What users observe
In preview:
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Only one file is processed
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Only one sheet is processed
Why this is intentional
Preview mode runs locally and synchronously.
To protect:
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Browser responsiveness
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Developer productivity
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Local system resources
Sparkflows enforces guardrails:
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First file only
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First sheet only
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Row limits applied
Batch mode difference
Batch execution removes all such limits.
Key takeaway
Preview is for validation, not volume.