apache/flink · error · IOException

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

Error message

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

What it means

FromElementsGeneratorFunction#serializeElements catches any Exception while serializing each element with the Flink TypeSerializer and wraps it in an IOException carrying the inner exception's message. This means the element type's serializer rejected one of the provided elements (type mismatch, non-serializable field, null where not allowed).

Source

Thrown at flink-connectors/flink-connector-datagen/src/main/java/org/apache/flink/connector/datagen/functions/FromElementsGeneratorFunction.java:107

    @VisibleForTesting
    @Nullable
    public TypeSerializer<OUT> getSerializer() {
        return serializer;
    }

    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();
    }

    @Override
    public void open(SourceReaderContext readerContext) throws Exception {
        ByteArrayInputStream bais = new ByteArrayInputStream(elementsSerialized);
        this.input = new DataInputViewStreamWrapper(bais);
    }

    @Override
    public OUT map(Long nextIndex) throws Exception {
        // Move iterator to the required position in case of failure recovery
        while (numElementsEmitted < nextIndex) {
            numElementsEmitted++;
            tryDeserialize(serializer, input);
        }
        numElementsEmitted++;

View on GitHub (pinned to 2f3c205e92)

Solutions

Ensure every element's runtime type matches the TypeInformation passed to the constructor (checkIterable already enforces the class, but nested fields may still mismatch).
Remove nulls or elements of inconsistent types from the collection.
If using a custom type, provide a correct TypeInformation and a serializer that handles all field values.
Inspect the wrapped cause for the exact field/type that failed serialization.

Example fix

// before: mixed types
new FromElementsGeneratorFunction<>(Types.POJO(MyPojo.class), new MyPojo(...), "oops");
// after: uniform type
new FromElementsGeneratorFunction<>(Types.POJO(MyPojo.class), new MyPojo(...), new MyPojo(...));

Defensive patterns

Strategy: validation

Validate before calling

// Verify all elements are compatible with the declared type before constructing:
for (OUT e : elements) {
    if (e == null) throw new IllegalArgumentException("null element not allowed");
    if (!typeInfo.getTypeClass().isInstance(e)) {
        throw new IllegalArgumentException("Element " + e + " is not a " + typeInfo.getTypeClass());
    }
}

Type guard

static <T> boolean allOfType(Iterable<T> elems, Class<T> clazz) {
    for (T e : elems) if (e == null || !clazz.isInstance(e)) return false;
    return true;
}

Try / catch

try {
    new FromElementsGeneratorFunction<>(typeInfo, elements);
} catch (IOException | RuntimeException e) {
    Throwable c = e.getCause() != null ? e.getCause() : e;
    throw new RuntimeException("Element serialization failed under " + typeInfo, c);
}

Prevention

Pass uniform element types matching the TypeInformation.
Avoid null elements unless the serializer explicitly supports them.
Provide an explicit TypeInformation rather than relying on inference for mixed collections.

When it happens

Trigger: Passing elements to FromElementsGeneratorFunction whose runtime type does not match the declared TypeInformation, or whose contents the generated/wired serializer cannot encode. Occurs in the constructor (trySerialize) and in setOutputType when the serializer changes.

Common situations: Mixing element types in varargs; passing null elements when the serializer forbids them; POJOs/POJO-like objects with fields the TypeSerializer cannot handle; custom serializers with bugs.

Related errors

AI-assisted analysis of apache/flink@2f3c205e92 (2026-08-14). Data as JSON: /api/errors/67ceed1f10a32e30. Report an issue: GitHub.