Design WhatsApp Chat — system design case study

A worked system design answer for WhatsApp Chat, timed at about 45 minutes — the length of the design portion of a real round. Reported asked at Meta, WhatsApp.

It walks the same sequence an interviewer expects: clarify the requirements, size the load, settle the API, choose the storage and the caching, then defend the trade-offs. It builds on the Networking & APIs, Databases, Queues & Async families and belongs to Networking & APIs.

Lessons alongside this case study

• Protocols — Core lesson.
• API Design — Core lesson.
• gRPC Deep Dive — Depth lesson.
• Advanced API Patterns — Depth lesson.
• Design WhatsApp Chat — Core lesson, worked case study, about 60 minutes, asked at Meta, WhatsApp.

Other worked case studies

• Design URL Shortener — about 45 minutes. Builds on Databases, Caching, Scalability.
• Design Rate Limiter — about 40 minutes. Builds on Caching, Scalability.
• Design Twitter Feed — about 45 minutes. Builds on Databases, Caching, Queues & Async.
• Design YouTube — about 45 minutes. Builds on Databases, Caching, Scalability.
• Design Uber — about 45 minutes. Builds on Search, Geo & Aggregation, Queues & Async, Databases.
• Design Ticketmaster — about 45 minutes. Builds on Coordination & Transactions, Databases, Caching.
• Design Payment System — about 45 minutes. Builds on Coordination & Transactions, Queues & Async, Databases.
• Design a Key-Value Store — about 45 minutes. Builds on Databases, Scalability.
• Design Dropbox — about 45 minutes. Builds on Databases, Queues & Async.
• Design a Distributed Cache — about 45 minutes. Builds on Caching, Scalability.
• Design a Webhook Delivery System — about 45 minutes. Builds on Queues & Async, Networking & APIs.
• Design a Notification System — about 45 minutes. Builds on Queues & Async, Scalability.
• Design URL Shortener at Massive Scale — about 45 minutes. Builds on Scalability, Search, Geo & Aggregation.
• Design Ticketmaster at Scale — about 45 minutes. Builds on Coordination & Transactions, Scalability.
• Design an Ad Click Aggregator — about 45 minutes. Builds on Search, Geo & Aggregation, Queues & Async.
• Design Uber Deep Dive — about 45 minutes. Builds on Search, Geo & Aggregation, Scalability.
• Design a Web Crawler — about 60 minutes. Builds on Queues & Async.
• Design a Job Scheduler — about 60 minutes. Builds on Queues & Async.
• Design Instagram — about 60 minutes. Builds on Scalability.
• Design TikTok Feed — about 60 minutes. Builds on Scalability.
• Design Google Docs — about 60 minutes. Builds on Coordination & Transactions.
• Design a Stock Exchange — about 60 minutes. Builds on Coordination & Transactions.
• Design a Top-K Leaderboard — about 60 minutes. Builds on Search, Geo & Aggregation.
• Design Google Maps — about 60 minutes. Builds on Search, Geo & Aggregation.
• Design a Recommendation System — about 60 minutes. Builds on AI Infrastructure.

The system design pattern families

• Foundations — The 45-minute interview framework, estimation with modern numbers, and CAP theorem. 6 lessons · about 2 hours · Interview Soon · 1 worked case study.
• Databases — SQL vs NoSQL decision tree, replication, sharding, and indexing strategies. 7 lessons · about 3 hours · Interview Soon · 2 worked case studies.
• Caching — Cache-aside, write-through, failure modes, and when NOT to cache. 5 lessons · about 1.5 hours · Interview Soon · 2 worked case studies.
• Queues & Async — Kafka vs SQS, delivery guarantees, idempotent consumers, and event-driven patterns. 10 lessons · about 1.5 hours · Interview Soon · 5 worked case studies.
• Scalability — Consistent hashing, read vs write scaling, fan-out strategies, and circuit breakers. 7 lessons · about 1.5 hours · Interview Soon · 4 worked case studies.
• Coordination & Transactions — Raft consensus, distributed locking, saga pattern, and conflict resolution. 9 lessons · about 1.5 hours · Interview Soon · 5 worked case studies.
• Search, Geo & Aggregation — Full-text search, geospatial indexing, stream processing, and probabilistic structures. 9 lessons · about 1.5 hours · Interview Soon · 5 worked case studies.
• Reliability & Operations — Monitoring, observability, deployment strategies, and graceful degradation. 2 lessons · about 3 hours · Deep Dive.
• Security & Auth — OAuth 2.0, JWT vs sessions, API security, and encryption. 2 lessons · about 3 hours · Deep Dive.
• AI Infrastructure — LLM serving, inference batching, RAG pipelines, and vector search at scale. 6 lessons · about 6 hours · Deep Dive · 1 worked case study.
• Real-World Architecture Studies — Deep dives into Discord, Uber, Stripe, Netflix, and Kafka production architectures. 5 lessons · about 4 hours · Deep Dive.

Related interview preparation

• Looking for the practice sheet? — Track your progress against every family as a checklist, with company tags and AI interview practice.
• Low Level Design course — The object-level companion: OOP, SOLID, the design patterns and the machine-coding round.
• DSA patterns course — The coding round: the algorithmic patterns behind the problems asked alongside system design.
• Latency and capacity numbers — The reference table of modern latency, throughput and storage figures used in estimation.
• AI mock interviews — Practise the spoken answer against an interviewer that pushes back on the trade-offs.