apache/beam · error

Unknown write type

Error message

Unknown write type %v

What it means

StateWriter.Write switches on the request's writeType (append/clear); any other value hits the default branch and fails. This indicates an internal enum/type mismatch when constructing a state write request in the Beam Go harness.

Solutions

1. Only use the public Append/Clear APIs rather than constructing StateWriterRequest directly.
2. Update the Beam Go SDK; if a new write type is unhandled, it is a version-skew bug between runner and SDK.
3. If in a fork, add a case for the new write type in StateWriter.Write before the default branch.
4. Check that request structs are fully initialized, not zero-valued.

Example fix

// before
req := &fnpb.StateRequest{WriteType: unknownType}
// after
w := channel.NewStateWriter(key, window)
w.Append(ctx, value)

switch r.writeType {
case channel.Append, channel.Clear:
    // ok
default:
    return fmt.Errorf("unsupported write type %v", r.writeType)
}

When it happens

Trigger: Invoking state writing machinery with a StateRequest whose writeType was not Append or Clear — effectively only from internal/runtime code paths or a struct initialized with a zero/unknown write type.

Common situations: Beam runtime bugs after adding a new write type without updating Write, hand-built fnpb state requests in tests or forks, zero-valued request structs passed to Write.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

• failed to decode special type, unknown type
• instruction no longer processing
• invalid bundle processing state
• bad coder kind
• bad CoGBK

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/b17077de25aaeda5. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/runtime/harness/statemgr.go:582

			InstructionId: string(r.instID),
			StateKey:      r.key,
			Request: &fnpb.StateRequest_Append{
				Append: &fnpb.StateAppendRequest{
					Data: toSend,
				},
			},
		}
	case writeTypeClear:
		req = &fnpb.StateRequest{
			// ID: set by StateChannel
			InstructionId: string(r.instID),
			StateKey:      r.key,
			Request: &fnpb.StateRequest_Clear{
				Clear: &fnpb.StateClearRequest{},
			},
		}
	default:
		return 0, errors.Errorf("Unknown write type %v", r.writeType)
	}

	_, err := localChannel.Send(req)
	if err != nil {
		return 0, err
	}
	return len(toSend), nil
}

// StateChannelManager manages data channels over the State API. A fixed number of channels
// are generally used, each managing multiple logical byte streams. Thread-safe.
type StateChannelManager struct {
	ports map[string]*StateChannel
	mu    sync.Mutex
}

// Open opens a R/W StateChannel over the given port.
func (m *StateChannelManager) Open(ctx context.Context, port exec.Port) (*StateChannel, error) {

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