apache/beam · error

StepConfig.OutputPerInput cannot be negative. Got

Error message

StepConfig.OutputPerInput cannot be negative. Got: %v

What it means

StepConfigBuilder.Build() panics when OutputPerInput is negative. This field controls how many output records each input produces in the synthetic step; only non-negative counts are valid. It is a fail-fast validation during pipeline graph construction.

Solutions

1. Set OutputPerInput to 0 or a positive value before Build().
2. Clamp computed values: if n < 0 { n = 0 }.
3. Replace -1 sentinels with explicit 0 or use an optional pointer field in your config layer.

Example fix

// before
b.OutputPerInput(-1) // meant 'no extra outputs'
// after
b.OutputPerInput(0)

if cfg.OutputPerInput < 0 {
    return fmt.Errorf("OutputPerInput must be >= 0, got %d", cfg.OutputPerInput)
}

When it happens

Trigger: Calling StepConfigBuilder.Build() after OutputPerInput(-1) or any negative value, typically from a bad computed value or a negative number in a JSON/env-driven config.

Common situations: Subtracting to compute a delta that goes negative; misconfigured synthetic load-testing pipelines; copying a config template and hand-editing a value to -1 meaning 'unset'.

Understand the failure class

Background: "Invalid value" and "allowed values are" config errors: what your library rejected and how to fix it — this error's family across 41 libraries.

Related errors

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• StepConfig.InitialSplits must be >= 1. Got
• AfterProcessingTime trigger set without a delay or…
• At least one subtrigger required for composite triggers.
• attempted to add namespace to missing coder id

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/8e3c05800166b3a2. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/io/synthetic/step.go:247

// N, then N initial restrictions will be created, each containing 1 element.
//
// Valid values are in the range of [1, ...] and the default value is 1. Values
// of 0 (and below) are invalid as they would result in dropping elements that
// are expected to be emitted.
func (b *StepConfigBuilder) InitialSplits(val int) *StepConfigBuilder {
	b.cfg.InitialSplits = val
	return b
}

// Build constructs the StepConfig initialized by this builder. It also performs
// error checking on the fields, and panics if any have been set to invalid
// values.
func (b *StepConfigBuilder) Build() StepConfig {
	if b.cfg.InitialSplits <= 0 {
		panic(fmt.Sprintf("StepConfig.InitialSplits must be >= 1. Got: %v", b.cfg.InitialSplits))
	}
	if b.cfg.OutputPerInput < 0 {
		panic(fmt.Sprintf("StepConfig.OutputPerInput cannot be negative. Got: %v", b.cfg.OutputPerInput))
	}
	return b.cfg
}

// StepConfig is a struct containing all the configuration options for a
// synthetic step. It should be created via a StepConfigBuilder, not by directly
// initializing it (the fields are public to allow encoding).
type StepConfig struct {
	OutputPerInput int
	FilterRatio    float64
	Splittable     bool
	InitialSplits  int
}

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