Design for Both Batch and Stream

Lambda Architecture in Practice Here's the reality the interviewer is testing: production data platforms almost always need both batch and streaming. Batch gives you correctness. Streaming gives you speed. The question is how you merge them without creating a maintenance nightmare. Your answer should acknowledge both sides and then explain the merge strategy. The Lambda architecture formalized this pattern: a speed layer (streaming) serves approximate results immediately, while a batch layer reprocesses everything for accuracy. A serving layer merges both views. The interviewer wants to hear you name "Lambda" explicitly and then immediately flag its biggest weakness: dual maintenance. The follow-up will be: "What happens when batch catches up?" Your answer: the speed layer results must be