golang mqtt publish and subscribe

Question

Does anybody know where I can get some example MQTT client Go (golang) code that does both publish and subscribe in an infinite loop ?

I am messaging with a Mosquitto broker running on MacOs.

In more detail...

Get a message from the network (a topic)
Compute something based on that message
Send the result of the computation back to the network (topic)

Here is the code I am using:

package main

import (
"fmt"
 MQTT "github.com/eclipse/paho.mqtt.golang"
"os"
"time"
)

var knt int

var f MQTT.MessageHandler = func(client MQTT.Client, msg MQTT.Message) 
{ 
   fmt.Printf("MSG: %s\n", msg.Payload())
   text:= fmt.Sprintf("this is result msg #%d!", knt)
   knt++
   token := client.Publish("nn/result", 0, false, text)
   token.Wait()
}

func main() {
   knt = 0

   opts := MQTT.NewClientOptions().AddBroker("tcp://localhost:1883")
   opts.SetClientID("mac-go")
   opts.SetDefaultPublishHandler(f)

   c := MQTT.NewClient(opts)
   if token := c.Connect(); token.Wait() && token.Error() != nil {
        panic(token.Error())
   }

  if token := c.Subscribe("nn/sensors", 0, nil); token.Wait() && 
     token.Error() != nil {
     fmt.Println(token.Error())
     os.Exit(1)
  }

  time.Sleep(3 * time.Second)
} //end of main

I looked through the GoDocs for some hint as to how to keep the connections open but nothing seems pertinent. I can certainly do an infinite loop over the 'subscribe' but that seems inefficient.

Asking for tutorials it's off topic for Stack Overflow, show us what you've already tried and somebody will help you fix it — hardillb
Ok. Thanks for the feedback. I found a solution... I think. I added the solution code to the question. It runs in a test environment. Now I have to try it in a real environment. — Phillip Neal

Phillip Neal Phillip Neal · Accepted Answer · 2018-02-21T12:40:34

I looked through the GoDocs for some hint as to how to keep the connections open but nothing seems pertinent. I can certainly do an infinite loop over the 'subscribe' but that seems inefficient.

Ok. Found a solution at . https://github.com/eclipse/paho.mqtt.golang/blob/master/cmd/stdoutsub/main.go. Essentially, I had to open up a channel for the subscribe. Here is the new code:

package main

import (
    "fmt"
    MQTT "github.com/eclipse/paho.mqtt.golang"
    "os"
    "os/signal"
    "syscall"
)

var knt int
var f MQTT.MessageHandler = func(client MQTT.Client, msg MQTT.Message) {
    fmt.Printf("MSG: %s\n", msg.Payload())
    text := fmt.Sprintf("this is result msg #%d!", knt)
    knt++
    token := client.Publish("nn/result", 0, false, text)
    token.Wait()
}

func main() {
    knt = 0
    c := make(chan os.Signal, 1)
    signal.Notify(c, os.Interrupt, syscall.SIGTERM)

    opts := MQTT.NewClientOptions().AddBroker("tcp://localhost:1883")
    opts.SetClientID("mac-go")
    opts.SetDefaultPublishHandler(f)
    topic := "nn/sensors"

    opts.OnConnect = func(c MQTT.Client) {
            if token := c.Subscribe(topic, 0, f); token.Wait() && token.Error() != nil {
                    panic(token.Error())
            }
    }
    client := MQTT.NewClient(opts)
    if token := client.Connect(); token.Wait() && token.Error() != nil {
            panic(token.Error())
    } else {
            fmt.Printf("Connected to server\n")
    }
    <-c
}

golang mqtt publish and subscribe

1 Answers