NetMQ How to detect slow subscribers using HighWatermark option and disconnect them by publisher?

Question

I put simply delay before subscriber reads next frame, so I expected that this will simulate slow subscriber and give some effects of HighWatermark option. I don't observe anything, subscriber doesn't skip (drop) any messages neither slow down the publisher. I have run 1 publisher and x subscribers.

I try to play with a pub-sub example taken from documentation https://netmq.readthedocs.io/en/latest/pub-sub/

Is there any way to detect that subscriber is slow? I mean the number of queued messages to be received exceeded the HighWatermark value. Shall I expect any exception or event in NetMqMonitor? I am also looking if there is a option to disconnect such a slow subscriber.

using System;
using System.Threading;
using NetMQ;
using NetMQ.Sockets;

namespace Sub
{
    class Program
    {
        static void Main()
        {
            Console.WriteLine("Subscriber started for Topic : {0}", topic);
            using (var subSocket = new SubscriberSocket())
            {
                subSocket.Options.ReceiveHighWatermark = 100;
                subSocket.Connect("tcp://localhost:12345");
                subSocket.Subscribe("topic1");
                Console.WriteLine("Subscriber socket connecting...");
                while (true)
                {
                    string messageTopicReceived = subSocket.ReceiveFrameString();
                    string messageReceived = subSocket.ReceiveFrameString();
                    Console.WriteLine($"{messageTopicReceived} {messageReceived}");
                    Thread.Sleep(50);
                }
            }
        }
    }
}

using System;
using System.Threading;
using NetMQ;
using NetMQ.Sockets;

namespace Pub
{
    class Program
    {
        static void Main()
        {
            using (var pubSocket = new PublisherSocket())
            {
                Console.WriteLine("Publisher socket binding...");
                pubSocket.Options.SendHighWatermark = 100;
                pubSocket.Bind("tcp://*:12345");
                Thread.Sleep(1000);
                for (var i = 0; i < 100000; i++)
                {
                    var msg = "msg-" + i;
                    Console.WriteLine("Sending message : {0}", msg);
                    pubSocket.SendMoreFrame("topic1").SendFrame(msg);
                    //Thread.Sleep(1);
                }
            }
        }
    }
}

You also have the tcp buffer that queue messages, and it depends on OS. Eventually if you publish faster than subscriber process publisher will drop messages. You can use the slow subscriber example from zguide.zeromq.org — somdoron, Feb 06 '20 at 11:53
In fact, the message was too small to observe any effects. I changed the `msg` to very long string and then I started observing that subscriber drops some messages. — ziomyslaw, Feb 25 '20 at 09:46

score 0 · Accepted Answer · answered Feb 25 '20 at 09:55

NetMqMonitor doesn't trigger events to indicate that some messages have been dropped. Subscriber has to verify the generation time and decide itself what to do. http://zguide.zeromq.org/php:chapter5#Slow-Subscriber-Detection-Suicidal-Snail-Pattern

Ps. To observe effect of ReceiveHighWatermark the example code from question has to be modified. The messages were too small.

NetMQ How to detect slow subscribers using HighWatermark option and disconnect them by publisher?

1 Answers1