Introduction to System Design

System design is the process of defining the architecture, interfaces, and data for a system that satisfies specific requirements. When designing systems at scale, we must move away from single-server setups and address scalability, availability, and reliability.

1. Vertical vs. Horizontal Scaling

When traffic grows, we have two primary options for scaling our infrastructure:

• Vertical Scaling (Scale Up): Adding more power (CPU, RAM, disk space) to an existing server. While simple to implement, it has a hard hardware ceiling and does not solve the Single Point of Failure (SPOF) problem.
• Horizontal Scaling (Scale Out): Adding more servers to our pool of resources. This is highly scalable and fault-tolerant but requires distributed system management and database consistency controls.

2. Load Balancing

To distribute client traffic across multiple horizontal servers, we introduce a Load Balancer.

Load balancers distribute user requests among a pool of application servers using routing algorithms:

1. Round Robin: Sequentially routes requests (Server A -> Server B -> Server C).
2. Least Connections: Routes to the server handling the fewest active connections.
3. IP Hash: Hashes the client's IP to assign them to a specific server (useful for session stickiness).

3. Eliminating Single Points of Failure (SPOF)

A Single Point of Failure is any part of a system that, if it fails, will stop the entire system from working. We eliminate SPOFs by introducing Redundancy:

• Running multiple active servers behind a load balancer.
• Implementing Database Replication (Primary-Replica setups) to prevent data loss.
• Using failover systems to automatically promote a standby resource if the active resource crashes.