Handling empty/invalid Mqtt Messages with Kafka Connect - apache-kafka

Question

I am trying to ingest data from Mqtt into Kafka. Unfortunately, some of those Mqtt-Messages are either empty or invalid JSON. I assume that is what leads to the following exception:
{
"name": "source_mqtt_alarms",
"connector": {
"state": "RUNNING",
"worker_id": "-redacted-:8083"
},
"tasks": [
{
"id": 0,
"state": "FAILED",
"worker_id": "-redacted-:8083",
"trace": "org.apache.kafka.connect.errors.ConnectException:
Tolerance exceeded in error handler\n\tat org.apache.kafka.connect.runtime.errors.RetryWithToleranceOperator.execAndHandleError(RetryWithToleranceOperator.java:196)\n\t
at org.apache.kafka.connect.runtime.errors.RetryWithToleranceOperator.execute(RetryWithToleranceOperator.java:122)\n\t
at org.apache.kafka.connect.runtime.WorkerSourceTask.convertTransformedRecord(WorkerSourceTask.java:314)\n\t
at org.apache.kafka.connect.runtime.WorkerSourceTask.sendRecords(WorkerSourceTask.java:340)\n\t
at org.apache.kafka.connect.runtime.WorkerSourceTask.execute(WorkerSourceTask.java:264)\n\t
at org.apache.kafka.connect.runtime.WorkerTask.doRun(WorkerTask.java:185)\n\t
at org.apache.kafka.connect.runtime.WorkerTask.run(WorkerTask.java:235)\n\t
at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)\n\t
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)\n\t
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)\n\t
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)\n\t
at java.base/java.lang.Thread.run(Thread.java:834)\n
Caused by: org.apache.kafka.connect.errors.DataException: Conversion error: null value for field that is required and has no default value\n\t
at org.apache.kafka.connect.json.JsonConverter.convertToJson(JsonConverter.java:611)\n\t
at org.apache.kafka.connect.json.JsonConverter.convertToJsonWithEnvelope(JsonConverter.java:592)\n\t
at org.apache.kafka.connect.json.JsonConverter.fromConnectData(JsonConverter.java:346)\n\t
at org.apache.kafka.connect.storage.Converter.fromConnectData(Converter.java:63)\n\t
at org.apache.kafka.connect.runtime.WorkerSourceTask.lambda$convertTransformedRecord$2(WorkerSourceTask.java:314)\n\t
at org.apache.kafka.connect.runtime.errors.RetryWithToleranceOperator.execAndRetry(RetryWithToleranceOperator.java:146)\n\t
at org.apache.kafka.connect.runtime.errors.RetryWithToleranceOperator.execAndHandleError(RetryWithToleranceOperator.java:180)\n\t
... 11 more\n"
}
],
"type": "source"
}
From what I've learned so far, it looks like the incoming (empty/invalid) messages do not contain values that are declared as non-optional, which leads to the exception above.
My question would be, where is the connector taking that expectation from? It says "null value for field that is required and has no default value", but how is that field required if the schema is (I assume) created per message?
Additional information:
I am using the Lenses.io Stream Reactor Mqtt Source Connector. The configuration is as follows:
{
"name": "source_mqtt_alarms",
"config": {
"topics": "alarms",
"connect.mqtt.kcql": "INSERT INTO alarms SELECT * FROM `-redacted-/+/alarms` WITHCONVERTER=`com.datamountaineer.streamreactor.connect.converters.source.JsonSimpleConverter`",
"connect.mqtt.client.id": "kafka_connect_alarms",
"tasks.max": 1,
"connector.class": "com.datamountaineer.streamreactor.connect.mqtt.source.MqttSourceConnector",
"connect.mqtt.service.quality": 2,
"connect.mqtt.hosts": "ssl://-redacted-:8883",
"connect.mqtt.ssl.ca.cert": "/usr/share/certs/cumu.crt",
"connect.mqtt.ssl.cert": "/usr/share/certs/mqtt.crt",
"connect.mqtt.ssl.key": "/usr/share/certs/mqtt.pem",
"value.converter": "org.apache.kafka.connect.json.JsonConverter",
"value.converter.schemas.enable": true,
"key.converter":"org.apache.kafka.connect.json.JsonConverter",
"key.converter.schemas.enable": true,
}
}
Edit: I just went through the logs of the Kafka Connect worker and it's giving a bit more information. Prior to the exception above, I get a lost of these:
[2021-05-26 08:27:19,552] ERROR Error handling message with id:0 on topic:-redacted-/alarms (com.datamountaineer.streamreactor.connect.mqtt.source.MqttManager)
java.util.NoSuchElementException: head of empty list
at scala.collection.immutable.Nil$.head(List.scala:430)
at scala.collection.immutable.Nil$.head(List.scala:427)
at com.datamountaineer.streamreactor.connect.converters.source.JsonSimpleConverter$.convert(JsonSimpleConverter.scala:76)
at com.datamountaineer.streamreactor.connect.converters.source.JsonSimpleConverter$.convert(JsonSimpleConverter.scala:70)
at com.datamountaineer.streamreactor.connect.converters.source.JsonSimpleConverter.convert(JsonSimpleConverter.scala:37)
at com.datamountaineer.streamreactor.connect.mqtt.source.MqttManager.messageArrived(MqttManager.scala:110)
at org.eclipse.paho.client.mqttv3.internal.CommsCallback.deliverMessage(CommsCallback.java:514)
at org.eclipse.paho.client.mqttv3.internal.CommsCallback.handleMessage(CommsCallback.java:417)
at org.eclipse.paho.client.mqttv3.internal.CommsCallback.run(CommsCallback.java:214)
at java.base/java.lang.Thread.run(Thread.java:834)