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System Design Architectures

Interactive technical blueprints, capacity estimates, database sharding strategies, and visual flowcharts for the top 30 system design interview questions.

Target Architectures
Architecture Blueprint

Design WhatsApp

High-concurrency instant messaging service with offline delivery queues and presence indicators.

1. System Requirements

Functional
  • Real-time one-on-one instant text messaging.
  • Delivery indicators (sent, delivered, read status).
  • Presence indicator (online, offline, last seen status).
  • Group chat support with up to 1,000 members.
Non-Functional / SLA
  • Extremely low latency message delivery (< 200ms).
  • Highly durable message delivery guarantees.
  • Scales to billions of active messaging connections.

2. Scale & Resource Estimates

Throughput: 2 Billion Total Users. 100 Billion Messages per day = 1.15 Million messages/sec average. Peak = 3 Million messages/sec.

Storage & RAM: 100B messages/day * 100 bytes = 10 Terabytes/day. Retained in chat history partitions.

3. System API Endpoints

// 1. Establish websocket connection
WS /chat/connect?user_id=usr-5012

// 2. Message Payload JSON
{
  "message_id": "msg-889342",
  "recipient_id": "usr-1102",
  "type": "text",
  "content": "Hello, see you at 5!",
  "timestamp": 1783708872
}

4. Database Schema Design

Table: messages (Cassandra Column Family - Sharded by chat_id)
- chat_id (UUID - Partition Key)
- message_id (TimeUUID - Clustering Key)
- sender_id (UUID)
- content (text)
- status (ENUM: sent, delivered, read)

Table: presence (Redis Cache - Key-Value)
- user_id (Key)
- last_active_timestamp (Integer)
- status (ENUM: online, offline)

5. Interactive High-Level Architecture

graph TD ClientA[Client A] -->|WebSocket| WebSocketA[WebSocket Manager A] ClientB[Client B] -->|WebSocket| WebSocketB[WebSocket Manager B] WebSocketA -->|Publish Message| Broker[Kafka Message Broker] Broker -->|Persist| MessageDB[(Cassandra Message Store)] Broker -->|Route| RouteService[Routing Service] RouteService -->|Check Active Location| RedisCache[(Redis Session Registry)] RouteService -->|Online| WebSocketB RouteService -->|Offline| PushService[Push Notifications APNS/FCM]

6. Scalability Deep Dive & Bottlenecks

Every active user maintains a persistent TCP connection via WebSockets. WebSocket Manager nodes keep track of active sessions in a shared Redis Session Registry. When a message is sent, the routing service checks the registry. If the user is offline, it stores the message in Cassandra and triggers mobile push notifications (FCM/APNS). Online status triggers pub/sub updates to followers' chat channels.

7. Production Scaling Strategies

WebSocket Connection Scaling: 2B users × 30% concurrent = 600M active connections. Run 60K WebSocket gateway nodes (10K connections each). Use consistent hashing on user_id for sticky routing. Gateways are stateless — session state lives in Redis.

Cassandra Partition Sizing: Partition messages by chat_id with TimeUUID clustering. Keep partitions under 100MB (~500K messages). For group chats, sub-partition by time bucket (weekly) to avoid wide partition anti-patterns.

Message Routing Fan-out: Group messages (up to 1000 members) fan out via Kafka topic per-group. Each WebSocket gateway subscribes to its assigned user partitions. Avoids O(N) push from a single node — Kafka handles the fan-out.

Presence System: Track online/offline in Redis with TTL-based heartbeats (30s). Presence updates are published via Redis Pub/Sub to subscribed contacts. Limit presence fan-out to mutual contacts only (not all 2B users) to bound write amplification.

Offline Message Queue: When a user reconnects, pull undelivered messages from Cassandra ordered by timestamp. Batch-deliver in chunks of 100. Mark as 'delivered' via async Kafka event to update sender-side delivery receipts.

Multi-Region: Deploy WebSocket gateways per-region (US, EU, Asia). Messages between regions route through dedicated Kafka MirrorMaker cross-region bridges. User-to-gateway affinity based on nearest PoP via GeoDNS.

8. Runnable Production Starter Code

Scaffold