Getting Persistence Already in Use error from MQTT

Question

I am getting the following exception from the mqtt broker when I am trying to create a new MqttClient. The error is here ---

Caused by: Persistence already in use (32200)
at org.eclipse.paho.client.mqttv3.persist.MqttDefaultFilePersistence.open(MqttDefaultFilePersistence.java:108) [mqtt-client-0.4.0.jar:]
at org.eclipse.paho.client.mqttv3.MqttAsyncClient.<init>(MqttAsyncClient.java:273) [mqtt-client-0.4.0.jar:]
at org.eclipse.paho.client.mqttv3.MqttClient.<init>(MqttClient.java:222) [mqtt-client-0.4.0.jar:]
at org.eclipse.paho.client.mqttv3.MqttClient.<init>(MqttClient.java:134) [mqtt-client-0.4.0.jar:]
at com.ericsson.asdp.virtualassist.notification.messaging.MQTTHandler.createClient(MQTTHandler.java:61) [classes:]
at com.ericsson.asdp.virtualassist.notification.messaging.MQTTMessagingService.receieve(MQTTMessagingService.java:52) [classes:]
... 44 more 

Here is the code for my java class receive() method from where I am trying to connect to mqtt ---

MqttClient subClient = null;
try {
    subClient = mqttHandler.createClient(userId, brokerURL);
    MQTTNotificationSubscriber notificationSub = new MQTTNotificationSubscriber(mqttHandler);
    notificationSub.setUserId(userId);

    subClient.setCallback(notificationSub);
    mqttHandler.subscribe(subClient, userId);
    // do something here
} catch (Exception e) {
    logger.error("Error in receive " + e.getMessage());
        throw new VirtualAssistServicesException(e.getMessage(), e);
}  finally {
    try {
        mqttHandler.disconnect(subClient);
    } catch (MqttException e) {
        throw new VirtualAssistServicesException(e.getMessage(), e);
    }
}

And here is the MQTTHandler class createClient() method ---

MqttClient subClient = null;

try {
    subClient = new MqttClient(brokerURL, clientId);

} catch (MqttException e) {

}

When I create the client for a userId first time it works. From second time onwards it fails with the above exception. I am using clean-session=false here.

If anyone has any idea please let me know. Thanks.

Answer 1

Looks like both clients are trying to use the same file for persistence.
Javadocs for MqttDefaultFilePersistence.open() says

Initialise the persistent store. If a persistent store exists for this client ID then open it, otherwise create a new one. If the persistent store is already open then just return. An application may use the same client ID to connect to many different servers, so the client ID in conjunction with the connection will uniquely identify the persistence store required.
Throws: MqttPersistenceException - if there was a problem opening the persistent store.

I suppose the file is already open, you must use a different clientId in your code for each one of your Mqtt clients.