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Lessons (20)

• Design an E-Commerce Checkout

  … inventory reservation, idempotent payment, order creation and durable events. ![Checkout: catalog/search are read paths while inventory, payment …

• The L6 System-Design Thinking Loop

  … twice. Peak Average traffic hides capacity limits. 2M checkouts in five minutes. Failure window Finds duplicate/lost-work …

• Frontend Testing Strategy

  … sign up, log in, checkout, payment. ts // e2e/checkout.spec.ts import { test, expect } from "@playwright/test"; test …

• Batch vs Stream and Data Modeling

  … Batch jobs need isolation so reporting cannot starve checkout. A denormalized projection needs an owner and a measurable …

• Resilience Patterns

  … A slow recommendations service can then exhaust only its pool, while checkout keeps working. mermaid flowchart LR subgraph …

• Timeouts, Retries & Backpressure

  … of retaining unlimited memory. - Shed optional work before checkout. - Route poison jobs to a visible DLQ. Watch: queue …

• Exactly-Once Payment Processing

  … deduplication. The invariant For one customer, amount, and checkout intent: charge at most once . A retry returns the …

• Comments & Formatting

  … separate ideas js // ❌ A wall of text function checkout(cart){const total=sum(cart);if(total 100){applyDiscount …

• Small Functions That Do One Thing

  … High level and low level mixed together function checkout(cart) { applyDiscounts(cart); let total = 0; for (const item …

• Topics, Partitions & Offsets

  … A Kafka topic is the store; its partitions are the checkout lanes. ![A topic split into partitions; the …

• Services & Ingress

  … gone after restart" P1[pod] end subgraph With["✅ Through a Service"] C2[checkout] -- "http://shop-api" S{{"Service …

• Onion Architecture & Clean vs Hexagonal vs Onion

  … Application services Use cases / workflows orchestrating the domain CheckoutService.placeOrder() 🟣 Outer ring: infrastructure, UI, tests Implementations of …

• Structural Patterns: How to Connect Objects

  … inventory, payment, shipping, email… class CheckoutFacade { constructor({ inventory, payments, shipping, mailer }) { Object.assign(this, { inventory, payments, shipping, mailer …

• I — Interface Segregation Principle

  … OrderReader) {} } // can't delete by accident class CheckoutService { constructor(private orders: OrderReader & OrderWriter) {} } Key takeaways - Many small, focused …

• Behavioral Patterns I: Observer, Strategy, Command, State

  … standard; return strategy(order); } mermaid flowchart LR Ctx["Checkout<br/ (context)"] -- S{{"strategy(order)"}} S -- A[standard: 5 …

• ConfigMaps, Secrets & Storage

  … LOG LEVEL: "info" FEATURE NEW CHECKOUT: "true" PAYMENTS URL: "http://payments-api" app-settings.json: { "currency": "EUR", "pageSize …

• API Gateway, BFF & Service Discovery

  … O2[orders instance 2] -- register + heartbeat Registry C[Checkout svc] -- "where is orders?" Registry Registry -- "10.0.1 …

• Deploying & Operating Microservices

  … without redeploying. js if (await flags.isEnabled("new-checkout", { userId: user.id, country: user.country })) { return renderNewCheckout(); } return …

• Observability: Logs, Metrics & Traces

  … GET /checkout (total 820 ms) dateFormat x axisFormat %L section api-gateway gateway :0, 820 section orders-svc …

• Scaling & Health Probes

  … Autoscaling The big idea A supermarket opens more checkout lanes when the queues get long, and closes some …