apache/beam · warning

Error while processing the element

Error message

Error while processing the element

What it means

KafkaWriteSchemaTransformProvider's ProcessElement wraps row-to-KV conversion in try/catch. When an element fails conversion and error handling is enabled, the exception is not propagated; instead the element is logged at WARN and routed to the error output tag with its error metadata. If handleErrors is false, the exception is rethrown as a RuntimeException and fails the bundle.

Source

Thrown at sdks/java/io/kafka/src/main/java/org/apache/beam/sdk/io/kafka/KafkaWriteSchemaTransformProvider.java:156

          TupleTag<KV<byte @Nullable [], T>> successTag) {
        this.conversionFn = conversionFn;
        this.errorCounter = Metrics.counter(KafkaWriteSchemaTransformProvider.class, name);
        this.handleErrors = handleErrors;
        this.errorSchema = errorSchema;
        this.successTag = successTag;
      }

      @ProcessElement
      public void process(@DoFn.Element Row row, MultiOutputReceiver receiver) {
        KV<byte @Nullable [], T> output = null;
        try {
          output = KV.of(null, conversionFn.apply(row));
        } catch (Exception e) {
          if (!handleErrors) {
            throw new RuntimeException(e);
          }
          errorsInBundle += 1;
          LOG.warn("Error while processing the element", e);
          receiver.get(ERROR_TAG).output(ErrorHandling.errorRecord(errorSchema, row, e));
        }
        if (output != null) {
          receiver.get(successTag).output(output);
        }
      }

      @FinishBundle
      public void finish() {
        errorCounter.inc(errorsInBundle);
        errorsInBundle = 0L;
      }
    }

    public static class ErrorCounterFn extends BaseKafkaWriterFn<byte[]> {
      public ErrorCounterFn(
          String name,
          SerializableFunction<Row, byte[]> toBytesFn,

View on GitHub (pinned to 12126d8942)

Solutions

1. Inspect the wrapped cause 'e' in the log to find the field/value that failed conversion.
2. Fix upstream data so rows match the declared write schema (types, nullability).
3. Consume the ERROR_TAG output of the transform and route those records to a dead-letter sink or repair logic.
4. If you want the pipeline to fail instead, disable error handling (handleErrors=false) so the exception propagates.

Example fix

// before: errors silently swallowed into ERROR_TAG
.apply(KafkaWriteSchemaTransformProvider ...)
// after: handle error records explicitly
PCollection<Row> errors = result.get(ERROR_TAG);
errors.apply("DeadLetter", IO.write(...));

if (row.getSchema() != null && row.getValue("payload") == null) {
  LOG.warn("Skipping row with null payload before Kafka write");
}

// Rely on the provider's error output instead of crashing:
WriteResult result = ...;
result.getErrors().apply("DeadLetter", FileIO.write());
// Or let it fail fast:
// handleErrors=false -> RuntimeException propagates and fails the bundle

When it happens

Trigger: conversionFn.apply(row) throws for a specific Row — e.g. a schema field is null where the Kafka serialization expects a value, a payload fails byte conversion, or the row does not match the configured Kafka write schema.

Common situations: Pipelines writing malformed or null-bearing rows to Kafka with the error-handling (dead-letter) configuration enabled; developers see the message in logs but some records silently go to the error output instead of the success topic.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

• field was received -- type mismatch
• Required org.apache.beam.sdk.schemas.Schema field has null
• Failed trying to process value for key %s.
• Failed trying to process extra files.
• The provided external bucket could not be matched to a known

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/fbfc70647554ce90. Report an issue: GitHub.