How to avoid subscribing to the same topic and fire the callback multiple times in HiveMQ Android Client?

Question

Expected behavior

I want to have a callback to listen to every topic I subscribe to just once per message sent. I mean, I want to subscribe to a topic 1000 times, but when a message is received, I want to listen to it just one time.

IDK if there is something I am doing wrong (I guess).

Actual behavior

I am developing a home security camera app.
I have a list of cameras that I own.
For every camera on the list, I subscribe to a topic.
Every 30s, I update the screen, and again I subscribe to a topic for every camera. This means a TOPIC could be subscribed many times.
Every time I receive a message on a topic, the callback fires messages about how many times the same topi was subscribed.

To Reproduce

Steps

haven a topic camera/123
subscribe the topic N times with the below method called subscribeWith
Send a message over camera/123
You will receive the message N times because the N time you subscribed to the topic

Reproducer code

Just variables

private var mqtt: Mqtt5AsyncClient? = null
private var username: String? = null
private var password: String? = null
private val serverHost: String,
private val serverPort: Int = 1883

Build the MQTT

private fun build() {
        if (mqtt != null) return

        mqtt = Mqtt5Client.builder()
                .identifier(identifier())
                .serverHost(serverHost)
                .serverPort(serverPort)
                .automaticReconnect()
                .applyAutomaticReconnect()
                .addConnectedListener { Timber.d("On Connected") }
                .addDisconnectedListener { onMQTTDisconnected(it) }
                .buildAsync()
    }

Connecting the MQTT

fun connect(username: String, password: String) {
        build()

        this.username = username
        this.password = password

        mqtt?.connectWith()
                ?.keepAlive(30)
                ?.sessionExpiryInterval(7200)
                ?.cleanStart(false)
                ?.simpleAuth()
                ?.username("abc")
                ?.password("123".toByteArray())
                ?.applySimpleAuth()
                ?.send()
    }

And then, subscribing a topic Every time I subscribe a topic I use these fun

fun subscribeWith(topic: String) {
        mqtt?.subscribeWith()
                ?.topicFilter(topic)
                ?.qos(MqttQos.AT_MOST_ONCE)
               ?.callback { t -> onConsumingTopic(t) }  <- I THINK THIS IS THE IMPORTANT THING
                ?.send()
                ?.whenComplete { ack, error -> onTopicConnected(ack, error, topic) }

    }

You are going to need to keep a list of topics that you have subscribed to. Check the list before adding a new topic. — hardillb, Dec 18 '20 at 11:48
exactly as i said. Subscribing multiple times. imaging this. Whe have a topic A and the MQTT and you do MQTT.subscribe(topic = A) MQTT.subscribe(topic = A) MQTT.subscribe(topic = A) the topic fires 3 times when receiving a message @Odysseus — Deneb Chorny, Dec 18 '20 at 13:31
@hardillb I thought about that, but I think it is just a workaround instead of the real behavior. MQTT says subscribing to the same topic means substitution and not duplication. Thanks — Deneb Chorny, Dec 18 '20 at 13:36
@DenebChorny And what is the purpose of subscribing the same topic multiple times from one client? — Odysseus, Dec 18 '20 at 13:51
It's not on purpose. By the nature of the app, when I subscribe to a topic I have no way of knowing if I previously subscribed to it (without storing a cache). But, still MQTT indicates that this does not generate duplication. @Odysseus — Deneb Chorny, Dec 18 '20 at 14:04
I see - then I agree with @hardillb that you should keep track of the topics already subsrcibed to prevent re-subscribing topics. But indeed the broker should overwrite already existing subsriptions if you re-subsribe them or do you have multiple client instances by accident? — Odysseus, Dec 18 '20 at 14:37
I don't think this happening on the broker, it's adding extra matching callbacks in the client. — hardillb, Dec 18 '20 at 14:38
Thanks. You are right @hardillb and Odysseus. The only way to have a callback is when subscribing to a topic and there is no way to verify beforehand subscribed topics. I hate this solutions bus is the only one here — Deneb Chorny, Dec 18 '20 at 14:56
@hardillb Good hint - then he might not use a lambda for callback as well — Odysseus, Dec 18 '20 at 15:31

score 2 · Answer 1 · answered Dec 18 '20 at 15:07

2

As mentioned in the comments, the only solution at the moment is to keep a list of the subscribed topics outside the MQTT client library and check it before subscribing to new topics.

answered Dec 18 '20 at 15:07

hardillb

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score 1 · Accepted Answer · answered Feb 10 '21 at 18:51

I found the correct answer.

There is no need to register a callback for every subscribe call nor using a global array handling the registered topics as this:

mqtt?.subscribeWith()
    ?.callback { t -> onConsumingTopic(t) }  <- This is not needed

Instead you could register one "global" callback for all messages, for example:

client.publishes(MqttGlobalPublishFilter.SUBSCRIBED, publish -> { ... } );

and then you can subscribe without providing a callback.

client.subscribeWith().topicFilter("test").qos(MqttQos.AT_LEAST_ONCE).send();

Complete example:

Building the MQTT

 mqtt = Mqtt5Client.builder()
                .identifier(identifier())
                .serverHost(serverHost)
                .serverPort(serverPort)
                .automaticReconnect()
                .applyAutomaticReconnect()
                .addConnectedListener { onMQTTConnected(it) }
                .addDisconnectedListener { onMQTTDisconnected(it) }
                .buildAsync()

        mqtt?.publishes(MqttGlobalPublishFilter.SUBSCRIBED) { onConsumingTopic(it) }