System Design Patterns: From Fundamentals to Real Systems

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Lambda and Kappa

1. The Incident

The store now has both: the nightly batch computing authoritative reports, and the streaming pipeline feeding the live dashboard. Both compute revenue. On Wednesday morning, finance notices they disagree: the dashboard's Tuesday total reads 412,880**; the batch report says **413,205.

Which is right? Embarrassingly: both, according to themselves. The streaming job dropped some late events (its watermark policy, working as designed) and had a one-minute dedupe gap during Tuesday's deploy (the [stream lesson's Q3](

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