Design Reddit - System Architecture

High-Level Architecture

[Clients] → [Load Balancer] → [API Gateway] → [Services]
                                                    ↓
                                    [Post Service, Comment Service,
                                     Vote Service, Subreddit Service]
                                                    ↓
                                    [PostgreSQL, Redis, Elasticsearch]

Core Services

1. Post Service

• Create, read, update, delete posts
• Store in PostgreSQL (partitioned by subreddit)
• Cache hot posts in Redis
• Publish events to Kafka

2. Comment Service

• Threaded comment storage
• Nested comment retrieval
• Comment tree caching
• Real-time comment updates

3. Vote Service

• Handle upvotes/downvotes
• Update karma scores
• Prevent vote manipulation
• Cache vote counts

4. Ranking Service

• Hot algorithm: score = (ups - downs) / (age + 2)^1.5
• Best algorithm: Wilson confidence interval
• Controversial: (ups + downs)^(min(ups,downs)/(ups+downs))
• Cache rankings (5 min TTL)

5. Subreddit Service

• Create and manage subreddits
• Moderator permissions
• Subreddit settings
• Community rules

6. Search Service

• Elasticsearch for full-text search
• Index posts and comments
• Autocomplete suggestions
• Faceted search

Technology Stack

• Backend: Python (Flask/Django)
• Database: PostgreSQL (posts, comments, users)
• Cache: Redis (rankings, hot data)
• Search: Elasticsearch
• Queue: Kafka (events)
• CDN: CloudFront (media)

Data Flow

Post Creation

1. User submits post
2. Post Service validates
3. Store in PostgreSQL
4. Publish to Kafka
5. Update subreddit stats
6. Invalidate cache
7. Index in Elasticsearch

Feed Generation

1. User requests subreddit feed
2. Check Redis cache
3. If miss: Query PostgreSQL
4. Apply ranking algorithm
5. Cache result (5 min TTL)
6. Return to client

This architecture provides scalability for Reddit's community-driven platform.