apache/flink · error · FlinkRuntimeException

Failed to open the GeneratorFunction

Error message

Failed to open the GeneratorFunction

What it means

Thrown by start() in GeneratingIteratorSourceReader when the user-provided GeneratorFunction.open(context) throws an exception. The original exception is wrapped in FlinkRuntimeException. This is the production-oriented iterator source reader that applies a GeneratorFunction to transform iterator values.

Source

Thrown at flink-connectors/flink-connector-datagen/src/main/java/org/apache/flink/connector/datagen/source/GeneratingIteratorSourceReader.java:69

    @Override
    protected O convert(E value) {
        try {
            return generatorFunction.map(value);
        } catch (Exception e) {
            String message =
                    String.format(
                            "A user-provided generator function threw an exception on this input: %s",
                            value.toString());
            throw new FlinkRuntimeException(message, e);
        }
    }

    @Override
    public void start(SourceReaderContext context) {
        try {
            generatorFunction.open(context);
        } catch (Exception e) {
            throw new FlinkRuntimeException("Failed to open the GeneratorFunction", e);
        }
    }

    @Override
    public void close() throws Exception {
        generatorFunction.close();
        super.close();
    }
}

View on GitHub (pinned to 2f3c205e92)

Solutions

  1. Inspect the wrapped cause exception in the FlinkRuntimeException for the root failure.
  2. Fix the root cause in the GeneratorFunction's open() method.
  3. Ensure external resources are available and accessible before job submission.
  4. Add proper error handling and retry logic in open() for transient failures.
Defensive patterns

Strategy: try-catch

Validate before calling

// Validate GeneratorFunction.open() works with a test context
GeneratorFunction<E, O> fn = ...;
fn.open(testContext);
fn.close();

Try / catch

try {
    sourceReader.start(context);
} catch (FlinkRuntimeException e) {
    if (e.getMessage().equals("Failed to open the GeneratorFunction")) {
        // inspect e.getCause() for the root failure
    }
    throw e;
}

Prevention

When it happens

Trigger: A custom GeneratorFunction whose open() method throws during source reader initialization. The reader calls generatorFunction.open(context) in its start() lifecycle phase.

Common situations: Generator function initializing external resources (database, network, file) in open() that fail; misconfigured SourceReaderContext; resource exhaustion or permission errors.

Related errors

AI-assisted analysis of apache/flink@2f3c205e92 (2026-08-14). Data as JSON: /api/errors/dc61ae83236215ed. Report an issue: GitHub.