10. System design for a full stack .NET developer

You are not expected to design Netflix. Expect: "Design an e-commerce order system / a URL shortener / a notification service." Use this structure:

1. Clarify requirements — functional (place order, track status) and non-functional (users, requests/sec, latency, availability, consistency).
2. Estimate — e.g. 10k orders/day ≈ 0.1/sec average, peaks ×20; data ~5 GB/year → one SQL database is fine.
3. API design — POST /orders, GET /orders/{id}, idempotency key on create.
4. Data model — Orders, OrderLines, Products, Customers; indexes for the main queries.
5. High-level design — SPA → CDN; API behind a load balancer (stateless, scale out); SQL primary + read replica; Redis cache; Service Bus queue for emails/invoices.
6. Deep dive — one hard part: payment + stock consistency (outbox pattern, idempotency), or caching strategy.
7. Reliability & ops — retries with backoff (Polly / Microsoft.Extensions.Http.Resilience), health checks, logging, monitoring, alerts.
8. Trade-offs — say what you would do differently at 100× scale.

Browser ─ CDN (SPA files)
   │
   └─ HTTPS ─> Load balancer ─> API (N instances) ─┬─> SQL Server (primary + read replica)
                                                  ├─> Redis cache
                                                  └─> Service Bus ─> Worker (emails, invoices)