apache/beam · error

emit parameters expected in method

Error message

emit parameters expected in method %v

What it means

validateEmits checks that if ProcessElement declares emit parameters, every other emitting method (StartBundle, FinishBundle) declares the same ones — and vice versa. When the method has no emits but ProcessElement does, this error fires with a message explaining emits must be consistent across methods.

Solutions

1. Add the same emit func parameter(s) to StartBundle/FinishBundle that ProcessElement declares
2. Or, if the method should not emit, remove mismatched expectations and keep emission in ProcessElement only
3. Keep emit signatures identical (type and order) across all emitting methods

Example fix

// before
func (f *MyFn) StartBundle(ctx context.Context) error { f.emitLater() ... }
func (f *MyFn) ProcessElement(s string, emit func(int)) error { ... }
// after
func (f *MyFn) StartBundle(ctx context.Context, emit func(int)) error { ... }

// if ProcessElement has emit funcs, StartBundle/FinishBundle must declare identical ones
pe, _ := reflect.TypeOf(fn).MethodByName("ProcessElement")
sb, hasSb := reflect.TypeOf(fn).MethodByName("StartBundle")
if hasSb && emitCount(pe.Type) > 0 && emitCount(sb.Type) == 0 {
  return errors.New("StartBundle must declare the same emit parameters as ProcessElement")
}

When it happens

Trigger: Declaring ProcessElement(s string, emit func(int)) but StartBundle/FinishBundle without the matching emit func parameter, or declaring a method expected to emit with zero emit parameters while ProcessElement has them.

Common situations: Emitting from StartBundle/FinishBundle (e.g. flushing state) but forgetting to add the emit parameter to those methods; refactoring ProcessElement to add an emit without updating StartBundle.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• Duplicate state key used by and . Ensure that state keys…
• emit parameter in method
• error writing state
• failed to find method
• invalid DoFn

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/820626396dc43e1a. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/graph/fn.go:704

	}
	return nil
}

// validateEmits compares the emits found in a DoFn method signature with the emits found in
// the signature for ProcessElement, and performs validation that those match. This function
// should only be used to validate methods that are expected to have the same emit parameters as
// ProcessElement.
func validateEmits(processFnEmits []funcx.FnParam, method *funcx.Fn, methodName string) error {
	posMethodEmits, numMethodEmits, ok := method.Emits()
	numProcessEmits := len(processFnEmits)

	// Handle cases where method has no emits.
	if !ok {
		if numProcessEmits == 0 { // We're good, expected no emits.
			return nil
		}
		// Error, missing emits.
		err := errors.Errorf("emit parameters expected in method %v", methodName)
		return errors.SetTopLevelMsgf(err,
			"Missing emit parameters in the %v method of a DoFn. "+
				"If emit parameters are present in %v those parameters must also be present in %v.",
			methodName, processElementName, methodName)
	}

	// Error if number of emits doesn't match.
	if numMethodEmits != numProcessEmits {
		err := errors.Errorf("number of emits in method %v does not match method %v: got %d, expected %d",
			methodName, processElementName, numMethodEmits, numProcessEmits)
		return errors.SetTopLevelMsgf(err,
			"Incorrect number of emit parameters in the %v method of a DoFn. "+
				"The emit parameters should match those of the %v method.",
			methodName, processElementName)
	}

	// Error if there's a type mismatch.
	methodEmits := method.Param[posMethodEmits : posMethodEmits+numMethodEmits]

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