Run a Full Mock System Design Interview With a Requirement Change
A mock system design interview designs a multi-region B2B document processing API in 35 to 45 minutes with one mid-interview requirement change. Learners practice the full round out loud in this hands-on project lesson. Builds: Document processing API with unit-labeled estimates, an async queue and worker path, and an isolated EU regional stack. Applies: Requirements discovery, trust boundaries, idempotent workers, regional isolation, load and recovery testing. Practice: Apply the EU Residency Change. Prerequisites: Async processing and DR patterns.
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The interviewer says, “Design a multi-region business-to-business document processing application programming interface.” We have 35–45 minutes, an interruption for a deep dive, and one requirement change halfway through. Let’s lead with a concise restatement: customers upload documents, receive an acceptance response quickly, and retrieve processing status and results later.
Ask questions that alter architecture rather than collecting trivia. Establish customer regions, average and peak upload rates, latency expectations, document size and retention, sensitivity classification, availability target, recovery expectations, and budget constraints. Write each answer as either a carried assumption or an item to validate later.
Create a back-of-the-envelope estimate, a rough calculation that constrains a design before detailed sizing. Label the units. For example, assume an average of 20 uploads/second, a peak of 200 uploads/second, 10 MB/upload, and 30-day retention. That implies about 2 GB/second of peak incoming data (requiring ...