Develop Reference

Akka: Trying to understand order of message delivery between Actors

I'm wondering why the messages are being printed in exactly the same order as in the code.
import akka.actor.AbstractActor
import akka.actor.ActorRef
import akka.actor.ActorSystem
import akka.actor.Props
import akka.event.Logging
import akka.event.LoggingAdapter
data class Request(val name: String)
class Device : AbstractActor() {
val log = Logging.getLogger(getContext().getSystem(), this);
override fun createReceive(): Receive {
return receiveBuilder().matchEquals("print") { x -> log.info("hello i'm a device") }
.match(Request::class.java) { x -> log.info("A " + x.name) }
.build()
}
companion object {
fun props(): Props {
return Props.create { Device() }
}
}
}
fun main(args: Array<String>) {
val system = ActorSystem.create("container")
val deviceA = system.actorOf(Device.props())
val deviceC = system.actorOf(Device.props())
val deviceD = system.actorOf(Device.props())
val deviceB = system.actorOf(Device.props())
deviceA.tell(Request("first "), deviceB)
deviceA.tell(Request("second"), deviceC)
deviceA.tell(Request("third"), deviceD)
}
It prints out:
/usr/lib/jvm/java-8-oracle/bin/java -Dvisualvm.id=32047598721041 -javaagent:/opt/intellij-idea-community/lib/idea_rt.jar=41737:/opt/intellij-idea-community/bin -Dfile.encoding=UTF-8 -classpath /usr/lib/jvm/java-8-oracle/jre/lib/charsets.jar:/usr/lib/jvm/java-8-oracle/jre/lib/deploy.jar:/usr/lib/jvm/java-8-oracle/jre/lib/ext/cldrdata.jar:/usr/lib/jvm/java-8-oracle/jre/lib/ext/dnsns.jar:/usr/lib/jvm/java-8-oracle/jre/lib/ext/jaccess.jar:/usr/lib/jvm/java-8-oracle/jre/lib/ext/jfxrt.jar:/usr/lib/jvm/java-8-oracle/jre/lib/ext/localedata.jar:/usr/lib/jvm/java-8-oracle/jre/lib/ext/nashorn.jar:/usr/lib/jvm/java-8-oracle/jre/lib/ext/sunec.jar:/usr/lib/jvm/java-8-oracle/jre/lib/ext/sunjce_provider.jar:/usr/lib/jvm/java-8-oracle/jre/lib/ext/sunpkcs11.jar:/usr/lib/jvm/java-8-oracle/jre/lib/ext/zipfs.jar:/usr/lib/jvm/java-8-oracle/jre/lib/javaws.jar:/usr/lib/jvm/java-8-oracle/jre/lib/jce.jar:/usr/lib/jvm/java-8-oracle/jre/lib/jfr.jar:/usr/lib/jvm/java-8-oracle/jre/lib/jfxswt.jar:/usr/lib/jvm/java-8-oracle/jre/lib/jsse.jar:/usr/lib/jvm/java-8-oracle/jre/lib/management-agent.jar:/usr/lib/jvm/java-8-oracle/jre/lib/plugin.jar:/usr/lib/jvm/java-8-oracle/jre/lib/resources.jar:/usr/lib/jvm/java-8-oracle/jre/lib/rt.jar:/home/dell/akka-quickstart-java/target/classes:/home/dell/.m2/repository/com/typesafe/akka/akka-actor_2.12/2.5.19/akka-actor_2.12-2.5.19.jar:/home/dell/.m2/repository/org/scala-lang/scala-library/2.12.8/scala-library-2.12.8.jar:/home/dell/.m2/repository/com/typesafe/config/1.3.3/config-1.3.3.jar:/home/dell/.m2/repository/org/scala-lang/modules/scala-java8-compat_2.12/0.8.0/scala-java8-compat_2.12-0.8.0.jar:/home/dell/.m2/repository/org/jetbrains/kotlin/kotlin-stdlib-jdk8/1.3.20/kotlin-stdlib-jdk8-1.3.20.jar:/home/dell/.m2/repository/org/jetbrains/kotlin/kotlin-stdlib/1.3.20/kotlin-stdlib-1.3.20.jar:/home/dell/.m2/repository/org/jetbrains/kotlin/kotlin-stdlib-common/1.3.20/kotlin-stdlib-common-1.3.20.jar:/home/dell/.m2/repository/org/jetbrains/annotations/13.0/annotations-13.0.jar:/home/dell/.m2/repository/org/jetbrains/kotlin/kotlin-stdlib-jdk7/1.3.20/kotlin-stdlib-jdk7-1.3.20.jar StuffKt
[INFO] [01/31/2019 18:43:16.058] [container-akka.actor.default-dispatcher-2] [akka://container/user/$a] A first
[INFO] [01/31/2019 18:43:16.059] [container-akka.actor.default-dispatcher-2] [akka://container/user/$a] A second
[INFO] [01/31/2019 18:43:16.059] [container-akka.actor.default-dispatcher-2] [akka://container/user/$a] A third
I was expecting the order to be different sometimes from ("first second third"), but it keeps printing out that same order on every run.
Is my expectation incorrect?

The documentation on message delivery order states
The rule more specifically is that for a given pair of actors,
messages sent directly from the first to the second will not be
received out-of-order. The word directly emphasizes that this
guarantee only applies when sending with the tell operator to the
final destination, not when employing mediators or other message
dissemination features (unless stated otherwise).
You are sending from the main method (outside of the actor system) to deviceA.
You are not sending from the deviceB, deviceC, or deviceD actors. Those are just being used as sender references, so that deviceA has someone to reply to.

ZMQ missing events being propagated in jeromq scala

I am new to ZeroMQ and seem to be losing messages in a loop in my begin() method.
I'm wondering if I am missing a piece where I am not queuing messages or something?
When I cause an event on my publisher, that sends two messages to my subscriber with a small gap in between, I seem not to be getting the second message that is relayed. What am I missing?
class ZMQSubscriber[T <: Transaction, B <: Block](
socket: InetSocketAddress,
hashTxListener: Option[HashDigest => Future[Unit]],
hashBlockListener: Option[HashDigest => Future[Unit]],
rawTxListener: Option[Transaction => Future[Unit]],
rawBlockListener: Option[Block => Future[Unit]]) {
private val logger = BitcoinSLogger.logger
def begin()(implicit ec: ExecutionContext) = {
val context = ZMQ.context(1)
// First, connect our subscriber socket
val subscriber = context.socket(ZMQ.SUB)
val uri = socket.getHostString + ":" + socket.getPort
//subscribe to the appropriate feed
hashTxListener.map { _ =>
subscriber.subscribe(HashTx.topic.getBytes(ZMQ.CHARSET))
logger.debug("subscribed to the transaction hashes from zmq")
}
rawTxListener.map { _ =>
subscriber.subscribe(RawTx.topic.getBytes(ZMQ.CHARSET))
logger.debug("subscribed to raw transactions from zmq")
}
hashBlockListener.map { _ =>
subscriber.subscribe(HashBlock.topic.getBytes(ZMQ.CHARSET))
logger.debug("subscribed to the hashblock stream from zmq")
}
rawBlockListener.map { _ =>
subscriber.subscribe(RawBlock.topic.getBytes(ZMQ.CHARSET))
logger.debug("subscribed to raw block")
}
subscriber.connect(uri)
subscriber.setRcvHWM(0)
logger.info("Connection to zmq client successful")
while (true) {
val notificationTypeStr = subscriber.recvStr(ZMQ.DONTWAIT)
val body = subscriber.recv(ZMQ.DONTWAIT)
Future(processMsg(notificationTypeStr, body))
}
}
private def processMsg(topic: String, body: Seq[Byte])(implicit ec: ExecutionContext): Future[Unit] = Future {
val notification = ZMQNotification.fromString(topic)
val res: Option[Future[Unit]] = notification.flatMap {
case HashTx =>
hashTxListener.map { f =>
val hash = Future(DoubleSha256Digest.fromBytes(body))
hash.flatMap(f(_))
}
case RawTx =>
rawTxListener.map { f =>
val tx = Future(Transaction.fromBytes(body))
tx.flatMap(f(_))
}
case HashBlock =>
hashBlockListener.map { f =>
val hash = Future(DoubleSha256Digest.fromBytes(body))
hash.flatMap(f(_))
}
case RawBlock =>
rawBlockListener.map { f =>
val block = Future(Block.fromBytes(body))
block.flatMap(f(_))
}
}
}
}

So this seems to have been solved by using a ZMsg.recvMsg() in the while-loop instead of
val notificationTypeStr = subscriber.recvStr(ZMQ.DONTWAIT)
val body = subscriber.recv(ZMQ.DONTWAIT)
I'm not sure why this works, but it does. So here is what my begin method looks like now
while (run) {
val zmsg = ZMsg.recvMsg(subscriber)
val notificationTypeStr = zmsg.pop().getString(ZMQ.CHARSET)
val body = zmsg.pop().getData
Future(processMsg(notificationTypeStr, body))
}
Future.successful(Unit)
}

What am I missing?
How the blocking v/s non-blocking modus operandi work :
The trick is in the (non-)blocking mode of the respective call to the .recv() method.
A second call to the subscriber.recv( ZMQ.DONTWAIT )-method thus returns immediately, so your second part, ( the body ) may and will legally contain nothing, even though your promise stated a pair of messages was indeed dispached from the publisher-side ( a pair of .send() method calls - one may also object, there are chances the sender was actually sending just one message, in a multi-part fashion - MCVE-code is not specific on this part ).
So, once you have moved your code from non-blocking mode ( in the O/P ) into a principally blocking-mode ( which locked / sync-ed the further flow of the code with the external event of an arrival of any plausibly formatted message, not returning earlier ), in:
val zmsg = ZMsg.recvMsg(subscriber) // which BLOCKS-till-a-1st-zmsg-arrived
both the further processed .pop()-ed parts just unload the components ( ref. the remark on actual ZMsg multi-part structure actually sent by the published-side, presented above )
Safety next :unlimited alloc-s v/s a mandatory blocking / dropping messages ?
the code surprised me on several points. Besides a rather very "late" call to the .connect()-method, compared to all the previous socket-archetype detailed settings ( that normally get arranged "after" a request to setup a connection ). While this may work fine, as intended, yet it exposes even tighter ( smaller ) time-window for the .Context()-instance to setup and (re-)negotiate all the relevant connection-details so as to become RTO.
One particular line attracted my attention: subscriber.setRcvHWM( 0 ) this is a version-archetype dependent trick. Yet, the value of zero causes an application to become vulnerable and I would not advise doing so in any production-grade application.

Configure Akka ClusterRouting programmatically

When configuring the Cluster Routing in the config file using:
akka.actor.deployment {
/jobDispatcher/singleton/workerRouter {
router = round-robin-pool
nr-of-instances = 5
cluster {
enabled = on
max-number-of-instances-per-node = 1
allow-local-routees = on
}
}
}
I can lookup a routed worker using:
ActorRef actor = context().actorOf( //
FromConfig.getInstance().props( //
Props.create(MyRoutedActor.class)), //
"workerRouter");
I would prefer configuring the Pool programmatically since I want to hide the details from my user.
However using:
ActorRef actor = context().actorOf(new ClusterRouterPool(new RoundRobinPool(5), //
new ClusterRouterPoolSettings(100, 1, true, "")) //
.props(Props.create(MyRoutedActor.class)),
"workerRouter");
does not route the calls to Routees in the cluster (only the local .
How do I configure the routing correctly?

Try to use ClusterRouterPool
Akka doc says [http://doc.akka.io/docs/akka/2.4/scala/cluster-usage.html]:
Pool - router that creates routees as child actors and deploys them on remote nodes. Each router will have its own routee instances. For example, if you start a router on 3 nodes in a 10-node cluster, you will have 30 routees in total if the router is configured to use one instance per node. The routees created by the different routers will not be shared among the routers. One example of a use case for this type of router is a single master that coordinates jobs and delegates the actual work to routees running on other nodes in the cluster.
The example http://doc.akka.io/docs/akka/2.4/java/cluster-usage.html#Router_with_Pool_of_Remote_Deployed_Routees
akka.actor.deployment {
/statsService/singleton/workerRouter {
router = consistent-hashing-pool
cluster {
enabled = on
max-nr-of-instances-per-node = 3
allow-local-routees = on
use-role = compute
}
}
}
The code to do programmatically (also from the akka docs):
int totalInstances = 100;
int maxInstancesPerNode = 3;
boolean allowLocalRoutees = false;
String useRole = "compute";
ActorRef workerRouter = getContext().actorOf(
new ClusterRouterPool(new ConsistentHashingPool(0),
new ClusterRouterPoolSettings(totalInstances, maxInstancesPerNode,
allowLocalRoutees, useRole)).props(Props
.create(StatsWorker.class)), "workerRouter3");
I have an akka cluster in scala, and this is my code:
val workerRouter = context.actorOf(
ClusterRouterGroup(AdaptiveLoadBalancingGroup(MixMetricsSelector), ClusterRouterGroupSettings( //RoundRobinGroup(Nil)
totalInstances = 1000, routeesPaths = List("/user/worker"),
allowLocalRoutees = true, useRole = Some("workerRole"))).props(),
name = "pool")

Test WebSocket in PlayFramework

I have a WebSocket in my Play application and I want to write a test for it, but I couldn't find any example on how to write such a test. I found a discussion in the play-framework Google group but there has been no activity recently.
So, are there any ideas on how to test WebSocket's in a Java test?

You can retrieve underlying Iteratee,Enumerator and test them directly. This way you don't need to use a browser. You need akka-testkit though, to cope with asynchronous nature of iteratees.
A Scala example:
object WebSocket extends Controller {
def websocket = WebSocket.async[JsValue] { request =>
Future.successful(Iteratee.ignore[JsValue] -> Enumerator.apply[JsValue](Json.obj("type" -> "error")))
}
}
class WebSocketSpec extends PlaySpecification {
"WebSocket" should {
"respond with error packet" in new WithApplication {
val request = FakeRequest()
var message: JsValue = null
val iteratee = Iteratee.foreach[JsValue](chunk => message = chunk)(Akka.system.dispatcher)
Controller.websocket().f(request)(Enumerator.empty[JsValue],iteratee)
TestKit.awaitCond(message == Json.obj("type" -> "error"), 1 second)
}
}
}

I test WebSockets code using Firefox:
https://github.com/schleichardt/stackoverflow-answers/commit/13d5876791ef409e092e4a097f54247d851e17dc#L8R14
For Java it works similar replacing 'HTMLUNIT' with 'FIREFOX': http://www.playframework.com/documentation/2.1.x/JavaFunctionalTest

Chrome provides a plugin to test websocket service.
Edit
So using the plugin (as shown in picture below) you can provide websocket url and the request data and send message to service. And message log shows the message sent from client and also service response.

Assume that you have a websocket library that returns the Future[Itearatee[JsValue, Unit], Enumerator[JsValue]] your controller uses
trait WSLib {
def connect: Future[Itearatee[JsValue, Unit], Enumerator[JsValue]]
}
And you wanna test this library.
Here is a context you can use:
trait WebSocketContext extends WithApplication {
val aSecond = FiniteDuration(1, TimeUnit.SECONDS)
case class Incoming(iteratee: Iteratee[JsValue, Unit]) {
def feed(message: JsValue) = {
iteratee.feed(Input.El(message))
}
def end(wait: Long = 100) = {
Thread.sleep(wait) //wait until all previous fed messages are handled
iteratee.feed(Input.EOF)
}
}
case class OutGoing(enum: Enumerator[JsValue]) {
val messages = enum(Iteratee.fold(List[JsValue]()) {
(l, jsValue) => jsValue :: l
}).flatMap(_.run)
def get: List[JsValue] = {
Await.result(messages, aSecond)
}
}
def wrapConnection(connection: => Future[Iteratee[JsValue, Unit], Enumerator[JsValue]]): (Incoming, OutGoing) = {
val (iteratee, enumerator) = Await.result(conn, aSecond)
(Incoming(iteratee), OutGoing(enumerator))
}
}
Then your tests can be written as
"return all subscribers when asked for info" in new WebSocketContext {
val (incoming, outgoing) = wrapConnection(myWSLib.connect)
incoming.feed(JsObject("message" => "hello"))
incoming.end() //this closes the connection
val responseMessages = outgoing.get //you only call this "get" after the connection is closed
responseMessages.size must equalTo(1)
responseMessages must contain(JsObject("reply" => "Hey"))
}
Incoming represent the messages coming from the client side, while the outgoing represents the messages sent from the server. To write test, you first feed in the incoming messages from incoming and then close the connection by calling incoming.end, then you get the complete list of outgoing messages from the outgoing.get method.

Retrieve multiple messages from SQS

I have multiple messages in SQS. The following code always returns only one, even if there are dozens visible (not in flight). setMaxNumberOfMessages I thought would allow multiple to be consumed at once .. have i misunderstood this?
CreateQueueRequest createQueueRequest = new CreateQueueRequest().withQueueName(queueName);
String queueUrl = sqs.createQueue(createQueueRequest).getQueueUrl();
ReceiveMessageRequest receiveMessageRequest = new ReceiveMessageRequest(queueUrl);
receiveMessageRequest.setMaxNumberOfMessages(10);
List<Message> messages = sqs.receiveMessage(receiveMessageRequest).getMessages();
for (Message message : messages) {
// i'm a message from SQS
}
I've also tried using withMaxNumberOfMessages without any such luck:
receiveMessageRequest.withMaxNumberOfMessages(10);
How do I know there are messages in the queue? More than 1?
Set<String> attrs = new HashSet<String>();
attrs.add("ApproximateNumberOfMessages");
CreateQueueRequest createQueueRequest = new CreateQueueRequest().withQueueName(queueName);
GetQueueAttributesRequest a = new GetQueueAttributesRequest().withQueueUrl(sqs.createQueue(createQueueRequest).getQueueUrl()).withAttributeNames(attrs);
Map<String,String> result = sqs.getQueueAttributes(a).getAttributes();
int num = Integer.parseInt(result.get("ApproximateNumberOfMessages"));
The above always is run prior and gives me an int that is >1
Thanks for your input

AWS API Reference Guide: Query/QueryReceiveMessage
Due to the distributed nature of the queue, a weighted random set of machines is sampled on a ReceiveMessage call. That means only the messages on the sampled machines are returned. If the number of messages in the queue is small (less than 1000), it is likely you will get fewer messages than you requested per ReceiveMessage call. If the number of messages in the queue is extremely small, you might not receive any messages in a particular ReceiveMessage response; in which case you should repeat the request.
and
MaxNumberOfMessages: Maximum number of messages to return. SQS never returns more messages than this value but might return fewer.

There is a comprehensive explanation for this (arguably rather idiosyncratic) behaviour in the SQS reference documentation.
SQS stores copies of messages on multiple servers and receive message requests are made to these servers with one of two possible strategies,
Short Polling : The default behaviour, only a subset of the servers (based on a weighted random distribution) are queried.
Long Polling : Enabled by setting the WaitTimeSeconds attribute to a non-zero value, all of the servers are queried.
In practice, for my limited tests, I always seem to get one message with short polling just as you did.

I had the same problem. What is your Receive Message Wait Time for your queue set to? When mine was at 0, it only returned 1 message even if there were 8 in the queue. When I increased the Receive Message Wait Time, then I got all of them. Seems kind of buggy to me.

I was just trying the same and with the help of these two attributes setMaxNumberOfMessages and setWaitTimeSeconds i was able to get 10 messages.
ReceiveMessageRequest receiveMessageRequest = new ReceiveMessageRequest(myQueueUrl);
receiveMessageRequest.setMaxNumberOfMessages(10);
receiveMessageRequest.setWaitTimeSeconds(20);
Snapshot of o/p:
Receiving messages from TestQueue.
Number of messages:10
Message
MessageId: 31a7c669-1f0c-4bf1-b18b-c7fa31f4e82d
...

receiveMessageRequest.withMaxNumberOfMessages(10);
Just to be clear, the more practical use of this would be to add to your constructor like this:
ReceiveMessageRequest receiveMessageRequest = new ReceiveMessageRequest(queueUrl).withMaxNumberOfMessages(10);
Otherwise, you might as well just do:
receiveMessageRequest.setMaxNumberOfMessages(10);
That being said, changing this won't help the original problem.

Thanks Caoilte!
I faced this issue also. Finally solved by using long polling follow the configuration here:
https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/sqs-configure-long-polling-for-queue.html
Unfortunately, to use long polling, you must create your queue as FIFO one. I tried standard queue with no luck.
And when receiving, need also set MaxNumberOfMessages. So my code is like:
ReceiveMessageRequest receive_request = new ReceiveMessageRequest()
.withQueueUrl(QUEUE_URL)
.withWaitTimeSeconds(20)
.withMaxNumberOfMessages(10);
Although solved, still feel too wired. AWS should definitely provide a more neat API for this kind of basic receiving operation.
From my point, AWS has many many cool features but not good APIs. Like those guys are rushing out all the time.

For small task list I use FIFO queue like stackoverflow.com/a/55149351/13678017
for example modified AWS tutorial
// Create a queue.
System.out.println("Creating a new Amazon SQS FIFO queue called " + "MyFifoQueue.fifo.\n");
final Map<String, String> attributes = new HashMap<>();
// A FIFO queue must have the FifoQueue attribute set to true.
attributes.put("FifoQueue", "true");
/*
* If the user doesn't provide a MessageDeduplicationId, generate a
* MessageDeduplicationId based on the content.
*/
attributes.put("ContentBasedDeduplication", "true");
// The FIFO queue name must end with the .fifo suffix.
final CreateQueueRequest createQueueRequest = new CreateQueueRequest("MyFifoQueue4.fifo")
.withAttributes(attributes);
final String myQueueUrl = sqs.createQueue(createQueueRequest).getQueueUrl();
// List all queues.
System.out.println("Listing all queues in your account.\n");
for (final String queueUrl : sqs.listQueues().getQueueUrls()) {
System.out.println(" QueueUrl: " + queueUrl);
}
System.out.println();
// Send a message.
System.out.println("Sending a message to MyQueue.\n");
for (int i = 0; i < 4; i++) {
var request = new SendMessageRequest()
.withQueueUrl(myQueueUrl)
.withMessageBody("message " + i)
.withMessageGroupId("userId1");
;
sqs.sendMessage(request);
}
for (int i = 0; i < 6; i++) {
var request = new SendMessageRequest()
.withQueueUrl(myQueueUrl)
.withMessageBody("message " + i)
.withMessageGroupId("userId2");
;
sqs.sendMessage(request);
}
// Receive messages.
System.out.println("Receiving messages from MyQueue.\n");
var receiveMessageRequest = new ReceiveMessageRequest(myQueueUrl);
receiveMessageRequest.setMaxNumberOfMessages(10);
receiveMessageRequest.setWaitTimeSeconds(20);
// what receive?
receiveMessageRequest.withMessageAttributeNames("userId2");
final List<Message> messages = sqs.receiveMessage(receiveMessageRequest).getMessages();
for (final Message message : messages) {
System.out.println("Message");
System.out.println(" MessageId: "
+ message.getMessageId());
System.out.println(" ReceiptHandle: "
+ message.getReceiptHandle());
System.out.println(" MD5OfBody: "
+ message.getMD5OfBody());
System.out.println(" Body: "
+ message.getBody());
for (final Entry<String, String> entry : message.getAttributes()
.entrySet()) {
System.out.println("Attribute");
System.out.println(" Name: " + entry
.getKey());
System.out.println(" Value: " + entry
.getValue());
}
}

Here's a workaround, you can call receiveMessageFromSQS method asynchronously.
bulkReceiveFromSQS (queueUrl, totalMessages, asyncLimit, batchSize, visibilityTimeout, waitTime, callback) {
batchSize = Math.min(batchSize, 10);
let self = this,
noOfIterations = Math.ceil(totalMessages / batchSize);
async.timesLimit(noOfIterations, asyncLimit, function(n, next) {
self.receiveMessageFromSQS(queueUrl, batchSize, visibilityTimeout, waitTime,
function(err, result) {
if (err) {
return next(err);
}
return next(null, _.get(result, 'Messages'));
});
}, function (err, listOfMessages) {
if (err) {
return callback(err);
}
listOfMessages = _.flatten(listOfMessages).filter(Boolean);
return callback(null, listOfMessages);
});
}
It will return you an array with a given number of messages