apache/beam · error

invalid pcollection

Error message

invalid pcollection

What it means

TryReshuffle requires a valid input PCollection; if col is the zero PCollection (e.g. returned from a failed or skipped construction), the reshuffle has nothing to reshuffle and the call fails with 'invalid pcollection', wrapped with the Reshard scope context.

Solutions

1. Check col.IsValid() before calling Reshuffle and fix the upstream transform that produced the invalid collection.
2. Always handle the error return of transform calls — do not use the zero PCollection they return on failure.
3. Print/log the PCollection's validity (and upstream errors) where it is produced to find the broken step.

Example fix

// before
out, _ := someTransform(s, in) // error ignored, out invalid
reshuffled := beam.Reshuffle(s, out) // invalid pcollection

// after
out, err := someTransform(s, in)
if err != nil {
	return err
}
reshuffled := beam.Reshuffle(s, out)

if !col.IsValid() {
	return fmt.Errorf("cannot Reshuffle: input pcollection is invalid (check upstream transform errors)")
}
out := beam.Reshuffle(s, col)

func validPCol(c beam.PCollection) bool { return c.IsValid() }

When it happens

Trigger: beam.Reshuffle(s, col) / TryReshuffle where col == PCollection{} — usually the result of ignoring an error from a transform that returned PCollection{}, or a struct field of type PCollection that was never assigned.

Common situations: Chaining transforms where an earlier step failed and returned an empty PCollection that was then passed onward; conditional logic producing a PCollection only on one branch; typos causing the wrong (uninitialized) variable to be passed.

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

• expected CoGBK, got
• Expected single input PCollection in reshuffle:
• Expected single output PCollection in reshuffle:
• failed to expand Reshuffle transform for edge
• SDK side reshuffle not yet supported

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/86cd2d2abdcd1929. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/gbk.go:136

// A Reshuffle will force a break in the optimized pipeline. Consequently,
// this operation should be used sparingly, only after determining that the
// pipeline without reshuffling is broken in some way and performing an extra
// operation is worth the cost.
func Reshuffle(s Scope, col PCollection) PCollection {
	return Must(TryReshuffle(s, col))
}

// TryReshuffle inserts a Reshuffle into the pipeline, and returns an error if
// the pcollection's unable to be reshuffled.
func TryReshuffle(s Scope, col PCollection) (PCollection, error) {
	addContext := func(err error, s Scope) error {
		return errors.WithContextf(err, "inserting Reshard in scope %s", s)
	}
	if !s.IsValid() {
		return PCollection{}, addContext(errors.New("invalid scope"), s)
	}
	if !col.IsValid() {
		return PCollection{}, addContext(errors.New("invalid pcollection"), s)
	}
	edge, err := graph.NewReshuffle(s.real, s.scope, col.n)
	if err != nil {
		return PCollection{}, addContext(err, s)
	}
	col.n.WindowingStrategy()
	ret := PCollection{edge.Output[0].To}
	ret.SetCoder(NewCoder(ret.Type()))
	return ret, nil
}

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