Ankit Kashyap

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Microsoft Senior Software Engineer – LLD Daily Plan

Focus: Low-Level Design (LLD) + OOPS + SOLID + Design Patterns
Target: Microsoft Redmond – Senior SDE
Duration: Dec 20 – Jan 31
Exclusions: Sundays, Dec 25, Dec 31, Jan 1
Daily Time: ~90 minutes

🎯 Microsoft LLD Expectations

  • Strong OOPS fundamentals
  • SOLID applied naturally
  • Correct use of design patterns (not pattern dumping)
  • Clean, extensible, readable code
  • Clear verbal explanation of trade-offs

PHASE 1: OOPS + SOLID + Design Thinking (FOUNDATION)

Day 1 – Dec 20

Topic: Microsoft LLD Interview Mindset

Checklist (60–75 min):

  • ☐ Understand MSFT LLD interview flow
  • ☐ Learn how to ask clarifying questions
  • ☐ Create reusable LLD template:
    • Requirements
    • APIs
    • Entities
    • Relationships
    • Core logic
  • ☐ Practice explaining a simple system aloud

Day 2 – Dec 21

Topic: OOPS Fundamentals (Applied)

Checklist (75–90 min):

  • ☐ Encapsulation (real-world example)
  • ☐ Abstraction vs Interface
  • ☐ Inheritance pitfalls
  • ☐ Polymorphism in real systems
  • ☐ Identify OOPS violations in bad code

Day 3 – Dec 22

Topic: SOLID Principles (Practical)

Checklist (75–90 min):

  • ☐ SRP with real refactor
  • ☐ OCP using interfaces
  • ☐ LSP violation example
  • ☐ ISP vs fat interfaces
  • ☐ DIP with constructor injection

Day 4 – Dec 23

Topic: Composition over Inheritance

Checklist (60–75 min):

  • ☐ Why inheritance breaks systems
  • ☐ Composition benefits
  • ☐ Strategy pattern intro
  • ☐ Design PaymentProcessor using strategies

Day 5 – Dec 24

Topic: Interfaces & Dependency Injection

Checklist (75–90 min):

  • ☐ Interface-first design
  • ☐ Manual dependency injection
  • ☐ Avoid concrete dependencies
  • ☐ Design NotificationService (Email, SMS, Push)

❌ Dec 25 – Christmas (OFF)

Day 6 – Dec 26

Topic: Error Handling & Edge Cases

Checklist (60–75 min):

  • ☐ Fail-fast vs graceful handling
  • ☐ Custom exceptions
  • ☐ Retry & timeout logic
  • ☐ Idempotency basics
  • ☐ Add error handling to NotificationService

Day 7 – Dec 27

Topic: Testability & Clean Code

Checklist (60–90 min):

  • ☐ Unit-test friendly design
  • ☐ Dependency injection for testing
  • ☐ Mock vs fake
  • ☐ Write test scenarios (no framework needed)

❌ Dec 28 – Sunday (OFF)

PHASE 2: DESIGN PATTERNS (MSFT-FAVORITE SET)

⚠️ Focus on when & why, not definitions

Day 8 – Dec 29

Pattern: Strategy + Factory

Checklist (75–90 min):

  • ☐ When to use Strategy
  • ☐ Factory vs Simple Factory
  • ☐ Implement payment method selection
  • ☐ Explain pattern choice

Day 9 – Dec 30

Pattern: Singleton (Proper & Improper)

Checklist (60–75 min):

  • ☐ Thread-safe singleton
  • ☐ When NOT to use singleton
  • ☐ Singleton alternatives
  • ☐ MSFT-style discussion

❌ Dec 31 – OFF
❌ Jan 1 – OFF

Day 10 – Jan 2

Pattern: Observer

Checklist (75–90 min):

  • ☐ Event-driven thinking
  • ☐ Loose coupling
  • ☐ Design notification subscription model
  • ☐ Trade-offs discussion

Day 11 – Jan 3

Pattern: Decorator

Checklist (60–75 min):

  • ☐ Avoid inheritance explosion
  • ☐ Add features dynamically
  • ☐ Implement logging / metrics decorator

❌ Jan 4 – Sunday (OFF)

Day 12 – Jan 5

Pattern: Command

Checklist (75–90 min):

  • ☐ Encapsulate actions
  • ☐ Undo/redo use cases
  • ☐ Apply to Job Scheduler / UI actions

Day 13 – Jan 6

Pattern: State

Checklist (75–90 min):

  • ☐ Replace conditionals
  • ☐ State transitions
  • ☐ Apply to Elevator states

PHASE 3: CORE LLD PROBLEMS (PATTERNS + SOLID)

Day 14 – Jan 7

Parking Lot – Part 1

Checklist (75–90 min):

  • ☐ Entities & enums
  • ☐ OOPS correctness
  • ☐ Identify patterns used

Day 15 – Jan 8

Parking Lot – Part 2

Checklist (75–90 min):

  • ☐ Allocation logic
  • ☐ Extensibility discussion
  • ☐ SOLID validation

Day 16 – Jan 9

LRU Cache

Checklist (75–90 min):

  • ☐ Clean API
  • ☐ Data structure choice
  • ☐ Thread-safety discussion
  • ☐ Trade-offs

❌ Jan 11 – Sunday (OFF)

Day 17 – Jan 12

Rate Limiter

Checklist (75–90 min):

  • ☐ Algorithm choice
  • ☐ Interface-based design
  • ☐ Multi-user extension

Day 18 – Jan 13

Elevator System

Checklist (75–90 min):

  • ☐ State pattern usage
  • ☐ Scheduler logic
  • ☐ Explain decision flow

PHASE 4: ADVANCED MSFT LLD

Day 19 – Jan 14

API Design (LLD Level)

Checklist (60–75 min):

  • ☐ REST contracts
  • ☐ Versioning
  • ☐ Error handling
  • ☐ Backward compatibility

Day 20 – Jan 15

Feature Flag System

Checklist (75–90 min):

  • ☐ Evaluation logic
  • ☐ Strategy / Factory usage
  • ☐ Clean APIs

Day 21 – Jan 16

Concurrency & Thread Safety

Checklist (75–90 min):

  • ☐ Locks & atomicity
  • ☐ Stateless vs stateful
  • ☐ Common race conditions

❌ Jan 18 – Sunday (OFF)

Day 22 – Jan 19

Refactoring Exercise

Checklist (75–90 min):

  • ☐ Identify violations
  • ☐ Apply SOLID
  • ☐ Explain refactor rationale

Day 23 – Jan 20

Trade-offs & Decision Defense

Checklist (60–75 min):

  • ☐ Why this design?
  • ☐ Why not alternatives?
  • ☐ Simplicity vs extensibility

Day 24 – Jan 21

Full Mock LLD Interview

Checklist (90 min):

  • ☐ Requirements (10 min)
  • ☐ Design (20 min)
  • ☐ Coding (30 min)
  • ☐ Explanation (20 min)
  • ☐ Self-review

PHASE 5: POLISHING & CONFIDENCE

Jan 22–26

  • ☐ Revisit weak patterns
  • ☐ Explain designs without code
  • ☐ Improve clarity & confidence

❌ Jan 25 – Sunday (OFF)

Jan 27–30

  • ☐ Light revision
  • ☐ Whiteboard clarity
  • ☐ Calm explanations

🏁 Microsoft LLD Rule

Patterns support design — they are not the design.
Readable > Clever
Explainable > Perfect

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