[KafkaTrigger] Errors while retrieving function metadata using BindingName in cardinality=Cardinality.MANY mode

[aloiva]

Kafka trigger function has these metadata: Offset (long), Topic (string), key, partition, timestamp and headers. I was able to retrieve all this data using a basic function:

public class KafkaTriggerJava {
  @FunctionName("KafkaTriggerJava")
  public void run(
      @KafkaTrigger(
              name = "kafkaTrigger",
              topic = "test-topic",
              brokerList = "localhost:9092",
              authenticationMode = BrokerAuthenticationMode.PLAIN,
              protocol = BrokerProtocol.PLAINTEXT,
              consumerGroup = "test-group")
          String kafkaEventData,
      @BindingName("Key") String key,
      @BindingName("Offset") int offset,
      @BindingName("Partition") int partition,
      final ExecutionContext context) {
    context.getLogger().info("Payload: " + kafkaEventData);
  }
}

Then I set cardinality=Cardinality.MANY and tried the following binding names:

           @BindingName("KeyArray") Object[] key,
           @BindingName("OffsetArray") Object[] offset,
           @BindingName("TopicArray") Object[] topic,
           @BindingName("PartitionArray") Object[] partition,
           @BindingName("TimestampArray") Object[] timestamp, 
           @BindingName("HeadersArray") String[] headers

While I was able to get some of the data, "OffsetArray" and "TopicArray" ran into data conversion errors.
Trace for "OffsetArray":

Exception: ClassCastException: class com.microsoft.azure.functions.shaded.com.google.protobuf.LongArrayList cannot be cast to class java.lang.String (com.microsoft.azure.functions.shaded.com.google.protobuf.LongArrayList is in unnamed module of loader 'app'; java.lang.String is in module java.base of loader 'bootstrap')
Stack: java.lang.ClassCastException: class com.microsoft.azure.functions.shaded.com.google.protobuf.LongArrayList cannot be cast to class java.lang.String (com.microsoft.azure.functions.shaded.com.google.protobuf.LongArrayList is in unnamed module of loader 'app'; java.lang.String is in module java.base of loader 'bootstrap')
[2025-10-29T05:03:58.349Z]      at com.microsoft.azure.functions.worker.binding.DataOperations.apply(DataOperations.java:99)
[2025-10-29T05:03:58.352Z]      at com.microsoft.azure.functions.worker.binding.DataSource.computeByType(DataSource.java:56)
[2025-10-29T05:03:58.354Z]      at com.microsoft.azure.functions.worker.binding.RpcCollectionLongDataSource.computeByType(RpcCollectionLongDataSource.java:10)
[2025-10-29T05:03:58.356Z]      at com.microsoft.azure.functions.worker.binding.DataSource.computeByName(DataSource.java:42)
[2025-10-29T05:03:58.359Z]      at com.microsoft.azure.functions.worker.binding.RpcCollectionLongDataSource.computeByName(RpcCollectionLongDataSource.java:10)
[2025-10-29T05:03:58.361Z]      at com.microsoft.azure.functions.worker.binding.BindingDataStore.getTriggerMetatDataByName(BindingDataStore.java:72)
[2025-10-29T05:03:58.363Z]      at com.microsoft.azure.functions.worker.broker.ParameterResolver.resolve(ParameterResolver.java:47)
[2025-10-29T05:03:58.365Z]      at com.microsoft.azure.functions.worker.broker.ParameterResolver.resolveArguments(ParameterResolver.java:22)
[2025-10-29T05:03:58.367Z]      at com.microsoft.azure.functions.worker.broker.EnhancedJavaMethodExecutorImpl.execute(EnhancedJavaMethodExecutorImpl.java:20)
[2025-10-29T05:03:58.370Z]      at com.microsoft.azure.functions.worker.chain.FunctionExecutionMiddleware.invoke(FunctionExecutionMiddleware.java:19)
[2025-10-29T05:03:58.372Z]      at com.microsoft.azure.functions.worker.chain.InvocationChain.doNext(InvocationChain.java:21)
[2025-10-29T05:03:58.374Z]      at com.microsoft.azure.functions.worker.broker.JavaFunctionBroker.invokeMethod(JavaFunctionBroker.java:198)
[2025-10-29T05:03:58.376Z]      at com.microsoft.azure.functions.worker.handler.InvocationRequestHandler.execute(InvocationRequestHandler.java:34)
[2025-10-29T05:03:58.378Z]      at com.microsoft.azure.functions.worker.handler.InvocationRequestHandler.execute(InvocationRequestHandler.java:10)
[2025-10-29T05:03:58.380Z]      at com.microsoft.azure.functions.worker.handler.MessageHandler.handle(MessageHandler.java:44)
[2025-10-29T05:03:58.383Z]      at com.microsoft.azure.functions.worker.JavaWorkerClient$StreamingMessagePeer.lambda$onNext$0(JavaWorkerClient.java:94)
[2025-10-29T05:03:58.385Z]      at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
[2025-10-29T05:03:58.387Z]      at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
[2025-10-29T05:03:58.390Z]      at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
[2025-10-29T05:03:58.392Z]      at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
[2025-10-29T05:03:58.394Z]      at java.base/java.lang.Thread.run(Thread.java:834)

Trace for "TopicArray":

Exception: ClassCastException: class com.microsoft.azure.functions.shaded.com.google.protobuf.LazyStringArrayList cannot be cast to class java.lang.String (com.microsoft.azure.functions.shaded.com.google.protobuf.LazyStringArrayList is in unnamed module of loader 'app'; java.lang.String is in module java.base of loader 'bootstrap')
Stack: java.lang.ClassCastException: class com.microsoft.azure.functions.shaded.com.google.protobuf.LazyStringArrayList cannot be cast to class java.lang.String (com.microsoft.azure.functions.shaded.com.google.protobuf.LazyStringArrayList is in unnamed module of loader 'app'; java.lang.String is in module java.base of loader 'bootstrap')
[2025-10-29T04:43:25.493Z]      at com.microsoft.azure.functions.worker.binding.DataOperations.apply(DataOperations.java:99)
[2025-10-29T04:43:25.495Z]      at com.microsoft.azure.functions.worker.binding.DataSource.computeByType(DataSource.java:56)
[2025-10-29T04:43:25.498Z]      at com.microsoft.azure.functions.worker.binding.RpcCollectionStringDataSource.computeByType(RpcCollectionStringDataSource.java:10)
[2025-10-29T04:43:25.500Z]      at com.microsoft.azure.functions.worker.binding.DataSource.computeByName(DataSource.java:42)
[2025-10-29T04:43:25.503Z]      at com.microsoft.azure.functions.worker.binding.RpcCollectionStringDataSource.computeByName(RpcCollectionStringDataSource.java:10)
[2025-10-29T04:43:25.506Z]      at com.microsoft.azure.functions.worker.binding.BindingDataStore.getTriggerMetatDataByName(BindingDataStore.java:72)
[2025-10-29T04:43:25.509Z]      at com.microsoft.azure.functions.worker.broker.ParameterResolver.resolve(ParameterResolver.java:47)
[2025-10-29T04:43:25.512Z]      at com.microsoft.azure.functions.worker.broker.ParameterResolver.resolveArguments(ParameterResolver.java:22)
[2025-10-29T04:43:25.515Z]      at com.microsoft.azure.functions.worker.broker.EnhancedJavaMethodExecutorImpl.execute(EnhancedJavaMethodExecutorImpl.java:20)
[2025-10-29T04:43:25.518Z]      at com.microsoft.azure.functions.worker.chain.FunctionExecutionMiddleware.invoke(FunctionExecutionMiddleware.java:19)
[2025-10-29T04:43:25.521Z]      at com.microsoft.azure.functions.worker.chain.InvocationChain.doNext(InvocationChain.java:21)
[2025-10-29T04:43:25.525Z]      at com.microsoft.azure.functions.worker.broker.JavaFunctionBroker.invokeMethod(JavaFunctionBroker.java:198)
[2025-10-29T04:43:25.528Z]      at com.microsoft.azure.functions.worker.handler.InvocationRequestHandler.execute(InvocationRequestHandler.java:34)
[2025-10-29T04:43:25.532Z]      at com.microsoft.azure.functions.worker.handler.InvocationRequestHandler.execute(InvocationRequestHandler.java:10)
[2025-10-29T04:43:25.536Z]      at com.microsoft.azure.functions.worker.handler.MessageHandler.handle(MessageHandler.java:44)
[2025-10-29T04:43:25.539Z]      at com.microsoft.azure.functions.worker.JavaWorkerClient$StreamingMessagePeer.lambda$onNext$0(JavaWorkerClient.java:94)
[2025-10-29T04:43:25.543Z]      at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
[2025-10-29T04:43:25.547Z]      at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
[2025-10-29T04:43:25.550Z]      at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
[2025-10-29T04:43:25.553Z]      at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
[2025-10-29T04:43:25.556Z]      at java.base/java.lang.Thread.run(Thread.java:834)

[arnabnandy7]

The issue is caused by using Object[] for batch metadata. The Java worker supports concrete collection types, but does not provide a conversion from the underlying protobuf collections to Object[].

Please use the metadata’s concrete types instead:

@BindingName("TopicArray") String[] topics,
@BindingName("OffsetArray") long[] offsets

String[] is supported for TopicArray, while long[] or Long[] is supported for OffsetArray.

When Object[] is requested, the conversion falls through to the generic scalar conversion path, resulting in the reported ClassCastException. We should clarify this limitation and the supported types in the documentation.