Stream Processing in ProductionFlink platforms, state internals, watermarks, exactly-once — the compute layer above the broker. The Kafka alumni ladder.Deliberately compute-layer: where the Kafka cohort taught the broker, this one teaches the processing — LinkedIn's 4-trillion-event pipelines and −94% unification, Alibaba's 4B records/sec Double-11 spine, and the state/time semantics that decide whether streaming results are actually correct.Week 3: Exactly-once & streaming SQLExactly-once is a chain — transactions, deterministic identity, idempotent sinks, LSN audits — and streaming SQL is a tower with three known leaks: hot keys, destroyed watermarks, retractions.

Figure 1 — the exactly-once chain and the SQL tower: guarantees are layered (transactions → deterministic IDs → idempotent sinks → audits), and SQL abstractions leak in three known places. By the end of this week you can say which layer catches which failure — and which trap your SQL job just hit

These events are money. Every impression your pipeline counts becomes a line on an advertiser's invoice; every click feeds a budget pacer deciding what to spend next minute. Lose events and you lose revenue. Count one twice and you overcharge a customer — and the failure modes stack: the job restarts mid-window, the message bus redelivers, a whole region fails over and replays. Your CEO asks the question in four words: *"can we double-bill?"* "The framework has exactly-once" is not an answer — exactly-once *where*, against *which* failure? What would a real answer look like?After this week you canThe full week 3 brief is part of LeetData Pro.Case studies

Uber — Real-Time Exactly-Once Ad Event Processing with Apache Flink, Kafka, and Pinot

Grab — The complete stream processing journey on FlinkSQL.

Netflix — Streaming SQL in Data Mesh

Confluent — How ksqlDB Works : Internal Architecture and Advanced Features

Tangbao — Flink SQL Development Experience Sharing

Guidewire — How We Cut Operating Costs by 80% While Ensuring Data Integrity at Scale

Exercises

Exactly-Once & Streaming SQL — Week 3 Check

The Replay That Double-Billed

Exactly-Once, Except When It Isn't

Duplicates Downstream

Retries Without Duplicates

Where Did the Messages Go?

The Poison-Pill Message