MQTT error Too many publishes in progress

MQTT error Too many publishes in progress

At 100 messages per second and higher, messages to MQTT are lost by the MQTT Client Publish Adapter and the following error is seen in the console log:

(fragment deploy): 2019-02-15 18:24:06.990000+0000 [24676:MQTT Call: paho372226896500375] INFO  OutputAdapter: Action: 
 Connected Object: LocalMQTTBroker Message: Connected to MQTT Broker using connection config: LocalMQTTBroker
(fragment deploy): 2019-02-15 18:24:06.488000+0000 [24676:Thread-24] ERROR OutputAdapter: Action: Publish Failed Object: 
 data/rt Message: Error trying to publish to topic: data/rt, Error: Too many publishes in progress, see log for full details
(fragment deploy): org.eclipse.paho.client.mqttv3.MqttException: Too many publishes in progress
(fragment deploy): at org.eclipse.paho.client.mqttv3.internal.ClientState.send(ClientState.java:513)
(fragment deploy): at org.eclipse.paho.client.mqttv3.internal.ClientComms.internalSend(ClientComms.java:158)

How can this be avoided?
  According to MQTT defect "Fix tokens for QoS 0 getting lost. #563" ( https://github.com/eclipse/paho.mqtt.java/pull/563 ) this is a known problem when using Qualify-of-Service (QoS) level zero (0).

There are two workarounds:

One workaround is to configure the MQTT publsher to have QoS=1 or QoS=2 in Connection Configuration setting:

<setting name="willQos" val="1" />

Use "1" or "2" to not lose messages.

A future update to the MQTT paho libraries to fix this will be incorporated into StreamBase product when available. To see if this has been addressed in a specific StreamBase release, please check the Release Notes.

Another workaround is to downgrade the Paho library version dependency in a StreamBase 10 project by editing the pom.xml to have:

        <dependency>
            <groupId>org.eclipse.paho</groupId>
            <artifactId>org.eclipse.paho.client.mqttv3</artifactId>
            <version>1.1.0</version>
        </dependency>

Note that since this is a downgrade, there may be other issues in the 1.1.0 version which you may need to understand.