CreateTracker has unexpected number of parameters: %v

Error message

CreateTracker has unexpected number of parameters: %v

What it means

The CreateTracker function of an SDF must take 1 or 2 parameters (the restriction, optionally plus the element) and return an sdf.RTracker. initCallFn rejects other arities with this error during invoker initialization, as no supported call path can invoke the function.

Source

Thrown at sdks/go/pkg/beam/core/runtime/exec/sdf_invokers_arity.go:254

			r0 := fnT.Call2x1(n.args[0], n.args[1])
			return r0.(sdf.RTracker), nil
		}

	case reflectx.Func1x2:
		n.call = func() (rt sdf.RTracker, err error) {
			r0, r1 := fnT.Call1x2(n.args[0])
			return r0.(sdf.RTracker), asError(r1)
		}

	case reflectx.Func2x2:
		n.call = func() (rt sdf.RTracker, err error) {
			r0, r1 := fnT.Call2x2(n.args[0], n.args[1])
			return r0.(sdf.RTracker), asError(r1)
		}

	default:
		if len(n.fn.Param) < 1 || len(n.fn.Param) > 2 {
			return errors.Errorf("CreateTracker has unexpected number of parameters: %v", len(n.fn.Param))
		}

		n.call = func() (rt sdf.RTracker, err error) {
			ret := n.fn.Fn.Call(n.args)

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

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

	return nil
}

View on GitHub (pinned to 12126d8942)

Solutions

Give CreateTracker 1-2 parameters, e.g. func(rest R) sdf.RTracker.
Remove unsupported parameters such as context.Context.
Return a concrete *RTracker implementation (a type satisfying sdf.RTracker) — the return type is validated separately.
Verify with a small beam.TestPipeline run that the DoFn validates end to end.

Example fix

// before
func (fn *mySdf) CreateTracker(ctx context.Context, rest watermarksInterval) *myTracker { ... }
// after
func (fn *mySdf) CreateTracker(rest watermarksInterval) *myTracker { ... }

Defensive patterns

Strategy: validation

Validate before calling

t := reflect.TypeOf(fn.CreateTracker)
n := t.NumIn()
if n < 1 || n > 2 {
    return fmt.Errorf("CreateTracker must take 1-2 parameters, has %d", n)
}

Try / catch

if err != nil {
    if strings.Contains(err.Error(), "CreateTracker has unexpected number of parameters") {
        // fix arity to 1-2
    }
    return err
}

Prevention

Use func(rest R) sdf.RTracker as the canonical CreateTracker shape.
Return a type that satisfies sdf.RTracker.
Validate the whole SDF signature set in a unit test before running on a runner.

When it happens

Trigger: Registering a CreateTracker fn with 0 parameters or 3+ parameters, e.g. func() sdf.RTracker or func(rest R, el T, ts typex.EventTime) sdf.RTracker; the default validation at sdf_invokers_arity.go:254 fires.

Common situations: Defining CreateTracker with no restriction parameter after refactoring, or adding a context.Context parameter which this hook does not support.

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/eb64d0b96da20892. Report an issue: GitHub.