Is ScriptEngine of Scala thread-safe? This is from Scala-2.12 based implementation:
import java.util.concurrent.Executors
import javax.script._
import scala.concurrent.duration._
import scala.concurrent.{Await, ExecutionContext, Future}
object Prac extends App {
val scripter = new ScriptEngineManager().getEngineByName("scala")
val exe = Executors.newFixedThreadPool(100)
implicit val e = ExecutionContext.fromExecutor(exe)
val ans = for {
x <- 1 to 100
} yield Future {
val bindings = scripter.createBindings()
bindings.put("x", x)
val script = scripter.eval(
"""
def sqrt(n:Int) = math.sqrt(n)
sqrt(x.asInstanceOf[Int])
""".stripMargin, bindings)
require(script == math.sqrt(x))
println(x+" - " + script)
}
Await.result(Future.sequence(ans), 20 seconds)
}
The above fails if run in parallel. Runs perfectly fine if run sequential.
As per this tweet confirmation by Adriaan Moors, it is not thread-safe.
Related
I am trying to create an (Akka HTTP) stream procsesing flow using the classes akka.stream.scaladsl.Source and Sink queues.
I am using a queue because I have a processing step in my flow that issues http requests and I want this step to take as many
items off the queue as there are max-open-requests, and stop taking off the queue once max-open-requests are in flight.
The result is that backpressure is applied when my connection pool is overloaded.
Below, I have a very simplified test that reflects the main logic of my app. In the test 'Stress Spec' (below)
I am simulating a number of simultaneous connections via which I will send a 'Source' of 'Requesto' objects
to the getResponses method of the class ServiceImpl.
In the processing step 'pullOffSinkQueue' you will note that I am incrementing a counter to see how many items
I have pulled off the queue.
The test will send Serviceimpl a set of requests whose cardinality is set to equal
streamedRequestsPerConnection * numSimultaneousConnections.
When I send 20 requests my test passes fine. In particular the count of requests pulled off the
Sink.queue will be equal to the number of requests I send out. However, if
I increase the number of requests I send to above 50 or so, I see consistent failures in the test.
I get a message such as the one below
180 was not equal to 200
ScalaTestFailureLocation: com.foo.StressSpec at (StressSpec.scala:116)
Expected :200
Actual :180
<Click to see difference>
This indicates that the number of items pulled off the queue does not equal the number of items put on the queue.
I have a feeling this might be due to the fact that my test is not properly waiting for all items put into the stream
to be processed. If anyone has any suggestions, I'd be all ears ! Code is below.
package com.foo
import java.util.concurrent.atomic.AtomicInteger
import akka.stream.ActorAttributes.supervisionStrategy
import akka.stream.{Attributes, Materializer, QueueOfferResult}
import akka.stream.Supervision.resumingDecider
import akka.stream.scaladsl.{Flow, Keep, Sink, Source}
import scala.concurrent.{ExecutionContext, Future}
import akka.NotUsed
import akka.actor.ActorSystem
import akka.event.{Logging, LoggingAdapter}
import akka.stream.ActorMaterializer
import akka.stream.scaladsl.{Sink, Source}
import org.scalatest.mockito.MockitoSugar
import org.scalatest.{FunSuite, Matchers}
import scala.collection.immutable
import scala.concurrent.duration._
import scala.concurrent.{Await, Future, _}
final case class Responso()
final case class Requesto()
object Handler {
val dbRequestCounter = new AtomicInteger(0)
}
class Handler(implicit ec: ExecutionContext, mat: Materializer) {
import Handler._
private val source =
Source.queue[(Requesto, String)](8, akka.stream.OverflowStrategy.backpressure)
private val sink =
Sink.queue[(Requesto, String)]().withAttributes(Attributes.inputBuffer(8, 8))
private val (sourceQueue, sinkQueue) = source.toMat(sink)(Keep.both).run()
def placeOnSourceQueue(ar: Requesto): Future[QueueOfferResult] = {
sourceQueue.offer((ar, "foo"))
}
def pullOffSinkQueue(qofr: QueueOfferResult): Future[Responso] = {
dbRequestCounter.incrementAndGet()
qofr match {
case QueueOfferResult.Enqueued =>
sinkQueue.pull().flatMap { maybeRequestPair: Option[(Requesto, String)] =>
Future.successful(Responso())
}
case error =>
println("enqueuing error: " + error)
Future.failed(new RuntimeException("enqueuing error: " + error))
}
}
}
class ServiceImpl(readHandler: Handler, writeHandler: Handler)
(implicit log: LoggingAdapter, mat: Materializer) {
private val readAttributeFlow: Flow[Requesto, Responso, NotUsed] = {
Flow[Requesto]
.mapAsyncUnordered(1)(readHandler.placeOnSourceQueue)
.mapAsyncUnordered(1)(readHandler.pullOffSinkQueue)
}
def getResponses(request: Source[Requesto, NotUsed]): Source[Responso, NotUsed] =
request
.via(readAttributeFlow)
.withAttributes(supervisionStrategy(resumingDecider))
}
class StressSpec
extends FunSuite
with MockitoSugar
with Matchers {
val streamedRequestsPerConnection = 10
val numSimultaneousConnections = 20
implicit val actorSystem: ActorSystem = ActorSystem()
implicit val materializer: ActorMaterializer = ActorMaterializer()
implicit val log: LoggingAdapter = Logging(actorSystem.eventStream, "test")
implicit val ec: ExecutionContext = actorSystem.dispatcher
import Handler._
lazy val requestHandler = new Handler()
lazy val svc: ServiceImpl =
new ServiceImpl(requestHandler, requestHandler)
test("can handle lots of simultaneous read requests") {
val totalExpected = streamedRequestsPerConnection * numSimultaneousConnections
def sendRequestAndAwaitResponse(): Unit = {
def getResponses(i: Integer) = {
val requestStream: Source[Requesto, NotUsed] =
Source(1 to streamedRequestsPerConnection)
.map { i =>
Requesto()
}
svc.getResponses(requestStream).runWith(Sink.seq)
}
val responses: immutable.Seq[Future[immutable.Seq[Responso]]] =
(1 to numSimultaneousConnections).map { getResponses(_) }
val flattenedResponses: Future[immutable.Seq[Responso]] =
Future.sequence(responses).map(_.flatten)
Await.ready(flattenedResponses, 1000.seconds).value.get
}
sendRequestAndAwaitResponse()
dbRequestCounter.get shouldBe(totalExpected)
}
}
I am trying to understand how i should be working with Source.queue & Sink.queue in Akka streaming.
In the little test program that I wrote below I find that I am able to successfully offer 1000 items to the Source.queue.
However, when i wait on the future that should give me the results of pulling all those items off the queue, my
future never completes. Specifically, the message 'print what we pulled off the queue' that we should see at the end
never prints out -- instead we see the error "TimeoutException: Futures timed out after [10 seconds]"
any guidance greatly appreciated !
import akka.actor.ActorSystem
import akka.event.{Logging, LoggingAdapter}
import akka.stream.scaladsl.{Flow, Keep, Sink, Source}
import akka.stream.{ActorMaterializer, Attributes}
import org.scalatest.FunSuite
import scala.collection.immutable
import scala.concurrent.duration._
import scala.concurrent.{Await, ExecutionContext, Future}
class StreamSpec extends FunSuite {
implicit val actorSystem: ActorSystem = ActorSystem()
implicit val materializer: ActorMaterializer = ActorMaterializer()
implicit val log: LoggingAdapter = Logging(actorSystem.eventStream, "basis-test")
implicit val ec: ExecutionContext = actorSystem.dispatcher
case class Req(name: String)
case class Response(
httpVersion: String = "",
method: String = "",
url: String = "",
headers: Map[String, String] = Map())
test("put items on queue then take them off") {
val source = Source.queue[String](128, akka.stream.OverflowStrategy.backpressure)
val flow = Flow[String].map(element => s"Modified $element")
val sink = Sink.queue[String]().withAttributes( Attributes.inputBuffer(128, 128))
val (sourceQueue, sinkQueue) = source.via(flow).toMat(sink)(Keep.both).run()
(1 to 1000).map( i =>
Future {
println("offerd" + i) // I see this print 1000 times as expected
sourceQueue.offer(s"batch-$i")
}
)
println("DONE OFFER FUTURE FIRING")
// Now use the Sink.queue to pull the items we added onto the Source.queue
val seqOfFutures: immutable.Seq[Future[Option[String]]] =
(1 to 1000).map{ i => sinkQueue.pull() }
val futureOfSeq: Future[immutable.Seq[Option[String]]] =
Future.sequence(seqOfFutures)
val seq: immutable.Seq[Option[String]] =
Await.result(futureOfSeq, 10.second)
// unfortunately our future times out here
println("print what we pulled off the queue:" + seq);
}
}
Looking at this again, I realize that I originally set up and posed my question incorrectly.
The test that accompanies my original question launches a wave
of 1000 futures, each of which tries to offer 1 item to the queue.
Then the second step in that test attempts create a 1000-element sequence (seqOfFutures)
where each future is trying to pull a value from the queue.
My theory as to why I was getting time-out errors is that there was some kind of deadlock due to running
out of threads or due to one thread waiting on another but where the waited-on-thread was blocked,
or something like that.
I'm not interested in hunting down the exact cause at this point because I have corrected
things in the code below (see CORRECTED CODE).
In the new code the test that uses the queue is called:
"put items on queue then take them off (with async parallelism) - (3)".
In this test I have a set of 10 tasks which run in parallel to do the 'enequeue' operation.
Then I have another 10 tasks which do the dequeue operation, which involves not only taking
the item off the list, but also calling stringModifyFunc which introduces a 1 ms processing delay.
I also wanted to prove that I got some performance benefit from
launching tasks in parallel and having the task steps communicate by passing their results through a
queue, so test 3 runs as a timed operation, and I found that it takes 1.9 seconds.
Tests (1) and (2) do the same amount of work, but serially -- The first with no intervening queue, and the second
using the queue to pass results between steps. These tests run in 13.6 and 15.6 seconds respectively
(which shows that the queue adds a bit of overhead, but that this is overshadowed by the efficiencies of running tasks in parallel.)
CORRECTED CODE
import akka.{Done, NotUsed}
import akka.actor.ActorSystem
import akka.event.{Logging, LoggingAdapter}
import akka.stream.scaladsl.{Flow, Keep, Sink, Source}
import akka.stream.{ActorMaterializer, Attributes, QueueOfferResult}
import org.scalatest.FunSuite
import scala.concurrent.duration._
import scala.concurrent.{Await, ExecutionContext, Future}
class Speco extends FunSuite {
implicit val actorSystem: ActorSystem = ActorSystem()
implicit val materializer: ActorMaterializer = ActorMaterializer()
implicit val log: LoggingAdapter = Logging(actorSystem.eventStream, "basis-test")
implicit val ec: ExecutionContext = actorSystem.dispatcher
val stringModifyFunc: String => String = element => {
Thread.sleep(1)
s"Modified $element"
}
def setup = {
val source = Source.queue[String](128, akka.stream.OverflowStrategy.backpressure)
val sink = Sink.queue[String]().withAttributes(Attributes.inputBuffer(128, 128))
val (sourceQueue, sinkQueue) = source.toMat(sink)(Keep.both).run()
val offers: Source[String, NotUsed] = Source(
(1 to iterations).map { i =>
s"item-$i"
}
)
(sourceQueue,sinkQueue,offers)
}
val outer = 10
val inner = 1000
val iterations = outer * inner
def timedOperation[T](block : => T) = {
val t0 = System.nanoTime()
val result: T = block // call-by-name
val t1 = System.nanoTime()
println("Elapsed time: " + (t1 - t0) / (1000 * 1000) + " milliseconds")
result
}
test("20k iterations in single threaded loop no queue (1)") {
timedOperation{
(1 to iterations).foreach { i =>
val str = stringModifyFunc(s"tag-${i.toString}")
System.out.println("str:" + str);
}
}
}
test("20k iterations in single threaded loop with queue (2)") {
timedOperation{
val (sourceQueue, sinkQueue, offers) = setup
val resultFuture: Future[Done] = offers.runForeach{ str =>
val itemFuture = for {
_ <- sourceQueue.offer(str)
item <- sinkQueue.pull()
} yield (stringModifyFunc(item.getOrElse("failed")) )
val item = Await.result(itemFuture, 10.second)
System.out.println("item:" + item);
}
val result = Await.result(resultFuture, 20.second)
System.out.println("result:" + result);
}
}
test("put items on queue then take them off (with async parallelism) - (3)") {
timedOperation{
val (sourceQueue, sinkQueue, offers) = setup
def enqueue(str: String) = sourceQueue.offer(str)
def dequeue = {
sinkQueue.pull().map{
maybeStr =>
val str = stringModifyFunc( maybeStr.getOrElse("failed2"))
println(s"dequeud value is $str")
}
}
val offerResults: Source[QueueOfferResult, NotUsed] =
offers.mapAsyncUnordered(10){ string => enqueue(string)}
val dequeueResults: Source[Unit, NotUsed] = offerResults.mapAsyncUnordered(10){ _ => dequeue }
val runAll: Future[Done] = dequeueResults.runForeach(u => u)
Await.result(runAll, 20.second)
}
}
}
I know I'm doing something wrong with mutable.ListBuffer but I can't figure out how to fix it (and a proper explanation of the issue).
I simplified the code below to reproduce the behavior.
I'm basically trying to run functions in parallel to add elements to a list as my first list get processed. I end up "losing" elements.
import java.util.Properties
import scala.collection.mutable.ListBuffer
import scala.concurrent.duration.Duration
import scala.concurrent.{Await, Future}
import scala.concurrent.{ExecutionContext}
import ExecutionContext.Implicits.global
object MyTestObject {
var listBufferOfInts = new ListBuffer[Int]() // files that are processed
def runFunction(): Int = {
listBufferOfInts = new ListBuffer[Int]()
val inputListOfInts = 1 to 1000
val fut = Future.traverse(inputListOfInts) { i =>
Future {
appendElem(i)
}
}
Await.ready(fut, Duration.Inf)
listBufferOfInts.length
}
def appendElem(elem: Int): Unit = {
listBufferOfInts ++= List(elem)
}
}
MyTestObject.runFunction()
MyTestObject.runFunction()
MyTestObject.runFunction()
which returns:
res0: Int = 937
res1: Int = 992
res2: Int = 997
Obviously I would expect 1000 to be returned all the time. How can I fix my code to keep the "architecture" but make my ListBuffer "synchronized" ?
I don't know what exact problem is as you said you simplified it, but still you have an obvious race condition, multiple threads modify a single mutable collection and that is very bad. As other answers pointed out you need some locking so that only one thread could modify collection at the same time. If your calculations are heavy, appending result in synchronized way to a buffer shouldn't notably affect the performance but when in doubt always measure.
But synchronization is not needed, you can do something else instead, without vars and mutable state. Let each Future return your partial result and then merge them into a list, in fact Future.traverse does just that.
import scala.concurrent.duration._
import scala.concurrent.{Await, Future}
import scala.concurrent.ExecutionContext.Implicits.global
def runFunction: Int = {
val inputListOfInts = 1 to 1000
val fut: Future[List[Int]] = Future.traverse(inputListOfInts.toList) { i =>
Future {
// some heavy calculations on i
i * 4
}
}
val listOfInts = Await.result(fut, Duration.Inf)
listOfInts.size
}
Future.traverse already gives you an immutable list with all your results combined, no need to append them to a mutable buffer.
Needless to say, you will always get 1000 back.
# List.fill(10000)(runFunction).exists(_ != 1000)
res18: Boolean = false
I'm not sure the above shows what you are trying to do correctly. Maybe the issue is that you are actually sharing a var ListBuffer which you reinitialise within runFunction.
When I take this out I collect all the events I'm expecting correctly:
import java.util.Properties
import scala.collection.mutable.ListBuffer
import scala.concurrent.duration.Duration
import scala.concurrent.{ Await, Future }
import scala.concurrent.{ ExecutionContext }
import ExecutionContext.Implicits.global
object BrokenTestObject extends App {
var listBufferOfInts = ( new ListBuffer[Int]() )
def runFunction(): Int = {
val inputListOfInts = 1 to 1000
val fut = Future.traverse(inputListOfInts) { i =>
Future {
appendElem(i)
}
}
Await.ready(fut, Duration.Inf)
listBufferOfInts.length
}
def appendElem(elem: Int): Unit = {
listBufferOfInts.append( elem )
}
BrokenTestObject.runFunction()
BrokenTestObject.runFunction()
BrokenTestObject.runFunction()
println(s"collected ${listBufferOfInts.length} elements")
}
If you really have a synchronisation issue you can use something like the following:
import java.util.Properties
import scala.collection.mutable.ListBuffer
import scala.concurrent.duration.Duration
import scala.concurrent.{ Await, Future }
import scala.concurrent.{ ExecutionContext }
import ExecutionContext.Implicits.global
class WrappedListBuffer(val lb: ListBuffer[Int]) {
def append(i: Int) {
this.synchronized {
lb.append(i)
}
}
}
object MyTestObject extends App {
var listBufferOfInts = new WrappedListBuffer( new ListBuffer[Int]() )
def runFunction(): Int = {
val inputListOfInts = 1 to 1000
val fut = Future.traverse(inputListOfInts) { i =>
Future {
appendElem(i)
}
}
Await.ready(fut, Duration.Inf)
listBufferOfInts.lb.length
}
def appendElem(elem: Int): Unit = {
listBufferOfInts.append( elem )
}
MyTestObject.runFunction()
MyTestObject.runFunction()
MyTestObject.runFunction()
println(s"collected ${listBufferOfInts.lb.size} elements")
}
Changing
listBufferOfInts ++= List(elem)
to
synchronized {
listBufferOfInts ++= List(elem)
}
Make it work. Probably can become a performance issue? I'm still interested in an explanation and maybe a better way of doing things!
I wrote this method:
import scala.concurrent._
import ExecutionContext.Implicits.global
import scala.util.{ Success, Failure }
object FuturesSequence extends App {
val f1 = future {
1
}
val f2 = future {
2
}
val lf = List(f1, f2)
val seq = Future.sequence(lf)
seq.onSuccess {
case l => println(l)
}
}
I was expecting Future.sequence to gather a List[Future] into a Future[List] and then wait for every futures (f1 and f2 in my case) to complete before calling onSuccess on the Future[List] seq in my case.
But after many runs of this code, it prints "List(1, 2)" only once in a while and I can't figure out why it does not work as expected.
Try this for once,
import scala.concurrent._
import java.util.concurrent.Executors
import scala.util.{ Success, Failure }
object FuturesSequence extends App {
implicit val exec = ExecutionContext.fromExecutor(Executors.newCachedThreadPool)
val f1 = future {
1
}
val f2 = future {
2
}
val lf = List(f1, f2)
val seq = Future.sequence(lf)
seq.onSuccess {
case l => println(l)
}
}
This will always print List(1,2). The reason is simple, the exec above is an ExecutionContext of threads (not daemon threads) where as in your example the ExecutionContext was the default one implicitly taken from ExecutionContext.Implicits.global which contains daemon threads.
Hence being daemon, the process doesn't wait for seq future to be completed and terminates. if at all seq does get completed then it prints. But that doesn't happen always
The application is exiting before the future is completes.
You need to block until the future has completed. This can be achieved in a variety of ways, including changing the ExecutionContext, instantiating a new ThreadPool, Thread.sleep etc, or by using methods on scala.concurrent.Await
The simplest way for your code is by using Await.ready. This blocks on a future for a specified amount of time. In the modified code below, the application waits for 5 seconds before exiting.
Note also, the extra import scala.concurrent.duration so we can specify the time to wait.
import scala.concurrent._
import scala.concurrent.duration._
import java.util.concurrent.Executors
import scala.util.{ Success, Failure }
object FuturesSequence extends App {
val f1 = future {
1
}
val f2 = future {
2
}
val lf = List(f1, f2)
val seq = Future.sequence(lf)
seq.onSuccess {
case l => println(l)
}
Await.ready(seq, 5 seconds)
}
By using Await.result instead, you can skip the onSuccess method too, as it will return the resulting list to you.
Example:
val seq: List[Int] = Await.result(Future.sequence(lf), 5 seconds)
println(seq)
I am attempting to asynchronously write messages to Amazon Kinesis using Scala Futures so I can load test an application.
This code works, and I can see data moving down my pipeline as well as the output printing to the console.
import com.amazonaws.services.kinesis.AmazonKinesisClient
import java.nio.CharBuffer
import java.nio.charset.Charset
import java.text.SimpleDateFormat
import java.util.{Date, TimeZone}
object KinesisDummyDataProducer extends App {
val kinesis = new AmazonKinesisClient(PipelineConfig.awsCredentials)
println("Connected")
lazy val encoder = Charset.forName("UTF-8").newEncoder()
lazy val tz = TimeZone.getTimeZone("UTC")
lazy val df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'Z")
df.setTimeZone(tz)
(1 to args(0).toInt).map(int => send(int)).map(msg => println(msg))
private def send(int: Int) = {
val msg = "{\"event_name\":\"test\",\"timestamp\":\"%s\",\"int\":%s}".format(df.format(new Date()), int.toString)
val bytes = encoder.encode(CharBuffer.wrap(msg))
encoder.flush(bytes)
kinesis.putRecord("PrimaryEventStream", bytes, "123")
msg
}
}
This code works with Scala Futures.
import scala.concurrent.future
import scala.concurrent.ExecutionContext.Implicits.global
def doIt(x: Int) = {Thread.sleep(1000); x + 1}
(1 to 10).map(x => future{doIt(x)}).map(y => y.onSuccess({case x => println(x)}))
You'll note that the syntax is nearly identical on the mapping of sequences. However, the follwoing does not work (i.e., it neither prints to the console nor sends data down my pipeline).
import com.amazonaws.services.kinesis.AmazonKinesisClient
import java.nio.CharBuffer
import java.nio.charset.Charset
import java.text.SimpleDateFormat
import java.util.{Date, TimeZone}
import scala.concurrent.future
import scala.concurrent.ExecutionContext.Implicits.global
object KinesisDummyDataProducer extends App {
val kinesis = new AmazonKinesisClient(PipelineConfig.awsCredentials)
println("Connected")
lazy val encoder = Charset.forName("UTF-8").newEncoder()
lazy val tz = TimeZone.getTimeZone("UTC")
lazy val df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'Z")
df.setTimeZone(tz)
(1 to args(0).toInt).map(int => future {send(int)}).map(f => f.onSuccess({case msg => println(msg)}))
private def send(int: Int) = {
val msg = "{\"event_name\":\"test\",\"timestamp\":\"%s\",\"int\":%s}".format(df.format(new Date()), int.toString)
val bytes = encoder.encode(CharBuffer.wrap(msg))
encoder.flush(bytes)
kinesis.putRecord("PrimaryEventStream", bytes, "123")
msg
}
}
Some more notes about this project. I am using Maven to do the build (from the command line), and running all of the above code (also from the command line) works just dandy.
My question is: Why with using the same syntax does my function 'send' appear to not be executing?