Interview Prep
System Design Interview Questions (2026): What Good Answers Cover
System design interviews are open-ended by design. The question is rarely what you build. It's how you navigate a problem with no single right answer. These questions cover the scenarios that come up most at mid-level and above.
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System Design Interview Questions (2026): What Good Answers Cover
How would you explain the difference between request headers and the request body?
Headers contain metadata about the request. Bodies contain the main request data. Authentication often uses headers. POST and PATCH commonly include bodies. Understanding structure improves debugging
How should autoscaling triggers be designed for queue consumers?
oldest age trigger. drain time. downstream capacity. partition limit
How should dead letter queues be used?
failed message isolation. retry before DLQ. alert and inspect. safe replay
How would you explain the business trade-off of choosing microservices to non-technical stakeholders?
team autonomy. operational cost. business outcomes. measurable success
How do you approach incident postmortems?
Clarify the problem and constraints. Use practical engineering trade-offs. Show collaboration and communication. Include validation, testing, or monitoring. Mention maintainability over time
What is the difference between Socket.IO and native WebSocket?
Native WebSocket is standard protocol. Socket.IO adds higher-level features. Socket.IO uses its own protocol. Choice depends on interoperability and features
What are common cache invalidation strategies?
TTL expiration. delete on write. versioned keys. event-driven invalidation
How do you give useful review comments without slowing delivery?
Clarify the problem and constraints. Use practical engineering trade-offs. Show collaboration and communication. Include validation, testing, or monitoring. Mention maintainability over time
What makes collaborative editing harder than basic chat?
Concurrent shared-state edits. Needs conflict resolution. OT or CRDT often used. Harder than append-only chat
What is a poison pill message, and how should systems handle it?
repeated permanent failure. quarantine or DLQ. schema validation. safe replay after fix
Common mistakes
- Designing before clarifying requirements
- Skipping scale estimates and tradeoffs
- Forcing one answer instead of weighing options
What interviewers weigh
- How you navigate an open-ended problem
- Tradeoffs on scale, storage, and caching
- How you justify your decisions