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Lessons (40)

• Frontend Testing Strategy

  What to test and how. The testing trophy, unit tests with Vitest, component tests with Testing Library, API …

• Hexagonal Architecture (Ports & Adapters)

  … Driving vs driven sides, a complete TypeScript example, testing with fake adapters, and how to structure the folders …

• Code Smells & Refactoring

  … nothing leaves, it just ends up in the right place. mermaid flowchart LR T1["✅ Tests pass"] -- S["Make …

• D — Dependency Inversion Principle

  … new SesMailer(awsConfig), }); // Tests: plain fakes, no network const sent = []; const testService = new OrderService({ orderRepository: { save: async () = {} }, mailer …

• Deploying & Operating Microservices

  … own tests and its own pipeline. mermaid flowchart LR C[💻 Commit] -- B["🔨 Build<br/ + unit tests"] -- I["🐳 Docker …

• Onion Architecture & Clean vs Hexagonal vs Onion

  … infrastructure, UI, tests Implementations of the interfaces, web framework, DB, external APIs SqlOrderRepository , controllers, EF Core / Prisma mermaid …

• Clean Architecture

  … No database, no network, milliseconds per test. js test("rejects duplicate emails", async () = { const userRepository = new InMemoryUserRepository([{ id …

• Clean Architecture in Depth

  … Entities, use cases, input and output ports, presenters, crossing boundaries with DTOs, "screaming" folder structure, testing, and common …

• MVC, MVP & MVVM

  … Keeping these three apart means you can redesign the screen without rewriting the news, and test the producer …

• YAGNI: You Aren't Gonna Need It

  … Clean, tested code is what lets you change safely later. - Do think ahead for decisions that are expensive …

• DRY: Don't Repeat Yourself

  … s]+\.[^@\s]+$/.test(email)) throw new Error("Invalid email"); // profile.js if (!/^[^@\s]+@[^@\s]+\.[^@\s]+$/.test(email)) throw …

• O — Open/Closed Principle

  … is risky Every time you open a working, tested function to add an else if , you risk breaking …

• DSA Interview Patterns Cheat Sheet

  … Interviews test whether you can recognise the pattern from the wording, then apply its template. Pick a technique …

• What Is Software Architecture?

  … in under a week Testability Can we verify it quickly? Business rules testable without a database Security Is …

• Design a File Storage Service

  … Metadata-owned ACL/capability Device sync Versioned change feed/conflict policy Object-store durability still needs recovery testing.

• Capacity, Availability, Latency and CAP

  … A 99.9% monthly target allows about 43 minutes of downtime; redundancy and tested recovery make the target …

• Creational Patterns: How to Make Objects

  … they make testing harder (tests share state) and hide dependencies. Prefer creating one instance in your composition root …

• I — Interface Segregation Principle

  … Any change to User can affect it, // and it is hard to reuse or test. function Avatar({ user …

• Debug Latency Like an Investigator

  … img/system-design-l6/latency-investigation.svg) Symptom Test first --- --- p50 and p99 rise Broad capacity, network, deploy …

• Transactions, Indexes & Concurrency

  … Restore-tested independent backups remain necessary.

• KISS: Keep It Simple, Stupid

  … C -- O[Slower onboarding] C -- T[Harder to test] B & R & O & T -- Cost["💸 Every change costs more …

• Storage Engines & Database Operations

  … Keep independent backups, define retention, encrypt them, and test restoring a realistic dataset into an isolated environment. Know …

• Layered (N-Tier) Architecture

  … So business rules end up shaped by database tables, and testing a service needs a real (or heavily …

• ACID, Transactions & Locks

  … constraints and short-lived locks. The bank-transfer test A transfer moves 100 from A to B. If …

• Choosing an Architecture

  … and driven sides Many entry points, swappable integrations, testability Ports for things that never change Onion Rings around …

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

  … ve used it: React class lifecycle methods, test frameworks ( beforeEach → test → afterEach ), NestJS lifecycle hooks. 💡 Modern alternative: pass …

• SOLID at a Glance

  Five design principles that keep object-oriented and modular code easy to extend, test and change. One picture …

• S — Single Responsibility Principle

  … since {formatMemberSince(user.createdAt)}</p </div ; } How to test for SRP Ask these questions about any class or …

• What Is Clean Code?

  … Does each function or class do one thing? Tested Can I change it and quickly know if I …

• Which Pattern Do I Need?

  … function first Singleton abuse Global state everywhere, flaky tests Dependency injection God object One class that knows and …

• Comments & Formatting

  … Code is executed and tested; comments are not. Comments that help Type Example --- --- Why (a decision) // Use insertion …

• React vs Vue vs Angular

  … everything built in (router, forms, HTTP, DI, testing), with strong conventions. Great for large enterprise teams. ![React, Vue …

• Small Functions That Do One Thing

  Short, focused functions are easy to name, test and reuse. Learn how to spot a function doing too …

• Binary Search

  … overflows on huge ints lo + (hi - lo) / 2 💡 Test with tiny arrays: [] , [1] , [1, 2] , and a target …

• Monolith vs Microservices

  … Transactions ✅ One ACID database transaction ❌ Sagas, eventual consistency Testing end to end ✅ Easy ❌ Hard Ops cost ✅ Low ❌ High …

• Docker & Containers

  … shop } volumes: [pgdata:/var/lib/postgresql/data] healthcheck: test: ["CMD-SHELL", "pg isready -U app"] interval: 5s cache …

• API Gateway, BFF & Service Discovery

  … in YAML 🚦 Traffic splitting Canary releases, A/B tests 📊 Telemetry Golden-signal metrics and traces for every call …

• Microkernel (Plugin) Architecture

  … the ecosystem Plug-ins are small, focused and testable in isolation Plug-in interactions can be hard to …

• REST API Design

  … From it you get interactive docs, client SDK generation, request validation and contract tests. yaml paths: /users/{id …

• Services & Ingress

  … pod… wget -qO- http://shop-api/health …to test from inside the cluster kubectl port-forward svc/shop …