apache/flink · error · IOException

Failed to deserialize an element from the source. If you are

Error message

Failed to deserialize an element from the source. If you are using user-defined serialization (Value and Writable types), check the serialization functions.
Serializer is {serializer}

What it means

FromElementsGeneratorFunction#tryDeserialize catches a non-EOF Exception during deserialization and wraps it in an IOException that includes the failing serializer's toString. This indicates the element bytes could not be decoded (corruption, type change, incompatible serializer) but the stream was not exhausted.

Source

Thrown at flink-connectors/flink-connector-datagen/src/main/java/org/apache/flink/connector/datagen/functions/FromElementsGeneratorFunction.java:137

    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++;
        return tryDeserialize(serializer, input);
    }

    private OUT tryDeserialize(TypeSerializer<OUT> serializer, DataInputView input)
            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 types), check the "
                            + "serialization functions.\nSerializer is "
                            + serializer,
                    e);
        }
    }

    // For backward compatibility: Supports legacy usage of
    // StreamExecutionEnvironment#fromElements() which lacked type information and relied on the
    // returns() method. See FLINK-21386 for details.
    @Override
    public void setOutputType(TypeInformation<OUT> outTypeInfo, ExecutionConfig executionConfig) {
        Preconditions.checkState(
                elements != null,
                "The output type should've been specified before shipping the graph to the cluster");
        checkIterable(elements, outTypeInfo.getTypeClass());
        TypeSerializer<OUT> newSerializer =

View on GitHub (pinned to 2f3c205e92)

Solutions

  1. If using Value/Writable types, audit the write/read field order and length consistency.
  2. Ensure the TypeInformation/serializer used at read time matches the one used when elements were serialized (do not change the type between construction and open).
  3. Inspect the wrapped cause and the printed serializer to pinpoint the field that failed.
  4. Avoid mutating the elements collection or serializer after construction.
Defensive patterns

Strategy: try-catch

Validate before calling

// Round-trip test the serializer before using it:
ByteArrayOutputStream out = new ByteArrayOutputStream();
DataOutputViewStreamWrapper w = new DataOutputViewStreamWrapper(out);
for (OUT e : elements) serializer.serialize(e, w);
DataInputViewStreamWrapper r = new DataInputViewStreamWrapper(
    new ByteArrayInputStream(out.toByteArray()));
for (OUT e : elements) {
    OUT got = serializer.deserialize(r);
    if (!Objects.equals(got, e)) throw new IllegalStateException("round-trip mismatch");
}

Try / catch

try {
    return serializer.deserialize(input);
} catch (IOException e) {
    // serializer.toString() is included in the message; check Value/Writable read/write parity
    throw e;
}

Prevention

When it happens

Trigger: Deserialization fails for a reason other than end-of-stream: the serializer changed between writing and reading, the byte buffer is corrupt, or a Value/Writable type's serialization methods are inconsistent.

Common situations: User-defined Value/Writable types with buggy read/write; changing the element type or TypeInformation after the elements were serialized; serializer state divergence across classloader boundaries.

Related errors

AI-assisted analysis of apache/flink@2f3c205e92 (2026-08-14). Data as JSON: /api/errors/8ae5dfb645c39ced. Report an issue: GitHub.