ++    ZStream.fromZIO(ZIO.sleep(500.millis)) ++ ZStream(4, 5) ++    ZStream.fromZIO(ZIO.sleep(10.millis)) ++    ZStream(6)).debounce(100.millis) // emit only after a pause of at least 100 ms// Output: 3, 6


Aggregation is the process of converting one or more elements of type A into elements of type B. This operation takes a transducer as an aggregation unit and returns another stream that is aggregated. We have two types of aggregation:

Synchronous Aggregation#

They are synchronous because the upstream emits an element when the transducer emits one. To apply a synchronous aggregation to the stream we can use ZStream#aggregate or ZStream#transduce operations.

Let's see an example of synchronous aggregation:

val stream = ZStream(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)val s1 = stream.transduce(ZTransducer.collectAllN(3))// Output Chunk(1,2,3), Chunk(4,5,6), Chunk(7,8,9), Chunk(10)
val s2 = stream.aggregate(ZTransducer.collectAllN(3))// Output Chunk(1,2,3), Chunk(4,5,6), Chunk(7,8,9), Chunk(10)

Sometimes stream processing element by element is not efficient, specially when we are working with files or doing I/O works; so we might need to aggregate them and process them in a batch way:

val source =  ZStream    .iterate(1)(_ + 1)    .take(200)    .tap(x =>      printLine(s"Producing Element $x")        .schedule(Schedule.duration(1.second).jittered)    )
val sink =   ZSink.foreach((e: Chunk[Int]) =>    printLine(s"Processing batch of events: $e")      .schedule(Schedule.duration(3.seconds).jittered)  )  val myApp =   source.aggregate(ZTransducer.collectAllN[Int](5)).run(sink)

Let's see one output of running this program:

Producing element 1Producing element 2Producing element 3Producing element 4Producing element 5Processing batch of events: Chunk(1,2,3,4,5)Producing element 6Producing element 7Producing element 8Producing element 9Producing element 10Processing batch of events: Chunk(6,7,8,9,10)Producing element 11Producing element 12Processing batch of events: Chunk(11,12)

Elements are grouped into Chunks of 5 elements and then processed in a batch way.

Asynchronous Aggregation#

Asynchronous aggregations, aggregate elements of upstream as long as the downstream operators are busy. To apply an asynchronous aggregation to the stream, we can use ZStream#aggregateAsync, ZStream#aggregateAsyncWithin, and ZStream#aggregateAsyncWithinEither operations.

For example, consider source.aggregateAsync(ZTransducer.collectAllN(5)).mapZIO(processChunks). Whenever the downstream (mapZIO(processChunks)) is ready for consumption and pulls the upstream, the transducer (ZTransducer.collectAllN(5)) will flush out its buffer, regardless of whether the collectAllN buffered all its 5 elements or not. So the ZStream#aggregateAsync will emit when downstream pulls:

val myApp =   source.aggregateAsync(ZTransducer.collectAllN[Int](5)).run(sink)

Let's see one output of running this program:

Producing element 1Producing element 2Producing element 3Producing element 4Processing batch of events: Chunk(1,2)Processing batch of events: Chunk(3,4)Producing element 5Processing batch of events: Chunk(5)Producing element 6Processing batch of events: Chunk(6)Producing element 7Producing element 8Producing element 9Processing batch of events: Chunk(7)Producing element 10Producing element 11Processing batch of events: Chunk(8,9)Producing element 12Processing batch of events: Chunk(10,11)Processing batch of events: Chunk(12)

The ZStream#aggregateAsyncWithin is another aggregator which takes a scheduler. This scheduler will consume all events produced by the given transducer. So the aggregateAsyncWithin will emit when the transducer emits or when the scheduler expires:

abstract class ZStream[-R, +E, +O] {  def aggregateAsyncWithin[R1 <: R, E1 >: E, P](    transducer: ZTransducer[R1, E1, O, P],    schedule: Schedule[R1, Chunk[P], Any]  ): ZStream[R1 with Clock, E1, P] = ???}

When we are doing I/O, batching is very important. With ZIO streams, we can create user-defined batches. It is pretty easy to do that with the ZStream#aggregateAsyncWithin operator. Let's see the below snippet code:

dataStream.aggregateAsyncWithin(   ZTransducer.collectAllN(2000),   Schedule.fixed(30.seconds) )

So it will collect elements into a chunk up to 2000 elements and if we have got less than 2000 elements and 30 seconds have passed, it will pass currently collected elements down the stream whether it has collected zero, one, or 2000 elements. So this is a sort of timeout for aggregation operation. This approach aggressively favors throughput over latency. It will introduce a fixed amount of latency into a stream. We will always wait for up to 30 seconds if we haven't reached this sort of boundary value.

Instead, thanks to Schedule we can create a much smarter adaptive batching algorithm that can balance between throughput and *latency. So what we are doing here is that we are creating a schedule that operates on chunks of records. What the Schedule does is that it starts off with 30-second timeouts for as long as its input has a size that is lower than 1000, now once we see an input that has a size look higher than 1000, we will switch to a second schedule with some jittery, and we will remain with this schedule for as long as the batch size is over 1000:

val schedule: Schedule[Has[Clock] with Has[Random], Chunk[Chunk[Record]], Long] =  // Start off with 30-second timeouts as long as the batch size is < 1000  Schedule.fixed(30.seconds).whileInput[Chunk[Chunk[Record]]](_.flatten.length < 100) andThen    // and then, switch to a shorter jittered schedule for as long as batches remain over 1000    Schedule.fixed(5.seconds).jittered.whileInput[Chunk[Chunk[Record]]](_.flatten.length >= 1000)    dataStream  .aggregateAsyncWithin(ZTransducer.collectAllN(2000), schedule)


To schedule the output of a stream we use ZStream#schedule combinator.

Let's space between each emission of the given stream:

val stream = Stream(1, 2, 3, 4, 5).schedule(Schedule.spaced(1.second))

Consuming a Stream#

import zio._import zio.Console._import
val result: RIO[Has[Console], Unit] = Stream.fromIterable(0 to 100).foreach(printLine(_))

Using a Sink#

To consume a stream using ZSink we can pass ZSink to the ZStream#run function:

val sum: UIO[Int] = ZStream(1,2,3).run(Sink.sum)

Using fold#

The ZStream#fold method executes the fold operation over the stream of values and returns a ZIO effect containing the result:

val s1: ZIO[Any, Nothing, Int] = ZStream(1, 2, 3, 4, 5).fold(0)(_ + _)val s2: ZIO[Any, Nothing, Int] = ZStream.iterate(1)(_ + 1).foldWhile(0)(_ <= 5)(_ + _)

Using foreach#

Using ZStream#foreach is another way of consuming elements of a stream. It takes a callback of type O => ZIO[R1, E1, Any] which passes each element of a stream to this callback:

ZStream(1, 2, 3).foreach(printLine(_))

Error Handling#

Recovering from Failure#

If we have a stream that may fail, we might need to recover from the failure and run another stream, the ZStream#orElse takes another stream, so when the failure occurs it will switch over to the provided stream:

val s1 = ZStream(1, 2, 3) ++"Oh! Error!") ++ ZStream(4, 5)val s2 = ZStream(7, 8, 9)
val stream = s1.orElse(s2)// Output: 1, 2, 3, 7, 8, 9

Another variant of orElse is ZStream#orElseEither, which distinguishes elements of the two streams using the Either data type. Using this operator, the result of the previous example should be Left(1), Left(2), Left(3), Right(6), Right(7), Right(8).

ZIO stream has ZStream#catchAll which is powerful version of ZStream#orElse. By using catchAll we can decide what to do based on the type and value of the failure:

val first =  ZStream(1, 2, 3) ++"Uh Oh!") ++    ZStream(4, 5) ++"Ouch")
val second = ZStream(6, 7, 8)val third = ZStream(9, 10, 11)
val stream = first.catchAll {  case "Uh Oh!" => second  case "Ouch"   => third}// Output: 1, 2, 3, 6, 7, 8

Recovering from Defects#

If we need to recover from all causes of failures including defects we should use the ZStream#catchAllCause method:

val s1 = ZStream(1, 2, 3) ++ ZStream.dieMessage("Oh! Boom!") ++ ZStream(4, 5)val s2 = ZStream(7, 8, 9)
val stream = s1.catchAllCause(_ => s2)// Output: 1, 2, 3, 7, 8, 9

Recovery from Some Errors#

If we need to recover from specific failure we should use ZStream#catchSome:

val s1 = ZStream(1, 2, 3) ++"Oh! Error!") ++ ZStream(4, 5)val s2 = ZStream(7, 8, 9)val stream = s1.catchSome {  case "Oh! Error!" => s2}// Output: 1, 2, 3, 7, 8, 9

And, to recover from a specific cause, we should use ZStream#catchSomeCause method:

val s1 = ZStream(1, 2, 3) ++ ZStream.dieMessage("Oh! Boom!") ++ ZStream(4, 5)val s2 = ZStream(7, 8, 9)val stream = s1.catchSomeCause { case Die(value) => s2 }

Recovering to ZIO Effect#

If our stream encounters an error, we can provide some cleanup task as ZIO effect to our stream by using the ZStream#onError method:

val stream =   (ZStream(1, 2, 3) ++ ZStream.dieMessage("Oh! Boom!") ++ ZStream(4, 5))    .onError(_ => printLine("Stream application closed! We are doing some cleanup jobs.").orDie)

Retry a Failing Stream#

When a stream fails, it can be retried according to the given schedule to the ZStream#retry operator:

val numbers = ZStream(1, 2, 3) ++   ZStream    .fromZIO(      Console.print("Enter a number: ") *> Console.readLine        .flatMap(x =>          x.toIntOption match {            case Some(value) => ZIO.succeed(value)            case None        =>"NaN")          }        )    )    .retry(Schedule.exponential(1.second))

From/To Either#

Sometimes, we might be working with legacy API which does error handling with the Either data type. We can absolve their error types into the ZStream effect using ZStream.absolve:

def legacyFetchUrlAPI(url: URL): Either[Throwable, String] = ???
def fetchUrl(    url: URL): ZStream[Any, Throwable, String] =   ZStream.fromZIO(    ZIO.attemptBlocking(legacyFetchUrlAPI(url))  ).absolve

The type of this stream before absolving is ZStream[Any, Throwable, Either[Throwable, String]], this operation let us submerge the error case of an Either into the ZStream error type.

We can do the opposite by exposing an error of type ZStream[R, E, A] as a part of the Either by using ZStream#either:

val inputs: ZStream[Has[Console], Nothing, Either[IOException, String]] =   ZStream.fromZIO(Console.readLine).either

When we are working with streams of Either values, we might want to fail the stream as soon as the emission of the first Left value:

// Stream of Either values that cannot failval eitherStream: ZStream[Any, Nothing, Either[String, Int]] =  ZStream(Right(1), Right(2), Left("failed to parse"), Right(4))
// A Fails with the first emission of the left valueval stream: ZStream[Any, String, Int] = eitherStream.rightOrFail("fail")

Refining Errors#

We can keep one or some errors and terminate the fiber with the rest by using ZStream#refineOrDie:

val stream: ZStream[Any, Throwable, Int] = Throwable)
val res: ZStream[Any, IllegalArgumentException, Int] =  stream.refineOrDie { case e: IllegalArgumentException => e }

Timing Out#

We can timeout a stream if it does not produce a value after some duration using ZStream#timeout, ZStream#timeoutError and timeoutErrorCause operators:

stream.timeoutError(new TimeoutException)(10.seconds)

Or we can switch to another stream if the first stream does not produce a value after some duration:

val alternative = ZStream.fromZIO(ZIO.attempt(???))stream.timeoutTo(10.seconds)(alternative)