SplitRestriction has unexpected number of parameters: %v

Error message

SplitRestriction has unexpected number of parameters: %v

What it means

The SplitRestriction function of an SDF must take between 2 and 4 parameters (element plus optional timestamp/etc., matching CreateInitialRestriction's inputs plus the restriction). initCallFn rejects any other arity with this error. The reported 'called by' newCreateInitialRestrictionInvoker reflects that all SDF invokers share the same init wiring.

Source

Thrown at sdks/go/pkg/beam/core/runtime/exec/sdf_invokers_arity.go:138

			r0, r1 := fnT.Call2x2(n.args[0], n.args[1])
			return r0, asError(r1)
		}

	case reflectx.Func3x2:
		n.call = func() (splits any, err error) {
			r0, r1 := fnT.Call3x2(n.args[0], n.args[1], n.args[2])
			return r0, asError(r1)
		}

	case reflectx.Func4x2:
		n.call = func() (splits any, err error) {
			r0, r1 := fnT.Call4x2(n.args[0], n.args[1], n.args[2], n.args[3])
			return r0, asError(r1)
		}

	default:
		if len(n.fn.Param) < 2 || len(n.fn.Param) > 4 {
			return errors.Errorf("SplitRestriction has unexpected number of parameters: %v", len(n.fn.Param))
		}

		n.call = func() (splits any, err error) {
			ret := n.fn.Fn.Call(n.args)

			switch len(ret) {
			case 1:
				return ret[0], nil
			case 2:
				return ret[0], asError(ret[1])
			}

			panic(fmt.Sprintf("SplitRestriction has unexpected number of return values: %v", len(ret)))
		}
	}

	return nil
}

View on GitHub (pinned to 12126d8942)

Solutions

Give SplitRestriction 2-4 parameters, e.g. func(el T, rest R) []R.
Always include the element (and any timestamp parameter matching CreateInitialRestriction) in the signature.
Remove extra parameters beyond the documented set.
Unit-test the SDF with beam.TestPipeline so signature errors surface before deployment.

Example fix

// before
func (fn *mySdf) SplitRestriction(rest watermarksInterval) []watermarksInterval { ... }
// after
func (fn *mySdf) SplitRestriction(el string, rest watermarksInterval) []watermarksInterval { ... }

Defensive patterns

Strategy: validation

Validate before calling

t := reflect.TypeOf(fn.SplitRestriction)
n := t.NumIn()
if n < 2 || n > 4 {
    return fmt.Errorf("SplitRestriction must take 2-4 parameters, has %d", n)
}

Try / catch

if err != nil {
    if strings.Contains(err.Error(), "SplitRestriction has unexpected number of parameters") {
        // fix arity to 2-4, element first
    }
    return err
}

Prevention

Always include the element parameter in SplitRestriction.
Mirror the parameter list of CreateInitialRestriction plus the restriction.
Run a local TestPipeline smoke test for every new SDF.

When it happens

Trigger: Registering a SplitRestriction fn with fewer than 2 parameters (e.g. only the restriction) or more than 4, so no generated fast path matches and the default case errors at sdf_invokers_arity.go:138.

Common situations: Omitting the element parameter when writing a stateless splitter, or adding extra config parameters after refactoring; also happens when parameter types don't match so the reflected signature is treated as unknown arity.

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/2a98f47edc731324. Report an issue: GitHub.