{"record":{"id":"4ab4ad950237bb96","repo":"apache/flink","slug":"failed-to-deserialize-an-element-from-the-source-4ab4ad","errorCode":null,"errorMessage":"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}","messagePattern":"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 (.+?)","errorType":"exception","errorClass":"IOException","httpStatus":null,"severity":"error","filePath":"flink-connectors/flink-connector-datagen/src/main/java/org/apache/flink/connector/datagen/functions/IndexLookupGeneratorFunction.java","lineNumber":154,"sourceCode":"            }\n        } catch (Exception e) {\n            throw new IOException(\"Serializing the source elements failed: \" + e.getMessage(), e);\n        }\n        this.elementsSerialized = baos.toByteArray();\n    }\n\n    private OUT tryDeserialize() throws IOException {\n        try {\n            return serializer.deserialize(input);\n        } catch (EOFException eof) {\n            throw new NoSuchElementException(\n                    \"Reached the end of the collection. This could be caused by issues with the \"\n                            + \"serializer or by calling the map() function more times than there \"\n                            + \"are elements in the collection. Make sure that you set the number \"\n                            + \"of records to be produced by the DataGeneratorSource equal to the \"\n                            + \"number of elements in the collection.\");\n        } catch (Exception e) {\n            throw new IOException(\n                    \"Failed to deserialize an element from the source. \"\n                            + \"If you are using user-defined serialization (Value and Writable \"\n                            + \"types), check the serialization functions.\\nSerializer is \"\n                            + serializer,\n                    e);\n        }\n    }\n\n    private void buildLookup() throws IOException {\n        for (long i = 0; i < numElements; i++) {\n            lookupMap.put(i, tryDeserialize());\n        }\n    }\n\n    private void trySerialize(Iterable<OUT> elements) {\n        try {\n            serializeElements(elements);\n        } catch (IOException e) {","sourceCodeStart":136,"sourceCodeEnd":172,"githubUrl":"https://github.com/apache/flink/blob/2f3c205e9266cb30240eb7f4fdab15cad629a70f/flink-connectors/flink-connector-datagen/src/main/java/org/apache/flink/connector/datagen/functions/IndexLookupGeneratorFunction.java#L136-L172","documentation":"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.","triggerScenarios":"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.","commonSituations":"A custom type's readFields() does not mirror write() correctly; serializer snapshot version mismatch after upgrading Flink; state corruption in the serialized byte array.","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."],"exampleFix":null,"handlingStrategy":"try-catch","validationCode":"// Verify custom Value/Writable type round-trips correctly\nMyValue original = new MyValue(...);\nByteArrayOutputStream baos = new ByteArrayOutputStream();\nDataOutputViewStreamWrapper out = new DataOutputViewStreamWrapper(baos);\ntypeInfo.createSerializer(config).serialize(original, out);\nDataInputViewStreamWrapper in = new DataInputViewStreamWrapper(\n    new ByteArrayInputStream(baos.toByteArray()));\nMyValue restored = typeInfo.createSerializer(config).deserialize(in);\nassert original.equals(restored); // verify symmetry","typeGuard":null,"tryCatchPattern":"try {\n    fn.open(readerContext);\n} catch (IOException e) {\n    if (e.getMessage().startsWith(\"Failed to deserialize an element\")) {\n        // inspect e.getCause() for the specific deserialization error\n        // check custom Value/Writable readFields() vs write() symmetry\n    }\n    throw e;\n}","preventionTips":["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."],"tags":["datagen","serialization","deserialization","custom-type","index-lookup"],"backgroundTag":null,"analyzedSha":"2f3c205e9266cb30240eb7f4fdab15cad629a70f","analyzedAt":"2026-08-14T08:48:24.518Z","schemaVersion":2},"datasetVersion":"2026-08-14T10:17:34.591Z"}