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All System Design Interview Questions

Practice the complete collection of System Design interview questions including theory, coding, MCQs and real interview problems.

Total Questions: 169Topics: 14Complete interview preparation

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All System Design Interview Questions (169)

Scroll through every important System Design question asked in real interviews. Includes MCQs, subjective questions, and coding prompts.

1. Which statement best describes the Single Responsibility Principle?

Difficulty: EasyType: MCQTopic: Foundations

A class should do exactly one thing or have one reason to change
A class should expose only one public method
A class must depend only on concrete classes
A class should be final and not extendable

SRP reduces ripple effects. When a class owns one responsibility, changes are localized and easier to test. This improves cohesion and long-term maintainability. In system design, the same idea guides service boundaries: one clear capability per service keeps coupling low and evolution safer.

Correct Answer: A class should do exactly one thing or have one reason to change

2. Open–Closed Principle encourages what practice when adding features?

Difficulty: MediumType: MCQTopic: Foundations

Modify existing classes directly

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All System Design Interview Questions

Overview

1. Which statement best describes the Single Responsibility Principle?

2. Open–Closed Principle encourages what practice when adding features?

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3. High-Level Design (HLD) typically focuses on which artifacts?

4. Which UML diagram best captures interactions over time between components?

5. Which pairing describes a healthy design goal?

6. In a network partition, a distributed store must choose between which two properties?

7. A typical benefit of microservices over a monolith is:

8. Which pattern encapsulates an algorithm family and lets you swap it at runtime?

9. Which is a common cache invalidation approach?

10. When would you denormalize an OLTP schema and what risks come with it?

11. Outline a crisp HLD structure you’d present in a 45-minute interview.

12. How do you turn an HLD API into an LLD design the team can implement next sprint?

13. You can either keep strict consistency on writes or serve always-on reads worldwide. Walk through how you’d reason and decide.

14. What is the primary purpose of a UML Use-Case diagram in system design interviews?

15. Which UML diagram best explains request flow, calls, and timing between services?

16. In a UML Class diagram, which relationship indicates a strong whole–part lifecycle ownership?

17. When do you prefer an Activity diagram over a Sequence diagram?

18. What does a UML Component diagram emphasize in system design?

19. A UML Deployment diagram is most useful for:

20. Which case is a good fit for a UML State Machine diagram?

21. In a Class diagram, what does multiplicity 1..* mean on an association end?

22. Which statement about a UML Dependency is accurate?

23. How do you decide between a Sequence diagram and an Activity diagram when explaining a design to interviewers?

24. Explain how you would use Component and Deployment diagrams together to present a microservices architecture.

25. How do aggregation and composition guide your class design in LLD for an Order module?

26. When would you introduce a State Machine diagram for an e-commerce order, and what benefits does it add?

27. Factory Method solves which problem in object creation?

28. When do you prefer Abstract Factory over Factory Method?

29. Builder is most useful when:

30. What is a practical risk of the Singleton pattern in large systems?

31. Adapter helps you:

32. How does Facade differ from Adapter?

33. Choose Decorator when you need to:

34. A virtual Proxy is commonly used to:

35. How do Strategy and State differ in intent?

36. Observer is a good fit when:

37. Explain how the Command pattern helps implement undo and audit in a workflow engine.

38. When would you prefer Template Method over Strategy for request validation pipelines?

39. How does Composite help model catalogs, folders, or organizational charts?

40. Describe the Circuit Breaker pattern and where you would place it in a microservices call path.

41. Show how you would add read-through caching to a repository using Decorator.

42. What should you state first in an HLD discussion before drawing components?

43. In capacity planning, which is the safest starting approach?

44. What is a good default for public read APIs under high traffic?

45. Which approach helps make POST operations safe to retry?

46. Which invalidation strategy is the most common first step?

47. When should a CDN be placed in your HLD?

48. Which benefit do message queues add to write paths?

49. Which algorithm is a common, simple choice for API rate limiting?

50. Which key choice reduces hot-partition risk for sharded writes?

51. Which trio forms a solid observability base in HLD?

52. Explain your API versioning strategy for a public service and how you deprecate safely.

53. You must choose between strong and eventual consistency for a user feed. Justify your choice and mitigation.

54. Describe a simple high availability plan for a database in your HLD.

55. How do you identify and optimize the critical request path in an HLD?

56. Your HLD must control cost while shipping fast. Outline a phased rollout plan.

57. When should you introduce a Data Transfer Object (DTO) instead of exposing domain entities directly?

58. Which is a practical benefit of immutable value objects in LLD?

59. Which class best follows Single Responsibility in a payments module?

60. How do you make a CreateOrder endpoint idempotent at the LLD level?

61. Where should you place input validation in a layered LLD?

62. What is the goal of the Repository pattern in LLD?

63. Which approach helps avoid lost updates in concurrent write scenarios?

64. At the code level, which structure is a simple base for a token bucket rate limiter?

65. Which pagination choice avoids high OFFSET cost at large page numbers?

66. Describe a clean error-handling strategy for a service layer. How do you map domain failures to API responses?

67. Outline an LLD for a Parking Lot system supporting multiple vehicle types and fee calculation.

68. Design the LLD for an Elevator system with multiple cars and scheduling.

69. Design an LLD for a Notification service supporting email and SMS with retries and templates.

70. Which practice improves thread-safety for shared caches inside a process?

71. At LLD level, what makes logs actionable for debugging a request?

72. What is the most reliable signal that a monolith should be split into microservices?

73. What is the best primary heuristic for defining a microservice boundary?

74. Why is the database-per-service pattern recommended?

75. Which trade-off best describes synchronous HTTP calls versus asynchronous messaging between services?

76. What problem does the Saga pattern address in microservices?

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