apache/flink · error · IOException

Serializing the source elements failed: {e.getMessage()}

Error message

Serializing the source elements failed: {e.getMessage()}

What it means

Thrown inside serializeElements of IndexLookupGeneratorFunction when TypeSerializer.serialize throws any exception for one of the elements. The original exception is caught and wrapped in an IOException with the prefix 'Serializing the source elements failed:'. This occurs during construction via trySerialize.

Source

Thrown at flink-connectors/flink-connector-datagen/src/main/java/org/apache/flink/connector/datagen/functions/IndexLookupGeneratorFunction.java:138

                throw new IllegalArgumentException(
                        "The elements in the collection are not all subclasses of "
                                + viewedAs.getCanonicalName());
            }
        }
    }

    private void serializeElements(Iterable<OUT> elements) throws IOException {
        Preconditions.checkState(serializer != null, "serializer not set");
        LOG.info("Serializing elements using  {}", serializer);
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        DataOutputViewStreamWrapper wrapper = new DataOutputViewStreamWrapper(baos);

        try {
            for (OUT element : elements) {
                serializer.serialize(element, wrapper);
            }
        } catch (Exception e) {
            throw new IOException("Serializing the source elements failed: " + e.getMessage(), e);
        }
        this.elementsSerialized = baos.toByteArray();
    }

    private OUT tryDeserialize() throws IOException {
        try {
            return serializer.deserialize(input);
        } catch (EOFException eof) {
            throw new NoSuchElementException(
                    "Reached the end of the collection. This could be caused by issues with the "
                            + "serializer or by calling the map() function more times than there "
                            + "are elements in the collection. Make sure that you set the number "
                            + "of records to be produced by the DataGeneratorSource equal to the "
                            + "number of elements in the collection.");
        } catch (Exception e) {
            throw new IOException(
                    "Failed to deserialize an element from the source. "
                            + "If you are using user-defined serialization (Value and Writable "

View on GitHub (pinned to 2f3c205e92)

Solutions

1. Examine the wrapped cause exception to identify which element and field failed.
2. Ensure all element fields are serializable by the Flink TypeSerializer for the declared TypeInformation.
3. Test the TypeSerializer round-trip in isolation before passing elements to the constructor.
4. For custom Value or Writable types, verify write/readFields symmetry.

Example fix

// before: custom Value type with broken write()
class MyValue implements Value { /* write() throws */ }
new IndexLookupGeneratorFunction<>(TypeInformation.of(MyValue.class), values);

// after: fix the write() method to serialize all fields correctly
class MyValue implements Value {
    public void write(DataOutputView out) throws IOException {
        out.writeUTF(name != null ? name : "");
    }
}

// Validate serialization before constructing
TypeSerializer<OUT> serializer = typeInfo.createSerializer(config.getSerializerConfig());
DataOutputViewStreamWrapper wrapper =
    new DataOutputViewStreamWrapper(new ByteArrayOutputStream());
for (OUT element : elements) {
    serializer.serialize(element, wrapper);
}

try {
    new IndexLookupGeneratorFunction<>(typeInfo, elements);
} catch (RuntimeException e) {
    if (e.getCause() instanceof IOException
        && e.getCause().getMessage().startsWith("Serializing the source elements failed")) {
        // inspect e.getCause().getCause() for root cause
    }
    throw e;
}

When it happens

Trigger: Constructing IndexLookupGeneratorFunction with elements that the serializer cannot handle — e.g., a POJO field that is not serializable, a custom Value type with a buggy write() method, or elements whose runtime class does not match the serializer.

Common situations: Custom types with broken serialization logic; POJOs containing non-serializable nested objects; TypeInformation that does not match the actual element types.

Related errors

• {e.getMessage()}
• The collection contains a null element
• Reached the end of the collection. This could be caused by i
• Failed to deserialize an element from the source. If you are
• The collection contains a null element

AI-assisted analysis of apache/flink@2f3c205e92 (2026-08-14). Data as JSON: /api/errors/c5a1395348586931. Report an issue: GitHub.