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