System Design Interview Master
Curated 20+ questions covering system architecture, load balancing, CAP theorem, replication, sharding, caching, and rate limiting.
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What is the difference between Vertical Scaling and Horizontal Scaling?
Vertical Scaling (scaling up) means adding more power (CPU, RAM) to an existing server machine. It is simple, has no network overhead, but has hardware limits and introduces a Single Point of Failure (SPOF). Horizontal Scaling (scaling out) means adding more machines/servers to your pool. It supports infinite scaling, provides high resilience, but requires load balancers and introduces distributed network complexity.
What is a Load Balancer, and how does it route traffic?
A Load Balancer is a device or software service that distributes incoming network traffic across a group of backend servers. This prevents any single server from becoming overloaded. Common routing algorithms include:
1. Round Robin: Sequential distribution.
2. Least Connections: Routes to the server with the fewest active sessions.
3. IP Hash: Routes based on client IP hash, ensuring a client always hits the same server (sticky sessions).
Explain the CAP Theorem.
The CAP Theorem states that a distributed system can guarantee at most two out of three characteristics simultaneously:
1. Consistency: Every read receives the most recent write or an error.
2. Availability: Every non-failing node returns a response (without guarantee of containing the latest write).
3. Partition Tolerance: The system continues to operate despite network partition drops or message delays.
Since network partitions are inevitable in distributed systems, you must choose between Consistency (CP) or Availability (AP) during a network partition.
What is a Content Delivery Network (CDN), and how does it work?
Explain Database Sharding vs Partitioning.
What is the difference between ACID and BASE properties?
What is a Message Queue (MQ), and why is it used?
Explain the difference between Monolithic and Microservices architectures.
What is DNS (Domain Name System)?
What is API Gateway, and what are its responsibilities?
Explain Eventual Consistency.
What is the Database Index, and how does it work?
Explain the difference between SQL and NoSQL databases.
What is the Circuit Breaker pattern?
Explain rate limiting algorithms.
What is a single point of failure (SPOF), and how do you eliminate it?
What is Database Replication, and what is the difference between synchronous and asynchronous replication?
Explain consistent hashing.
What is DNS Anycast?
How do you design a URL Shortening service (like Bit.ly)?
Technical Interview Mastery Blueprint
Proven methodologies and frameworks to succeed in technical and architecture rounds.
1. The Engineering STAR Technique
When responding to architectural or situational questions, structure your answer using Situation, Technical Constraint, Action, and Measurable Result. Rather than jumping straight to syntax, articulate the business problem, why certain patterns (e.g. caching, queuing, immutability) were selected over alternatives, and what latency or throughput improvements were achieved.
2. Defending Architectural Trade-Offs
Senior interviewers evaluate how you weigh trade-offs. Always explain why you chose an approach: e.g., memory overhead vs CPU cycle speed, consistency vs availability (CAP theorem), or normalized schemas vs denormalized fast reads. Stating the drawbacks of your own proposal demonstrates maturity and production experience.
3. Clean Code & Edge-Case Proactivity
In live-coding rounds, communicate your thinking aloud before typing. State edge cases explicitly: null or undefined inputs, extreme boundary values, concurrency races, and network timeouts. Writing modular, well-named functions with deterministic unit tests separates junior candidates from staff-level engineers.
4. Antipatterns to Avoid
Avoid reciting memorized definitions without practical context. Never dismiss non-functional requirements such as security (CSRF, injection, token rotation), observability (logging, metrics), and accessibility (ARIA, semantic standards). Always ask clarifying scope questions before writing complex implementations.
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