apache/flink · error · IOException

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

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

Thrown by tryDeserialize in IndexLookupGeneratorFunction when deserialization fails with an exception other than EOFException. The message advises checking user-defined serialization (Value and Writable types) and includes the serializer's toString() for diagnostics. The original exception is chained as the cause.

Solutions

Inspect the chained cause exception for the specific deserialization failure reason.
Verify the custom type's readFields/write symmetry — every field written must be read in the same order and format.
Check if the TypeSerializer version is compatible across Flink upgrades.
Test serialize+deserialize round-trip in a unit test for the custom type.

Defensive patterns

Strategy: try-catch

Validate before calling

// Verify custom Value/Writable type round-trips correctly
MyValue original = new MyValue(...);
ByteArrayOutputStream baos = new ByteArrayOutputStream();
DataOutputViewStreamWrapper out = new DataOutputViewStreamWrapper(baos);
typeInfo.createSerializer(config).serialize(original, out);
DataInputViewStreamWrapper in = new DataInputViewStreamWrapper(
    new ByteArrayInputStream(baos.toByteArray()));
MyValue restored = typeInfo.createSerializer(config).deserialize(in);
assert original.equals(restored); // verify symmetry

Try / catch

try {
    fn.open(readerContext);
} catch (IOException e) {
    if (e.getMessage().startsWith("Failed to deserialize an element")) {
        // inspect e.getCause() for the specific deserialization error
        // check custom Value/Writable readFields() vs write() symmetry
    }
    throw e;
}

Prevention

For custom Value/Writable types, ensure readFields mirrors write exactly.
Write a unit test that serializes then deserializes every element and asserts equality.
Keep serializer version stable across Flink upgrades, or implement TypeSerializerSnapshot migration.

When it happens

Trigger: During buildLookup or map deserialization, serializer.deserialize(input) throws a non-EOF exception. This indicates corrupt serialized data, a serializer version mismatch, or a bug in a custom Value/Writable readFields() method.

Common situations: A custom type's readFields() does not mirror write() correctly; serializer snapshot version mismatch after upgrading Flink; state corruption in the serialized byte array.

Related errors

AI-assisted analysis of apache/flink@2f3c205e92 (2026-08-14). Data as JSON: /api/errors/4ab4ad950237bb96. Report an issue: GitHub.

Appendix: source

Thrown at flink-connectors/flink-connector-datagen/src/main/java/org/apache/flink/connector/datagen/functions/IndexLookupGeneratorFunction.java:154

            }
        } 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 "
                            + "types), check the serialization functions.\nSerializer is "
                            + serializer,
                    e);
        }
    }

    private void buildLookup() throws IOException {
        for (long i = 0; i < numElements; i++) {
            lookupMap.put(i, tryDeserialize());
        }
    }

    private void trySerialize(Iterable<OUT> elements) {
        try {
            serializeElements(elements);
        } catch (IOException e) {

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