apache/beam · error

unable to encode type

Error message

unable to encode type: %v

What it means

The RowEncoderBuilder cannot build a reflective encoder for the given Go type: its kind is not in the supported set for Beam schema row encoding. Same family as the decoder counterpart but on the encode path.

Solutions

1. Restrict element types to schema-encodable kinds (primitives, strings, bytes, slices, maps, arrays, structs)
2. Provide a custom coder via beam.EncodedCustomCoder or register a schema creator/type
3. Strip or transform unsupported fields before encoding

Example fix

// before
type Point struct { Mutex sync.Mutex }
// after
type Point struct { X, Y float64 }

func encodable(t reflect.Type) bool {
    switch t.Kind() {
    case reflect.Chan, reflect.Func, reflect.Interface, reflect.UnsafePointer:
        return false
    }
    return true
}

func isRowEncodable(v any) bool { return encodable(reflect.TypeOf(v)) }

if _, err := (coder.RowEncoderBuilder{}).Build(t); err != nil {
    return fmt.Errorf("type %T not row-encodable: %w", v, err)
}

When it happens

Trigger: Calling EncoderForSlice / encoderForStructReflect / containerEncoderForType on a type whose kind (e.g. chan, func, unsafe.Pointer, nested unsupported element) has no encoder case.

Common situations: A PCollection element struct contains an unsupported field type and the user relies on default schema coding.

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

• unable to decode type
• bad coder kind
• bad decode
• bad decoding function
• bad encode

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/14345e9d8900a654. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/graph/coder/row_encoder.go:260

		return typeEncoderFieldReflect{encode: iterableEncoder(t, encf)}, nil
	case reflect.Array:
		encf, err := b.containerEncoderForType(t.Elem())
		if err != nil {
			return typeEncoderFieldReflect{}, err
		}
		return typeEncoderFieldReflect{encode: iterableEncoder(t, encf)}, nil
	case reflect.Map:
		encK, err := b.containerEncoderForType(t.Key())
		if err != nil {
			return typeEncoderFieldReflect{}, err
		}
		encV, err := b.containerEncoderForType(t.Elem())
		if err != nil {
			return typeEncoderFieldReflect{}, err
		}
		return typeEncoderFieldReflect{encode: mapEncoder(t, encK, encV)}, nil
	}
	return typeEncoderFieldReflect{}, errors.Errorf("unable to encode type: %v", t)
}

func (b *RowEncoderBuilder) containerEncoderForType(t reflect.Type) (typeEncoderFieldReflect, error) {
	encf, err := b.encoderForSingleTypeReflect(t)
	if err != nil {
		return typeEncoderFieldReflect{}, err
	}
	if t.Kind() == reflect.Ptr {
		return typeEncoderFieldReflect{encode: NullableEncoder(encf.encode), addr: encf.addr}, nil
	}
	return encf, nil
}

type typeEncoderReflect struct {
	debug  []string
	fields []typeEncoderFieldReflect
}

View on GitHub (pinned to 12126d8942)