kafka-connect : Getting error in distributed confi

I get task error for a distributed configuration for a connector sink cassandra. I was running the command :

curl -s localhost:8083/connectors/cassandraSinkConnector2/status | jq

to get the status

{
  "name": "cassandraSinkConnector2",
  "connector": {
    "state": "RUNNING",
    "worker_id": localhost:8083"
  },
  "tasks": [
    {
      "id": 0,
      "state": "FAILED",
      "worker_id": "localhost:8083",
      "trace": "org.apache.kafka.common.KafkaException: Failed to construct kafka consumer\n\tat org.apache.kafka.clients.consumer.KafkaConsumer.<init>(KafkaConsumer.java:811)\n\tat org.apache.kafka.clients.consumer.KafkaConsumer.<init>(KafkaConsumer.java:624)\n\tat org.apache.kafka.clients.consumer.KafkaConsumer.<init>(KafkaConsumer.java:605)\n\tat org.apache.kafka.connect.runtime.Worker.buildWorkerTask(Worker.java:505)\n\tat org.apache.kafka.connect.runtime.Worker.startTask(Worker.java:441)\n\tat org.apache.kafka.connect.runtime.distributed.DistributedHerder.startTask(DistributedHerder.java:865)\n\tat org.apache.kafka.connect.runtime.distributed.DistributedHerder.access$1600(DistributedHerder.java:110)\n\tat org.apache.kafka.connect.runtime.distributed.DistributedHerder$13.call(DistributedHerder.java:880)\n\tat org.apache.kafka.connect.runtime.distributed.DistributedHerder$13.call(DistributedHerder.java:876)\n\tat java.util.concurrent.FutureTask.run(FutureTask.java:266)\n\tat java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)\n\tat java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)\n\tat java.lang.Thread.run(Thread.java:748)\nCaused by: org.apache.kafka.common.KafkaException: io.confluent.monitoring.clients.interceptor.MonitoringConsumerInterceptor ClassNotFoundException exception occurred\n\tat org.apache.kafka.common.config.AbstractConfig.getConfiguredInstances(AbstractConfig.java:357)\n\tat org.apache.kafka.common.config.AbstractConfig.getConfiguredInstances(AbstractConfig.java:332)\n\tat org.apache.kafka.common.config.AbstractConfig.getConfiguredInstances(AbstractConfig.java:319)\n\tat org.apache.kafka.clients.consumer.KafkaConsumer.<init>(KafkaConsumer.java:701)\n\t... 12 more\nCaused by: java.lang.ClassNotFoundException: io.confluent.monitoring.clients.interceptor.MonitoringConsumerInterceptor\n\tat java.net.URLClassLoader.findClass(URLClassLoader.java:382)\n\tat java.lang.ClassLoader.loadClass(ClassLoader.java:424)\n\tat org.apache.kafka.connect.runtime.isolation.PluginClassLoader.loadClass(PluginClassLoader.java:104)\n\tat java.lang.ClassLoader.loadClass(ClassLoader.java:357)\n\tat java.lang.Class.forName0(Native Method)\n\tat java.lang.Class.forName(Class.java:348)\n\tat org.apache.kafka.common.utils.Utils.loadClass(Utils.java:338)\n\tat org.apache.kafka.common.utils.Utils.newInstance(Utils.java:327)\n\tat org.apache.kafka.common.config.AbstractConfig.getConfiguredInstances(AbstractConfig.java:355)\n\t... 15 more\n"
    }
  ],
  "type": "sink"

Stack trace:

"trace": "org.apache.kafka.common.KafkaException: Failed to construct kafka consumer
    at org.apache.kafka.clients.consumer.KafkaConsumer.<init>(KafkaConsumer.java:811)
    at org.apache.kafka.clients.consumer.KafkaConsumer.<init>(KafkaConsumer.java:624)
    at org.apache.kafka.clients.consumer.KafkaConsumer.<init>(KafkaConsumer.java:605)
    at org.apache.kafka.connect.runtime.Worker.buildWorkerTask(Worker.java:505)
    at org.apache.kafka.connect.runtime.Worker.startTask(Worker.java:441)
    at org.apache.kafka.connect.runtime.distributed.DistributedHerder.startTask(DistributedHerder.java:865)
    at org.apache.kafka.connect.runtime.distributed.DistributedHerder.access$1600(DistributedHerder.java:110)
    at org.apache.kafka.connect.runtime.distributed.DistributedHerder$13.call(DistributedHerder.java:880)
    at org.apache.kafka.connect.runtime.distributed.DistributedHerder$13.call(DistributedHerder.java:876)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at java.lang.Thread.run(Thread.java:748)
Caused by: org.apache.kafka.common.KafkaException: io.confluent.monitoring.clients.interceptor.MonitoringConsumerInterceptor ClassNotFoundException exception occurred
    at org.apache.kafka.common.config.AbstractConfig.getConfiguredInstances(AbstractConfig.java:357)
    at org.apache.kafka.common.config.AbstractConfig.getConfiguredInstances(AbstractConfig.java:332)
    at org.apache.kafka.common.config.AbstractConfig.getConfiguredInstances(AbstractConfig.java:319)
    at org.apache.kafka.clients.consumer.KafkaConsumer.<init>(KafkaConsumer.java:701)
    ... 12 more
Caused by: java.lang.ClassNotFoundException: io.confluent.monitoring.clients.interceptor.MonitoringConsumerInterceptor
    at java.net.URLClassLoader.findClass(URLClassLoader.java:382)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at org.apache.kafka.connect.runtime.isolation.PluginClassLoader.loadClass(PluginClassLoader.java:104)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:348)
    at org.apache.kafka.common.utils.Utils.loadClass(Utils.java:338)
    at org.apache.kafka.common.utils.Utils.newInstance(Utils.java:327)
    at org.apache.kafka.common.config.AbstractConfig.getConfiguredInstances(AbstractConfig.java:355)
    ... 15 more

You can find below the configuration of the connector.


{
  "name": "cassandraSinkConnector2",
  "config": {
    "connector.class": "io.confluent.connect.cassandra.CassandraSinkConnector",
    "tasks.max": "1",
    "topics": "appartenance_de",
    "cassandra.contact.points": "localhost",
    "cassandra.kcql": "INSERT INTO app_test SELECT * FROM app_de",
    "cassandra.port": "9042",
    "cassandra.keyspace": "dev_dkks",
    "cassandra.username": "superuser",
    "cassandra.password": "password",
    "cassandra.write.mode": "insert",
    "value.converter.schemas.enable": "true",
    "value.converter": "io.confluent.connect.avro.AvroConverter",
    "value.converter.schema.registry.url": "http://localhost:8081",
    "name": "cassandraSinkConnector2"
  },
  "tasks": [
    {
      "connector": "cassandraSinkConnector2",
      "task": 0
    }
  ],
  "type": "sink"
}

New error:

org.apache.kafka.connect.errors.ConnectException: Exiting WorkerSinkTask due to unrecoverable exception.
    at org.apache.kafka.connect.runtime.WorkerSinkTask.deliverMessages(WorkerSinkTask.java:560)
    at org.apache.kafka.connect.runtime.WorkerSinkTask.poll(WorkerSinkTask.java:321)
    at org.apache.kafka.connect.runtime.WorkerSinkTask.iteration(WorkerSinkTask.java:224)
    at org.apache.kafka.connect.runtime.WorkerSinkTask.execute(WorkerSinkTask.java:192)
    at org.apache.kafka.connect.runtime.WorkerTask.doRun(WorkerTask.java:175)
    at org.apache.kafka.connect.runtime.WorkerTask.run(WorkerTask.java:219)
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at java.lang.Thread.run(Thread.java:748)
Caused by: org.apache.kafka.connect.errors.DataException: Record with a null key was encountered.  This connector requires that records from Kafka contain the keys for the Cassandra table. Please use a transformation like org.apache.kafka.connect.transforms.ValueToKey to create a key with the proper fields.
    at io.confluent.connect.cassandra.CassandraSinkTask.put(CassandraSinkTask.java:86)
    at org.apache.kafka.connect.runtime.WorkerSinkTask.deliverMessages(WorkerSinkTask.java:538)
    ... 10 more
"

The root error is

java.lang.ClassNotFoundException: io.confluent.monitoring.clients.interceptor.MonitoringConsumerInterceptor

The Monitoring Interceptors are part of Confluent Platform. You can either disable their use in your Kafka Connect worker config, or better, make sure that the /usr/share/java/monitoring-interceptors/monitoring-interceptors-5.2.1.jar JAR is available to your Kafka Connect worker.

The new error you're seeing is

org.apache.kafka.connect.errors.DataException: 
Record with a null key was encountered.  This connector requires that records from Kafka contain the keys for the Cassandra table. 
Please use a transformation like org.apache.kafka.connect.transforms.ValueToKey to create a key with the proper fields.

I'd suggest using a Single Message Transform as suggested in the error to correctly key your data. You can see an example of doing this here and the documentation for the transform here.