InitialWatermarkEstimatorState fn %v has unexpected number o

Error message

InitialWatermarkEstimatorState fn %v has unexpected number of parameters: %v

What it means

The InitialWatermarkEstimatorState function of an SDF must match a supported signature; the reflection invoker only implements specific arities. If the function has an unsupported number of parameters, initCallFn returns this error during invoker construction (newCreateWatermarkEstimatorInvoker setup).

Source

Thrown at sdks/go/pkg/beam/core/runtime/exec/sdf_invokers.go:455

	default:
		switch len(n.fn.Param) {
		case 3:
			n.call = func(rest any, elms *FullValue) any {
				n.args[0] = elms.Timestamp
				n.args[1] = rest
				n.args[2] = elms.Elm
				return n.fn.Fn.Call(n.args)[0]
			}
		case 4:
			n.call = func(rest any, elms *FullValue) any {
				n.args[0] = elms.Timestamp
				n.args[1] = rest
				n.args[2] = elms.Elm
				n.args[3] = elms.Elm2
				return n.fn.Fn.Call(n.args)[0]
			}
		default:
			return errors.Errorf("InitialWatermarkEstimatorState fn %v has unexpected number of parameters: %v",
				n.fn.Fn.Name(), len(n.fn.Param))
		}
	}
	return nil
}

// Invoke calls InitialWatermarkEstimatorState given a restriction and returns an sdf.RTracker.
func (n *iwesInvoker) Invoke(rest any, elms *FullValue) any {
	return n.call(rest, elms)
}

// Reset zeroes argument entries in the cached slice to allow values to be
// garbage collected after the bundle ends.
func (n *iwesInvoker) Reset() {
	for i := range n.args {
		n.args[i] = nil
	}
}

View on GitHub (pinned to 12126d8942)

Solutions

Match InitialWatermarkEstimatorState to a supported signature such as func(wstate W, ts typex.EventTime, rest R, el T) W or its smaller arities.
Remove extra parameters not part of the documented signature set.
Pass auxiliary configuration via the DoFn struct fields instead of extra function parameters.
Compare against the sdf package docs/examples for InitialWatermarkEstimatorState.

Example fix

// before
func (fn *mySdf) InitialWatermarkEstimatorState(ctx context.Context, wstate W, ts typex.EventTime) W { ... }
// after
func (fn *mySdf) InitialWatermarkEstimatorState(wstate W, ts typex.EventTime) W { ... }

Defensive patterns

Strategy: validation

Validate before calling

t := reflect.TypeOf(fn.InitialWatermarkEstimatorState)
if t.NumIn() > 4 {
    return fmt.Errorf("InitialWatermarkEstimatorState has unsupported arity %d", t.NumIn())
}

Try / catch

if err != nil {
    if strings.Contains(err.Error(), "InitialWatermarkEstimatorState fn") {
        // align the signature with a supported form
    }
    return err
}

Prevention

Copy signatures verbatim from sdf package examples.
Avoid adding context/config parameters to SDF watermark hooks.
Validate DoFn signatures in tests before deployment.

When it happens

Trigger: Registering an InitialWatermarkEstimatorState fn with a signature outside the supported forms, e.g. taking 3+ parameters or an odd combination of watermark state, timestamp, element, and restriction. The supported call paths pass at most (state, ts, elms...) and the default case errors at sdf_invokers.go:455.

Common situations: Developers adding an extra context or config parameter to InitialWatermarkEstimatorState, or mixing up the ordering so the reflected signature no longer matches any generated case.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/34acbf3bdd603db1. Report an issue: GitHub.