Messages published out-of-order after reconnecting

[janvinas]

I've made this sample program to test what happens when the publisher loses and recovers connection:

package main

import (
	"fmt"
	"strconv"
	"time"

	mqtt "github.com/eclipse/paho.mqtt.golang"
)

func main() {
	opts := mqtt.NewClientOptions().
		AddBroker("127.0.1.1:1883").
		SetOrderMatters(true).
		SetAutoReconnect(true).
		SetClientID("sender").
		SetCleanSession(false)

	client := mqtt.NewClient(opts)

	if token := client.Connect(); token.Wait() && token.Error() != nil {
		panic(token.Error())
	}

	i := 0

	for {
		client.Publish("test", 1, false, strconv.Itoa(i))
		i++
		time.Sleep(1 * time.Second)
	}
}

I start the program and then I block outbound traffic with:

iptables -A OUTPUT -p tcp --dport 1883 -d 127.0.1.1 -j DROP

When the connection is recovered (I remove the iptables rule), the messages are sent out of order, even though SetOrderMatters is true: (capture from the loopback interface)

Is this intended behaviour? How can I make sure packets are sent in order? I want them to arrive in order at the subscriber.

Thanks,
Jan.

[MattBrittan]

@janvinas - you are correct, the packet order is not maintained following a reconnect when using the default MemoryStore (this makes no attempt to store, or use, the time a packet was stored). There are a few options that do:

• OrderedMemoryStore does track the order so should help with this.
• FileStore sorts by the file modification time, so packet order should be retained (within the resolution of the file system times).

Note that I don't rely on this feature myself so am not really sure how well ordered delivery works (the need to use a different store should probably be noted in the docs).

[janvinas]

I've tested OrderedMemoryStore and it seems to work well! I agree it could be nice to add a little note in the documentation.

Thank you,
Jan.