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Is MqttClient thread safe? [message #1693648]

Mon, 27 April 2015 09:26

Shisei Hanai

Messages: 1
Registered: April 2015

Junior Member

Hi, I am using Paho for Java 1.0.2. My application subscribes events from devices and send back reply as device commands. So I want to use MqttClient instance in callback MqttCallback.messageArrived(String topic, MqttMessage message).

private volatile MqttClient client;

client = create client...
client.setCallback(new MqttCallback() {
  public void messageArrived(String topic, MqttMessage message) {
    client.publish(...); // Send out command.
  }
...
});

I assume the callback is called by other thread than the thread that create MqttClient instance. Is it safe to use client in messageArrived method?

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Re: Is MqttClient thread safe? [message #1693950 is a reply to message #1693648]

Wed, 29 April 2015 12:52

Ian Craggs

Messages: 68
Registered: March 2013

Member

Yes, the callback is called by another thread.

From the Javadoc for MqttCallback:

1) This method is invoked synchronously by the MQTT client. An acknowledgment is not sent back to the server until this method returns cleanly.

2) It is possible to send a new message within an implementation of this callback (for example, a response to this message), but the implementation must not disconnect the client, as it will be impossible to send an acknowledgment for the message being processed, and a deadlock will occur.

So, yes, you should be able to call publish() within the callback. The resulting message will be sent before any acknowledgement to the incoming message.

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Re: Is MqttClient thread safe? [message #1711199 is a reply to message #1693950]

Wed, 14 October 2015 06:26

paul stanton

Messages: 86
Registered: July 2009

Member

FYI I have just reproduced a deadlock caused by publishing from the subscription callback. I resolved this by publishing in a new thread.

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