Design for Both Batch and Stream

Kappa Architecture and Reprocessing at Scale Lambda's fatal flaw is dual maintenance. Every business logic change ships to two codepaths. Every bug exists in two places. The interviewer wants to hear you name this problem first, then present Kappa as the solution: eliminate the batch layer entirely. Everything flows through a single streaming pipeline, and reprocessing is done by replaying the event log from a historical offset. The follow-up will be: "What's the prerequisite for Kappa?" Your answer: a replayable, ordered event log with sufficient retention. In practice, this means Kafka with retention set to the full reprocessing window - often 30-90 days, sometimes longer. If you can't retain the raw events long enough to replay, Kappa is off the table. Mention tiered storage as the enab