unexpected number of parameters in method

Error message

unexpected number of parameters in method %v. got: %v, want number in range: 0 to 1. Check that the signature conforms to the expected signature for %v.

What it means

The CreateWatermarkEstimator method accepts 0 or 1 parameters (at most one, e.g. an initial estimator state). Beam rejects the DoFn when the method declares more than one parameter, telling you to match the expected signature.

Solutions

Reduce CreateWatermarkEstimator to at most one parameter: either no parameters or exactly the initial state type InitialWatermarkEstimatorState() returns.
Move any extra configuration into the DoFn's fields rather than method parameters.
If you want a stateful estimator, also define InitialWatermarkEstimatorState and WatermarkEstimatorState methods consistently.

Example fix

// before
func (fn *f) CreateWatermarkEstimator(state StateT, cfg Cfg) *myEstimator {...}
// after
func (fn *f) CreateWatermarkEstimator(state StateT) *myEstimator {...}

Defensive patterns

Strategy: validation

Validate before calling

func wmCreatorArity(fn interface{}) error {
    m, ok := reflect.TypeOf(fn).MethodByName("CreateWatermarkEstimator")
    if ok && m.Type.NumIn() > 2 { // receiver + params
        return fmt.Errorf("CreateWatermarkEstimator has %d params, want 0 or 1", m.Type.NumIn()-1)
    }
    return nil
}

Prevention

CreateWatermarkEstimator takes either nothing or exactly the initial state; inject config via struct fields.
Copy signatures from the sdf package docs/examples rather than writing them freehand.

When it happens

Trigger: Defining CreateWatermarkEstimator with two or more parameters, e.g. CreateWatermarkEstimator(state StateT, config ConfigT) int64.

Common situations: Passing extra context/config arguments by mistake; misunderstanding which arguments Beam injects into creator methods.

Related errors

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/efdf8c9ba115240a. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/graph/fn.go:1183

			processElementName, pos)
		return false, errors.SetTopLevelMsgf(err, "Method %v has an sdf.WatermarkEstimator parameter at index %v, "+
			"but is not part of a watermark estimating DoFn. sdf.WatermarkEstimator is invalid in %v in "+
			"non-watermark estimating DoFns.",
			processElementName, pos, processElementName)
	}

	return isWatermarkEstimating, nil
}

// validateWatermarkSig validates that all watermark related functions are valid
func validateWatermarkSig(fn *Fn, numMainIn int) error {
	returnNum := 1 // TODO(BEAM-3301): Enable optional error params in SDF methods.

	watermarkEstimatorT := reflect.TypeOf((*sdf.WatermarkEstimator)(nil)).Elem()
	method := fn.methods[createWatermarkEstimatorName]

	if len(method.Param) > 1 {
		err := errors.Errorf("unexpected number of params in method %v. got: %v, want number in range: 0 to 1",
			createWatermarkEstimatorName, len(method.Param))
		return errors.SetTopLevelMsgf(err, "unexpected number of parameters in method %v. "+
			"got: %v, want number in range: 0 to 1. Check that the signature conforms to the expected signature for %v.",
			createWatermarkEstimatorName, len(method.Param), createWatermarkEstimatorName)
	} else if len(method.Param) == 1 {
		err := validateStatefulWatermarkSig(fn, numMainIn)
		if err != nil {
			return err
		}
	} else {
		if _, ok := fn.methods[initialWatermarkEstimatorStateName]; ok {
			err := errors.Errorf("stateful watermark estimation method %v is present, "+
				"but CreateWatermarkEstimator doesn't take in a state parameter.", initialWatermarkEstimatorStateName)
			return err
		}
		if _, ok := fn.methods[watermarkEstimatorStateName]; ok {
			err := errors.Errorf("stateful watermark estimation method %v is present, "+
				"but CreateWatermarkEstimator doesn't take in a state parameter.", watermarkEstimatorStateName)

View on GitHub (pinned to 12126d8942)