JeroMQ send heartbeat from same thread

Question

I have a problem with PUB/SUB mode, where SUB sometimes randomly (once a day or so) stops receiving data. I wan to add some custom heartbeat message from PUB to SUB every second, but I have to send it from same thread because PUB is not thread safe. To do that I can use some java Timer/Scheduler to do it every second but problem is that Timer/Scheduler creates a new thread. I am receiving data from external websocket in PUB, and I do not see a way to send heartbeat everyone is talking about. Only way would be adding proxy using XPUB-XSUB...

Any idea how could it be done?

Answer 1

Q : _{Any idea how could it be done?}

A :
Yes, several - before anything else, ZeroMQ has internal .poll()-er, that can listen for socket-related events, not longer than there stated amount of [ms], so a private event-aware loop can be implemented this way ( be it using a stumped inproc://voidPAIR-socket PAIR-archetype ( which bears indeed a Zero-overhead for not having any transport-class protocol / stack ) receiving

Or using a this naive-enough structured, yet segmented control-loop. ZeroMQ can inject "job-ToDo-command"(s) into the control-loop, yet on no receiving any such real work command, the control-loop can .send() a "soft"-heartbeat in due times, leaving customisable amounts of rest/sleep, re-.send() and its own "soft"-flagged termination signalling.

Bear with me, this is indeed a "naive-enough"-DEMO running (live).

// Control-loop experts will tear any remaining hairs if seeing this...

;o)

public class Main
{
    public static void main(String[] args) throws InterruptedException
    {                                                           // DEMO
        long    aNAP          =  500;                           // DEMO  500 [ms] ~ 0.5 [s]
        long    aNEXT         = 1000;                           // DEMO 1000 [ms] ~ 1.0 [s]
        long    aLAST         = 5000;                           // DEMO 5000 [ms] ~ 5.0 [s]
        long    aNextFireTIME = aNEXT + System.currentTimeMillis();
        long    aLastFireTIME = aLAST + aNextFireTIME;
        
        String  ZERO_MSG_PAYLOAD = "";
        
        boolean aTerminateFLAG = true;
        
        System.out.print(   System.currentTimeMillis() );
        System.out.println( " starts now..." );
        
        while(  aTerminateFLAG )
        {
            while ( aNextFireTIME > System.currentTimeMillis() )    
            {                                                   // DEMO
            // test & serve any critical events' contexts
            // do some useful work
            // do  any slice of low-prio maintenance work
               Thread.sleep( Math.min( aNAP,                    // DEMO
                                       aNextFireTIME - System.currentTimeMillis()
                                       )
                               );
            // sleep to take a rest, releasing CPU for other tasks
            // also may test here any incoming service-socket messages to get handled
            // using .poll( aZmqBuiltInPollerTIMEOUT );
               System.out.print(   System.currentTimeMillis() );
               System.out.println( " aNAP taken till now..." );
               System.out.flush();
            }
            if ( aLastFireTIME < System.currentTimeMillis() )   // DEMO
            {
                aTerminateFLAG = false;
            }
            aNextFireTIME = aNEXT + System.currentTimeMillis(); // DEMO
            
            System.out.print(   System.currentTimeMillis() );
            System.out.println( " PUB.send( ZERO_MSG_PAYLOAD, 0 ); # a ZERO-sized 'soft' HeartBeat message" );
            System.out.println( aNextFireTIME );
            System.out.flush();
        }
    }
}

---

1641539382710 starts now...
1641539383211 aNAP taken till now...
1641539383710 aNAP taken till now...
1641539383710 PUB.send( ZERO_MSG_PAYLOAD, 0 ); # a ZERO-sized 'soft' HeartBeat message
1641539384710
1641539384211 aNAP taken till now...
1641539384710 aNAP taken till now...
1641539384710 PUB.send( ZERO_MSG_PAYLOAD, 0 ); # a ZERO-sized 'soft' HeartBeat message
1641539385710
1641539385210 aNAP taken till now...
1641539385712 aNAP taken till now...
1641539385712 PUB.send( ZERO_MSG_PAYLOAD, 0 ); # a ZERO-sized 'soft' HeartBeat message
1641539386712
1641539386212 aNAP taken till now...
1641539386712 aNAP taken till now...
1641539386712 PUB.send( ZERO_MSG_PAYLOAD, 0 ); # a ZERO-sized 'soft' HeartBeat message
1641539387712
1641539387212 aNAP taken till now...
1641539387720 aNAP taken till now...
1641539387720 PUB.send( ZERO_MSG_PAYLOAD, 0 ); # a ZERO-sized 'soft' HeartBeat message
1641539388720
1641539388228 aNAP taken till now...
1641539388720 aNAP taken till now...
1641539388721 PUB.send( ZERO_MSG_PAYLOAD, 0 ); # a ZERO-sized 'soft' HeartBeat message
1641539389721
...