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System Design Interview Guide 2026

Master every system design interview question. Step-by-step design walkthroughs for real systems — URL shortener, chat apps, payment systems — plus the core concepts every senior engineer must know.

All Topics Design Problems Core Concepts Databases

How to ace a system design interview

  • Clarify requirements first — ask about scale (users, QPS, data size), read/write ratio, and latency targets before drawing anything.
  • Estimate scale — back-of-envelope calculations: storage, bandwidth, requests/sec. Shows systems thinking.
  • Start high-level — clients → load balancer → API servers → databases. Then drill into each component.
  • Identify bottlenecks — proactively mention where the design will struggle and how you'd fix it (caching, sharding, CDN).
  • Trade-offs are the answer — interviewers want to see that you understand why, not just what.
Start Here — Interview Framework
Must Read First
System Design Interview Guide 2026 — How to Approach Any Question

A framework for answering any system design question in 45 minutes. Covers: gathering requirements, capacity estimation, high-level design, API design, data model, deep dive, and trade-offs.

Requirements Gathering Capacity Estimation HLD → LLD Trade-off Discussion
Senior Read the framework
Recommended Study Path
1
Read the framework — learn the 45-min template
2
Core concepts — CAP theorem, consistent hashing, sharding
3
Classic problems — URL shortener, rate limiter, chat system
4
Advanced problems — payment system, notification system, Twitter feed
5
DB decisions — SQL vs NoSQL, API design
Classic Design Problems
Classic Problem
Design a URL Shortener (like Bitly)
~15 min read · Mid–Senior
Step-by-step design of a URL shortener handling 100M URLs and 1B redirects/day. Covers hashing strategies, database schema, caching layer, and analytics.
Base62 encoding Redis caching NoSQL vs SQL Bloom filter
Mid–Senior Read full design
Classic Problem
Design a Chat Application (like WhatsApp)
~18 min read · Senior
Real-time messaging system for 500M users. Covers WebSocket vs long polling, message storage, group chat, online presence, and end-to-end encryption.
WebSockets Message queue Fan-out Cassandra
Senior Read full design
Classic Problem
Design a Rate Limiter
~12 min read · Mid–Senior
API rate limiter used by every major platform. Covers token bucket, leaky bucket, fixed window, sliding window algorithms, and distributed rate limiting with Redis.
Token bucket Sliding window Redis scripts Distributed
Mid–Senior Read full design
Classic Problem
Design a Notification System
~14 min read · Mid–Senior
Multi-channel notification system sending push/email/SMS to 100M users. Covers fan-out strategies, priority queues, third-party integrations, and retry mechanisms.
Push notifications Kafka fan-out Priority queue Retry/DLQ
Mid–Senior Read full design
Advanced Problem
Design a Payment System
~18 min read · Senior
Fault-tolerant payment system handling millions of transactions. Covers idempotency, double-entry ledger, exactly-once semantics, reconciliation, and PCI-DSS considerations.
Idempotency Double-entry Saga pattern PCI-DSS
Senior Read full design
Classic Problem
Design Search Autocomplete
~13 min read · Mid–Senior
Google-scale search suggestion system returning results in <100ms. Covers trie data structure, prefix caching, ranking algorithms, and real-time updates from trending searches.
Trie Prefix cache Ranking Data pipeline
Mid–Senior Read full design
Advanced Problem
Design Twitter / Social Media Feed
~16 min read · Senior
Social news feed system handling celebrity users and 500M daily active users. Covers fan-out on write vs read, hybrid approach for hot users, and feed ranking.
Fan-out on write Fan-out on read Celebrity problem Feed ranking
Senior Read full design
Core Concepts Every Engineer Must Know

CAP Theorem Explained

Consistency, Availability, Partition tolerance — why you can only pick 2, with real database examples (Zookeeper, Cassandra, DynamoDB).

Read guide

Consistent Hashing Explained

How consistent hashing distributes load across servers with minimal redistribution when nodes are added or removed.

Read guide

Database Sharding — Complete Guide

Horizontal vs vertical partitioning, sharding strategies (range, hash, directory), and how to handle cross-shard queries and hotspots.

Read guide

SQL vs NoSQL — When to Use Which

Relational vs document vs columnar vs key-value databases. Decision framework for choosing the right database for your use case.

Read guide

Microservices vs Monolith

When to decompose into microservices vs stay monolithic. Migration strategies, inter-service communication, and when monolith wins.

Read guide

API Design: REST vs GraphQL vs gRPC

When to use each API paradigm. REST conventions, GraphQL for flexible queries, gRPC for internal microservice communication.

Read guide

Load Balancing Algorithms Explained

Round robin, least connections, IP hash, weighted, consistent hash — when each algorithm works best and how to configure them.

Read guide

Complete your senior interview prep