System Design Concepts

1. Core Principles

1.1 Scalability

• Vertical Scaling (Scale Up): Adding power to a single machine.
• Horizontal Scaling (Scale Out): Adding more machines to a pool.

1.2 Reliability

• System continues to work correctly even when things go wrong.

1.3 Availability

• System is operational at a given point in time (e.g., 99.99% uptime).

2. Key Components

2.1 Load Balancing

• Distributes traffic across servers (Nginx, HAProxy, AWS ELB).
• Algorithms: Round Robin, Least Connections, IP Hash.

2.2 Caching

• Client Side: Browser cache.
• CDN: Content Delivery Network (Cloudfront, Akamai) for static assets.
• Server Side: Redis, Memcached for DB query results or session data.
• Strategies: Cache-Aside, Write-Through, Write-Back.

2.3 Databases

• SQL (Relational): MySQL, Postgres. ACID compliance.
• NoSQL (Non-Relational):
  • Key-Value: DynamoDB, Redis.
  • Document: MongoDB.
  • Wide Column: Cassandra.
  • Graph: Neo4j.
• Replication: Master-Slave, Master-Master.
• Sharding: Partitioning data across servers.

2.4 Asynchronous Processing

• Message Queues: RabbitMQ, Kafka, SQS. Decoupling services.
• Event Driven Architecture: Pub/Sub models.

3. Communication Protocols

• REST: Standard HTTP verbs (GET, POST).
• GraphQL: Flexible queries.
• gRPC: High performance, Protobuf based.
• WebSockets: Real-time bi-directional.

4. Theorums

• CAP Theorem: Consistency, Availability, Partition Tolerance (Pick 2).
  • CP: Banking.
  • AP: Social Media timelines.

5. Common Design Problems

• URL Shortener (TinyURL).
• Rate Limiter.
• Web Crawler.
• Notification System.
• Chat Application (WhatsApp).