Data Practical Notes

Nov 2, 2026 · Big Data

Exactly-Once Semantics: Why 'Processed Once' Isn't That Simple in Data Pipelines

This article explains exactly-once processing semantics in data engineering, contrasting it with at-most-once and at-least-once, detailing why retries cause duplicates, the roles of checkpoints, idempotency, and transactions, why technical exactly-once doesn't guarantee business correctness, and the four critical failure tests needed before production deployment.