Streaming

Computing over unbounded data as it arrives: stateful operators, event time and late data, consistent snapshots, and exactly-once through crashes.

Explainers

• Design a Stream Processor with Exactly-Once — From a single consumer with a dictionary to a dataflow of operators that survives a crash without double-counting: checkpoint barriers, watermarks for late data, a keyed state backend, and a two-phase-commit sink — drawn, computed, and animated.
• Design Ad-Click Aggregation (Lambda vs Kappa) — Count a billion clicks a day two ways at once — a seconds-fresh dashboard and a billing-grade total that reconciles to the penny. Idempotent dedup, watermarks, late-event restatement, and why the replayable log lets one Kappa path replace two.