apache/beam · error

Unable to deduce single value from " + valueSet

Error message

Unable to deduce single value from " + valueSet

What it means

onlyValueOr collected multiple candidate values in a Set and the caller-supplied comparator failed to deem them equivalent, so no single representative value could be deduced. The valueSet and the comparator are the inputs at fault; this is a generic consistency helper used during postApplyTransform to reconcile per-PCollection metadata (e.g. windowing or coders) that must agree.

Solutions

1. Inspect the printed set to see which two values disagree, then make the producing transforms consistent (same windowing/coder on all inputs).
2. Pass a comparator that treats the differing values as equivalent if the discrepancy is benign and you control the call site.
3. Report a bug if identical-looking values still fail the comparator, since structural equality may be at fault.

When it happens

Trigger: Thrown at sdks/typescript/src/apache_beam/internal/pipeline.ts:343 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/318ef096d1f3f8a7. Report an issue: GitHub.

Appendix: source

Thrown at sdks/typescript/src/apache_beam/internal/pipeline.ts:343

function objectMap(obj, func) {
  return Object.fromEntries(Object.entries(obj).map(([k, v]) => [k, func(v)]));
}

function onlyValueOr<T>(
  valueSet: Set<T>,
  defaultValue: T,
  comparator: (a: T, b: T) => boolean = (a, b) => false,
) {
  if (valueSet.size === 0) {
    return defaultValue;
  } else if (valueSet.size === 1) {
    return valueSet.values().next().value;
  } else {
    const candidate = valueSet.values().next().value;
    for (const other of valueSet) {
      if (!comparator(candidate, other)) {
        throw new Error("Unable to deduce single value from " + valueSet);
      }
    }
    return candidate;
  }
}

requireForSerialization(`${packageName}/internal/pipeline`, exports);

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