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Lessons (40)

• Async JavaScript: Promises, async/await and Node's Event Loop

  From callbacks to promises to async/await: running work in parallel, handling errors, timeouts and cancellation with AbortController …

• TypeScript in Practice

  … It reads your code before it runs and says "you promised this was a User , but here it …

• Advanced TypeScript for Frontend

  … Promise<ApiResult<T { const response = await fetch(url); return response.json() as Promise<ApiResult<T ; } Generics describe the …

• I — Interface Segregation Principle

  … Promise<Order ; list(filter: Filter): Promise<Order[] ; } interface OrderWriter { save(order: Order): Promise<void ; delete(id: string): Promise …

• Hexagonal Architecture (Ports & Adapters)

  … bookId: string): Promise<boolean ; markBorrowed(bookId: string): Promise<void ; } export interface LoanRepository { save(loan: Loan): Promise<void ; countActiveFor …

• JavaScript Essentials: Event Loop, Closures & this

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

• L — Liskov Substitution Principle

  … changing height also changes width](/img/solid/lsp-square.svg) Square broke a promise that Rectangle made: "setting …

• Capacity Planning, SLOs and Error Budgets

  … An SLO is the target we promise ourselves. An error budget is how much failure that promise permits …

• 500 Users Upload Documents at Once: Design the Background Job Pipeline

  … the API promises a fast answer, the database promises an honest status, the queue promises delivery at least …

• Multi-Region and Disaster Recovery

  … Do not choose it only because it sounds more available. Promise Design consequence --- --- RPO = 0 for orders Synchronous …

• Clean Architecture in Depth

  … save(order: Order): Promise<void ; } export interface PaymentGateway { charge(customerId: string, amountCents: number): Promise<{ paymentId: string } ; } export interface …

• Design a Chat Application

  … Offline push comes from the durable event, not an in-memory gateway. Define the message promise For a …

• SQL vs NoSQL Databases

  … the four promises Letter Promise Example --- --- --- A tomicity All or nothing Transfer $100: debit AND credit both happen …

• TypeScript Essentials

  … Promise<T { const response = await fetch(url); if (!response.ok) throw new Error( HTTP ${response.status} ); return response …

• ACID, Transactions & Locks

  … recovered from WAL/replication. Consistency is not a promise that every replica is instantly current; it means the …

• Event-Driven Architecture

  … E) = Promise<void ; class EventBus { private handlers = new Map<string, Handler<any [] (); on<E extends { type: string } (type …

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

  … 15+, params is a Promise export default async function ProductPage({ params }: { params: Promise<{ id: string } }) { const { id } = await …

• Time, Consistency and Distributed Transactions

  Reason about clocks, ordering, split brain, locks, CRDTs and transactions without promising impossible global certainty. The problem in …

• Real-Time, Pub/Sub, Webhooks and CDC

  … Do not promise one global order across all orders. A consumer receiving version 8 after version 9 should …

• System Design Interview Framework

  … is no stated audience, load, group size, delivery promise or retention policy. What must be true Turn the …

• Design a File Storage Service

  … Authorize download requests and choose signed-link lifetimes consistent with the revocation promise. Walk through a failure and …

• Database Relations & Foreign Keys

  … It is a database promise: an order belongs to one customer; an order contains many items; a student …

• Database Internals, Replication and Scaling

  … true Scale reads while preserving the specific consistency promise for read-after-write and protecting database resources. Practical …

• Scalable Frontend Architecture

  … boundary. ts export async function getOrder(id: string): Promise<Order { const response = await http.get( /orders/${id} ); return …

• Domain-Driven Design: Tactical Patterns

  … OrderRepository { get(id: OrderId): Promise<Order ; // loads the WHOLE aggregate save(order: Order): Promise<void ; // saves the WHOLE …

• Design a Ride-Sharing Service

  … perhaps exclude drivers whose last update is older than 30 seconds, rather than promising exact live positions. Measure …

• Design an E-Commerce Checkout

  … invariant comes first For an order, the important promise is: stock is never oversold and a customer action …

• Message Delivery: Queues, Kafka, Ordering and DLQs

  … needs idempotent writes plus transaction boundaries. Do not promise global ordering unless you can name the ordering key …

• What Is Artificial Intelligence? A Developer's Guide

  … new order + saved model → estimated delivery time A prediction is an estimate, not a promise. Changes in traffic …

• The L6 System-Design Thinking Loop

  … name the promise, show the normal path, break one dependency, then explain recovery and evidence.

• API Gateway, BFF & Service Discovery

  … async (req, res) = { const [user, orders, recs] = await Promise.all([ usersClient.get(req.user.id), ordersClient.recent(req …

• Resilience Patterns

  … Math.random() backoff; // spread clients out await new Promise((r) = setTimeout(r, jitter)); } } } Retry rules: - ✅ Retry transient errors …

• React Hooks in Practice

  … pending state, result, errors useOptimistic Show the expected result instantly, before the server confirms use Read a promise …

• Cross-Service Data: How to Join Without a Database JOIN

  … responses in memory. js const [order, user] = await Promise.all([ orders.get( /orders/${orderId} ), users.get( /users/${userId …

• JavaScript Language Fundamentals

  … math.js"; const { readFile } = await import("node:fs/promises"); // dynamic import ES modules CommonJS --- --- --- Syntax import / export require …

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

  … class CsvImporter extends DataImporter { read(path) { return fs.promises.readFile(path, "utf8"); } parse(text) { return text.split("\n …

• Node.js and Express

  … err.message, // never leak internals }); }); Express 5 (the current major version) forwards rejected promises from async handlers to …

• The Hot SKU Alert Problem

  … Define whether the product promises every price version or only the latest meaningful change. 5. Use durable idempotency …

• Python Fundamentals

  … It's promising, but most production code still assumes the GIL. Testing with pytest python test pricing.py …

• Rust: Types, Errors and Concurrency

  … trait is a set of methods a type promises to have, like an interface. rust trait Shape { fn …