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Lessons (40)

• Service Communication: Sync vs Async

  … If they don't pick up, you're stuck. - Email (asynchronous): you send a message and carry on …

• Backend Languages: Node.js, Go, Python & Rust

  … Express js import express from "express"; const app = express(); app.get("/users/:id", async (req, res) = { const user …

• Hexagonal Architecture (Ports & Adapters)

  … Loan[] = []; async save(loan: Loan) { this.loans.push(loan); } async countActiveFor(memberId: string) { return this.loans.filter((l …

• Design a URL Shortener

  … durable mappings, cache-backed redirects and asynchronous analytics](/img/system-design-l6/url-shortener.svg) Generate a Base62 …

• D — Dependency Inversion Principle

  … A higher-order function works just as well: js const makePlaceOrder = ({ saveOrder, sendEmail }) = async (order) = { await saveOrder(order …

• Clean Architecture

  … null; } async save(user) { / INSERT … RETURNING id / } } // adapters/UserController.js export const userController = (registerUser) = async (req, res) = { const …

• Frontend Testing Strategy

  … 201 })), ]; tsx it("lists products from the API", async () = { render(<ProductsPage / ); expect(await screen.findByText("Keyboard")).toBeInTheDocument(); // findBy …

• Clean Architecture in Depth

  … IdGenerator, ) {} async execute(request: PlaceOrderRequest): Promise<PlaceOrderResponse { const order = Order.create(this.ids.next(), request.customerId, request.lines …

• How Browsers Render a Page

  … Use defer or async . - Changing geometry triggers expensive layout; transform and opacity are cheap. - Batch DOM reads and …

• Next.js Rendering: CSR, SSR, SSG, ISR & Server Components

  … the default when a page uses no request-specific data export default async function Page() { / … / } // ISR: regenerate at …

• MVC, MVP & MVVM

  … routes/products.js (Express) router.get("/products/:id", async (req, res) = { const product = await Product.findById(req.params …

• CQRS & Event Sourcing

  … rules and state changes async function changeShippingAddress({ orderId, address }) { const order = await orderRepository.load(orderId); order.changeAddress(address …

• Small Functions That Do One Thing

  … one function doing everything js async function registerUser(req) { // validate if (!req.body.email !req.body.email.includes …

• Structural Patterns: How to Connect Objects

  … each wraps a function and returns a function with the same shape const withLogging = (fn, name) = async (...args …

• Layered (N-Tier) Architecture

  … OrderService) {} async create(req: Request, res: Response) { const order = await this.orders.placeOrder(req.body.customerId, req.body …

• Message Queues & RabbitMQ

  … true }); channel.prefetch(10); // at most 10 unacknowledged messages per worker channel.consume("emails", async (msg) = { const job …

• Serverless Architecture

  … create a thumbnail export const handler = async (event: S3Event) = { for (const record of event.Records) { const image = await …

• Premiere-Night Video Streaming

  … Heartbeat progress to Redis for fast resume and persist asynchronously to durable storage. Include a timestamp/sequence so …

• Clean Error Handling

  … js // routes just throw app.get("/users/:id", async (req, res) = { const user = await users.findById(req.params …

• Replication, Partitioning & Sharding

  … Prefer colocating related data, asynchronous aggregates and explicit workflows. Do not shard because it sounds scalable; exhaust indexing …

• JavaScript Essentials: Event Loop, Closures & this

  … Split heavy work into chunks, or move it to a Web Worker . async/await is promises underneath js …

• Delivery Guarantees: At-Most, At-Least & Exactly Once

  … remember processed event IDs (same transaction as the business change) await db.transaction(async (tx) = { const inserted = await …

• Kafka Producers

  … 5 }); await producer.connect(); export async function publishOrderPlaced(order) { await producer.send({ topic: "shop.orders.placed", acks: -1 …

• Consumers & Consumer Groups

  … orders.placed" }); await consumer.run({ autoCommit: false, eachMessage: async ({ topic, partition, message }) = { const eventId = message.headers.eventId.toString …

• Event-Driven Architecture

  … Handler<E ) { this.handlers.set(type, [...(this.handlers.get(type) ?? []), handler]); } async publish(event: { type: string }) { await Promise …

• Vertical Slice Architecture & the Modular Monolith

  … simple, direct SQL export async function getOrderHistory(db: Pool, customerId: string) { const { rows } = await db.query( SELECT id …

• Caching & Redis

  … SET product:42 (expires in 300 s) end js async function getProduct(id) { const key = product:${id} ; const …

• Data in Microservices: Sagas & the Outbox Pattern

  … publish unsent outbox rows, then mark them as sent setInterval(async () = { const rows = await db.query( "SELECT FROM …

• Behavioral Patterns II: Chain, Template, Iterator, Mediator, Memento, Visitor

  … fixed order of steps async run(source) { const raw = await this.read(source); const records = this.parse(raw …

• SOLID at a Glance

  … injected abstractions this.repository = repository; this.notifier = notifier; } async publish(report, exporter) { const file = exporter.export(report); // L …

• KISS: Keep It Simple, Stupid

  … new NotificationContext(user, TemplateRegistry.get("welcome"))); js // ✅ KISS async function sendWelcomeEmail(user) { await mailer.send({ to: user.email …

• How a Web Request Works

  … Node.js (Express) endpoint app.get("/products/:id", async (req, res) = { const product = await productService.getById(req.params …

• What Is Apache Kafka?

  … topic: "orders", fromBeginning: true }); await consumer.run({ eachMessage: async ({ partition, message }) = { const order = JSON.parse(message.value.toString …

• TypeScript Essentials

  … string undefined ts // A typed API helper async function fetchJson<T (url: string): Promise<T { const response = await …

• API Gateway, BFF & Service Discovery

  … one endpoint shaped exactly for the home screen app.get("/mobile/home", async (req, res) = { const [user, orders …

• Deploying & Operating Microservices

  … in Node.js process.on("SIGTERM", () = { server.close(async () = { // stop accepting new requests, finish current ones await db …

• What Is Software Architecture?

  … system shaped? Monolith or microservices, layers, sync vs async, which database, where the business logic lives 🔴 High Design …

• REST API Design

  … return the SAVED response (no second charge ✅) js app.post("/payments", async (req, res) = { const key = req.get …

• Authentication, Authorization & API Security

  … is logged in" app.get("/orders/:id", authenticate, async (req, res) = { const order = await orders.findById(req.params …

• Observability: Logs, Metrics & Traces

  … res.statusCode })); next(); }); app.get("/metrics", async (req, res) = res.type("text/plain").send(await client.register.metrics …