Idempotent At-Least-Once Ingest

Concepts covered: paIdempotency, paDeduplication

The general idempotent-write playbook is the subject of Lesson 5 (partition overwrite, MERGE on a business key, DELETE-then-INSERT). This section narrows the playbook to the ingestion seam, where at-least-once delivery from the source forces deduplication on a message key. Most ingestion systems offer at-least-once delivery. Kafka consumers reprocess after rebalancing. Webhook senders retry until they get a 2xx. SFTP partners re-upload the same file when their cron job retries. Pull jobs that crash mid-write rerun the same window. The world the ingestion layer lives in is not exactly-once; it is at-least-once with deduplication required at the sink. Pipelines that pretend otherwise produce duplicate rows that take weeks to find. Why At-Least-Once Is the Default The Dedupe Key An idempotent

About This Interactive Section

This section is part of the Ingestion Patterns: Intermediate lesson on DataDriven, a free data engineering interview prep platform. Each section includes explanations, worked examples, and hands-on code challenges that execute in real time. SQL queries run against a live PostgreSQL database. Python runs in a sandboxed Docker container. Data modeling problems validate against interactive schema canvases. All content is framed around what data engineering interviewers actually test at companies like Meta, Google, Amazon, Netflix, Stripe, and Databricks.

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