🔧 Low-Level Design & Machine Coding

Many companies (Flipkart, Swiggy, Uber, Atlassian, PhonePe, Razorpay, Amazon) have an LLD / machine coding round: 60–120 min to build working, extensible, tested code.

Foundations

• OOP: encapsulation, abstraction, polymorphism, composition over inheritance
• SOLID, DRY, KISS, YAGNI, Law of Demeter
• UML class and sequence diagrams (just enough)
• Design patterns you actually use:
  • Creational: Factory, Abstract Factory, Builder, Singleton (and its problems)
  • Structural: Adapter, Decorator, Facade, Proxy, Composite
  • Behavioral: Strategy, Observer, State, Command, Chain of Responsibility, Template Method, Iterator
• Concurrency in LLD: thread-safe classes, locks per entity

Machine-coding checklist (per round)

1. Clarify requirements (write them down), agree on scope
2. Identify entities → relationships → class diagram (5 min)
3. Define interfaces first (strategies, repositories)
4. Implement the core flow end to end → then extensions
5. In-memory storage behind a repository interface
6. Handle edge cases + concurrency
7. Write a driver/main + a few unit tests
8. Explain the extensibility points

Problem list

• Parking lot · [ ] Splitwise · [ ] Snake & Ladder · [ ] Elevator system · [ ] Tic-tac-toe / chess
• LRU / LFU cache · [ ] Rate limiter · [ ] In-memory KV store with transactions · [ ] Logger framework
• Pub-sub / message queue · [ ] Task scheduler · [ ] Vending machine (State pattern)
• BookMyShow · [ ] Library management · [ ] ATM · [ ] Hotel booking · [ ] Cab booking
• Inventory management · [ ] Payment wallet · [ ] Notification service · [ ] Cricbuzz/scoreboard
• File system (Composite) · [ ] Meeting room scheduler · [ ] Distributed ID generator

🧪 Labs (🟢 warm-up → 🟡 core → 🔴 hard → ⚫ boss)

• 🟡 2 timed machine-coding rounds per week in Phases 1–2, then 1 per week
• 🔴 Orbit LLD set: workflow engine (State + Command + Strategy), LLM gateway middleware (Decorator + Chain), tool registry (Registry + Adapter), rate limiter (Strategy), prompt template engine (Interpreter)
• ⚫ Implement one LLD problem in both Java and Go; compare the designs

🧠 Cognitive tasks

• Self-review after 7 days: find 3 smells in your own LLD solution
• Extensibility test: add a new requirement to a finished solution in 15 min; did your design survive?

🛰️ Orbit integration

• Class models for Workflow, Step, Run, StepExecutor, RetryPolicy, StepState

Go deeper

🧩 GoF Patterns (Java & Go) · Anti-Patterns & Code Smells

Resources

• Head First Design Patterns 2e ⭐ · refactoring.guru (patterns + refactoring, free) ⭐
• Refactoring 2e (Fowler) · Clean Code (take it with a grain of salt) · A Philosophy of Software Design ⭐
• GitHub “awesome-low-level-design” · workat.tech machine coding problems