ZMQ missing events being propagated in jeromq scala

Question

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(_))
        }
    }
  }
}

score 0 · Answer 1 · edited Apr 23 '18 at 10:41

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)
  }

score 0 · Answer 2 · edited May 11 '18 at 13:02

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.

ZMQ missing events being propagated in jeromq scala

2 Answers2

How the blocking v/s non-blocking modus operandi work :

Safety next :unlimited alloc-s v/s a mandatory blocking / dropping messages ?

How the blocking ^v/_s non-blocking modus operandi work :

Safety next :
unlimited alloc-s ^v/_s a mandatory blocking / dropping messages ?