Github user trixpan commented on the issue:
https://github.com/apache/nifi/pull/1521
**JPercivall**
I actually already committed general MQTT processors [1] and they have some
good "best practices" with regards to publishing/consuming messages and MQTT I
think would be good to follow here. The first few I have are below:
First in regards to the internal queue of messages, there needs to be some
kind of limit on it. If a user doesn't set a fast enough trigger duration the
queue could become enormous and, since all the messages are held in memory,
potentially bringing NiFi to a halt. Here is the property I add to
ConsumeMQTT[2].
This leads to the second, what to do with the internal queue when the
processor is stopped. There will potentially be some data left in the internal
queue when the processor is stopped and if that data is not handled it will
more than likely be lost. In the OnStopped I actually transfer all messages in
the queue (just like is done int he OnTrigger)[3].
In the PutAWSIoT processor, you read the attributes from the FlowFile to
essentially overwrite Processor defined properties. Being able to have
different QoS/Topics/Retained different messages is very important and the most
user friendly way to do this is through Expression Language. Check out how I do
it here[4]. This gives the user to potentially dynamically select the
QoS/Topic/Retained and doesn't do things automatically for the user.
[1]
https://github.com/apache/nifi/tree/f47af1ce8336c9305916f00738976f3505b01b0b/nifi-nar-bundles/nifi-mqtt-bundle/nifi-mqtt-processors/src/main/java/org/apache/nifi/processors/mqtt
[2]
https://github.com/apache/nifi/blob/f47af1ce8336c9305916f00738976f3505b01b0b/nifi-nar-bundles/nifi-mqtt-bundle/nifi-mqtt-processors/src/main/java/org/apache/nifi/processors/mqtt/ConsumeMQTT.java#L111
[3]
https://github.com/apache/nifi/blob/f47af1ce8336c9305916f00738976f3505b01b0b/nifi-nar-bundles/nifi-mqtt-bundle/nifi-mqtt-processors/src/main/java/org/apache/nifi/processors/mqtt/ConsumeMQTT.java#L233
[4]
https://github.com/apache/nifi/blob/f47af1ce8336c9305916f00738976f3505b01b0b/nifi-nar-bundles/nifi-mqtt-bundle/nifi-mqtt-processors/src/main/java/org/apache/nifi/processors/mqtt/PublishMQTT.java#L63
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