System Design Interview Questions

System Design Interview Questions

A curated set of representative system design interview prompts across difficulty levels, with notes on what interviewers are actually evaluating for each.

What is Load Balancing?

Load balancing is the practice of distributing incoming network traffic across multiple backend servers so no single server is overwhelmed, improving availabil…

What is Caching in System Design?

Caching is storing copies of frequently accessed data in a fast, temporary layer so future requests are served from it instead of the slower original source, r…

Horizontal vs Vertical Scaling

Vertical scaling (scaling up) means adding more power — CPU, RAM or disk — to a single machine, while horizontal scaling (scaling out) means adding more machin…

What is a CDN (Content Delivery Network)?

A CDN (Content Delivery Network) is a geographically distributed network of edge servers that cache and serve content from a location close to each user, reduc…

What is a Message Queue?

A message queue is a middleware component that lets a producer publish messages which sit durably in a buffer until a consumer pulls and processes them, decoup…

SQL vs NoSQL for Scale: Which Should You Choose?

SQL databases scale best when data is relational and consistency matters, typically via vertical scaling or read replicas, while NoSQL databases are built to s…

What is Rate Limiting?

Rate limiting is the practice of capping how many requests a client can make to a service within a given time window, protecting the system from overload and a…

What is Consistent Hashing?

Consistent hashing is a distribution technique that maps both data keys and servers onto the same hash ring, so that when a server is added or removed only a s…

What is an API Gateway?

An API gateway is a single entry point that sits in front of a system’s backend services, routing client requests to the right service while centrally handling…

What is Eventual Consistency?

Eventual consistency is a consistency model in which, if no new updates are made to a piece of data, all replicas will converge to the same value over time, th…

What is a Microservices Architecture?

A microservices architecture structures an application as a collection of small, independently deployable services, each owning a specific business capability…

Monolith vs Microservices: What Is the Difference?

A monolith is a single, unified codebase and deployment unit for an entire application, while microservices split that same application into many small, indepe…

What is a Reverse Proxy?

A reverse proxy is a server that sits in front of one or more backend servers and forwards client requests to them, returning the response back to the client a…

What is Idempotency in System Design?

Idempotency means that performing the same operation multiple times produces the same result as performing it once, so retries or duplicate requests do not cau…

How to Design a URL Shortener?

A URL shortener maps a long URL to a short unique code by generating a base62 identifier (via a counter, hash, or random string), storing the mapping in a key-…

Write-Through vs Write-Back Cache: What is the Difference?

Write-through caching writes to the cache and the underlying database synchronously on every write, trading extra write latency for strong consistency, while w…

What is Database Replication in System Design?

Database replication is the process of copying data from a primary database to one or more replica databases so the system gains redundancy for fault tolerance…

What is a Single Point of Failure (SPOF)?

A single point of failure (SPOF) is any component in a system whose failure alone causes the entire system, or a critical part of it, to become unavailable, an…

What is the Publish-Subscribe (Pub/Sub) Pattern?

The publish-subscribe (pub/sub) pattern is a messaging model where publishers send messages to a topic without knowing who receives them, and subscribers regis…

How Would You Design a Twitter-Style News Feed?

A Twitter-style news feed is built with a fan-out architecture that precomputes each follower’s timeline on write for most users, falls back to a fan-out-on-re…

How Would You Design Instagram?

Instagram is designed around three separable concerns — durable object storage for photos and videos behind a CDN, a metadata service backed by a sharded datab…

How Would You Design a WhatsApp-Style Chat System?

A WhatsApp-style chat system is built around persistent WebSocket connections held open by connection-gateway servers, a message service that durably stores an…

How Would You Design a Ride-Hailing System Like Uber?

A ride-hailing system like Uber is built around a geospatial indexing layer (typically a quadtree or geohash grid) that tracks nearby available drivers in near…

How Would You Design a Video Streaming Platform Like Netflix?

A Netflix-style streaming platform is built around an asynchronous transcoding pipeline that converts each uploaded video into multiple bitrate renditions and…