apache/beam · error

Unexpected composite type

Error message

Unexpected composite type: %v

What it means

inferCoder in sdks/go/pkg/beam/coder.go panics with "Unexpected composite type: %v" when it encounters a composite (multi-component) FullType whose kind is not one of the supported composite kinds (KV, CoGBK, WindowedValue, ShardedKey, etc.). This is a defensive programmer-error panic: the Beam type system should have rejected the type earlier, so reaching the default branch means an unhandled typex kind reached coder inference. It aborts pipeline construction immediately.

Solutions

1. Identify the kind from the panic message; if it is a custom or unsupported kind, change the PCollection element type to a supported one (KV, struct, slice, etc.).
2. Align the Beam SDK version so inferCoder knows the composite kind (it was likely added in a newer release).
3. If you maintain a fork, add a case to inferCoder mapping the missing kind to the correct coder.Coder Kind.
4. Isolate which transform introduced the type and adjust its type hints.

Example fix

// before
default:
    panic(fmt.Sprintf("Unexpected composite type: %v", t))
// after
case typex.MyCustomKind:
    return &coder.Coder{Kind: coder.MyCustomKind, T: t, Components: c}, nil
default:
    panic(fmt.Sprintf("Unexpected composite type: %v", t))

if t == nil || !isSupportedKind(t.Kind()) {
    return nil, fmt.Errorf("unsupported composite type for coder inference: %v", t)
}
c, err := beam.NewCoder(t.Type())
if err != nil {
    return nil, err
}

func isSupportedKind(k typex.Kind) bool {
    switch k {
    case typex.KVKind, typex.CoGBKKind, typex.WindowedValueKind, typex.ShardedKeyKind:
        return true
    }
    return false
}

func inferCoderSafe(t typex.FullType) (c *coder.Coder, err error) {
    defer func() {
        if r := recover(); r != nil {
            err = fmt.Errorf("coder inference panicked for type %v: %v", t, r)
        }
    }()
    return beam.NewCoder(t.Type())
}

When it happens

Trigger: Calling NewElementEncoder, NewElementDecoder, NewCoder, inferCoders, or a transform like TryCombinePerKey with a composite FullType kind not covered by inferCoder's switch — e.g. a custom typex kind registered by an extension, or a kind added in a newer Beam release while running an older coder.go.

Common situations: Upgrading Beam so pipeline data flows through a composite type the installed coder.go does not handle; custom DoFn/PCollection element types using exotic typex kinds; SDK/worker version skew.

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

• inferCoder: unknown container kind
• Unexpected type
• coder must not be nil
• err
• expected WV coder for user state

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/be592d855a3be0f5. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/coder.go:271

		if err != nil {
			return nil, err
		}

		switch t.Type() {
		case typex.KVType:
			return &coder.Coder{Kind: coder.KV, T: t, Components: c}, nil
		case typex.CoGBKType:
			return &coder.Coder{Kind: coder.CoGBK, T: t, Components: c}, nil
		case typex.WindowedValueType:
			// TODO(herohde) 4/15/2018: do we ever infer W types now that PCollections
			// are non-windowed? We either need to know the windowing strategy or
			// we should remove this case.
			return &coder.Coder{Kind: coder.WindowedValue, T: t, Components: c, Window: coder.NewGlobalWindow()}, nil
		case typex.ShardedKeyType:
			return &coder.Coder{Kind: coder.ShardedKey, T: t, Components: c}, nil

		default:
			panic(fmt.Sprintf("Unexpected composite type: %v", t))
		}
	default:
		panic(fmt.Sprintf("Unexpected type: %v", t))
	}
}

func inferCoders(list []FullType) ([]*coder.Coder, error) {
	var ret []*coder.Coder
	for _, t := range list {
		c, err := inferCoder(t)
		if err != nil {
			return nil, err
		}
		ret = append(ret, c)
	}
	return ret, nil
}

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