apache/beam · error

bad base type

Error message

bad base type

What it means

In the Beam Go SDK's graphx type serialization, encodeType recursively encodes reflect.Type values into protobuf form. When encoding a pointer's base (element) type fails, the inner error is wrapped with 'bad base type' and contextualized with 'encoding pointer %v'. It indicates the pointed-to type itself is not serializable by graphx.

Solutions

1. Change the pointed-to type to an encodable kind (struct, slice, primitive)
2. Wrap the map/array in a struct field, per https://github.com/apache/beam/issues/23101
3. Inspect the full wrapped error chain (base error + 'encoding pointer %v' context) to find the offending type
4. Register a custom coder for the type via beam.CustomCoder if the type must be kept

Example fix

// before
var x *[10]int
pc := beam.ParDo(s, extractFn, beam.Create(s, x))
// after
type IntArray struct { V [10]int }
pc := beam.ParDo(s, extractFn, beam.Create(s, IntArray{}))

func isEncodableType(t reflect.Type) bool {
	switch t.Kind() {
	case reflect.Ptr:
		return isEncodableType(t.Elem())
	case reflect.Map, reflect.Array:
		return false
	case reflect.Slice, reflect.Struct:
		if t.Kind() == reflect.Struct {
			for i := 0; i < t.NumField(); i++ {
				if !isEncodableType(t.Field(i).Type) { return false }
			}
		}
		return true
	default:
		return graphx.TryEncodeSpecial(t) != 0 || isKnownKind(t.Kind())
	}
}

func isEncodable(t reflect.Type) bool { return isEncodableType(t) }

When it happens

Trigger: Pipelines whose graph contains a pointer type (e.g. *T as an input/output or nested inside another type) where T itself fails encodeType — for example *map[string]int or *[10]int, since Map and Array are unencodable at top level.

Common situations: Users declare DoFn parameters or PCollection elements as pointers to inherently unencodable types (maps, arrays) instead of wrapping them in a struct; surfaces during pipeline submission/marshalling of the model graph.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• bad channel direction
• bad element type
• bad field type
• bad parameter type
• bad return type

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/5448c8548fa1fb1c. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/runtime/graphx/serialize.go:502

		return &v1pb.Type{Kind: v1pb.Type_FUNC, ParameterTypes: in, ReturnTypes: out, IsVariadic: t.IsVariadic()}, nil

	case reflect.Chan:
		elm, err := encodeType(t.Elem())
		if err != nil {
			wrapped := errors.Wrap(err, "bad element type")
			return nil, errors.WithContextf(wrapped, "encoding channel %v", t)
		}
		dir, err := encodeChanDir(t.ChanDir())
		if err != nil {
			wrapped := errors.Wrap(err, "bad channel direction")
			return nil, errors.WithContextf(wrapped, "encoding channel %v", t)
		}
		return &v1pb.Type{Kind: v1pb.Type_CHAN, Element: elm, ChanDir: dir}, nil

	case reflect.Ptr:
		elm, err := encodeType(t.Elem())
		if err != nil {
			wrapped := errors.Wrap(err, "bad base type")
			return nil, errors.WithContextf(wrapped, "encoding pointer %v", t)
		}
		return &v1pb.Type{Kind: v1pb.Type_PTR, Element: elm}, nil

	case reflect.Map, reflect.Array:
		return nil, errors.Errorf("unencodable type '%v', try to wrap the type as a field in a struct, see https://github.com/apache/beam/issues/23101 for details", t.Kind())

	default:
		return nil, errors.Errorf("unencodable type '%v'", t.Kind())
	}
}

func tryEncodeSpecial(t reflect.Type) (v1pb.Type_Special, bool) {
	switch t {
	case reflectx.Error:
		return v1pb.Type_ERROR, true
	case reflectx.Context:
		return v1pb.Type_CONTEXT, true

View on GitHub (pinned to 12126d8942)