System Design Practice Exam

Horizontal scaling (or scaling out) means adding more machines to your resource pool to handle increased load. For example, going from 1 server to 10 servers behind a load balancer.

Advantages:

• Near-infinite scalability
• Better fault tolerance — one server failing doesn't take down the system
• Can use commodity hardware

Vertical scaling (or scaling up) means adding more power (CPU, RAM, disk) to an existing machine.

Advantages:

• Simpler architecture — no distributed coordination needed
• No code changes required

• Hardware limits — there's a ceiling to how much you can upgrade
• Single point of failure
• Downtime during upgrades

A load balancer distributes incoming network traffic across multiple backend servers to ensure no single server is overwhelmed.

It sits between clients and servers, improving:

• Availability — routes around failed servers
• Throughput — parallel processing across servers
• Latency — directs to least-loaded server

Common algorithms:

• Round Robin — requests are distributed sequentially across servers
• Weighted Round Robin — servers with higher capacity get more requests
• Least Connections — routes to the server with fewest active connections
• IP Hash — routes based on client IP, ensuring session stickiness
• Random — randomly selects a server
• Least Response Time — routes to the fastest-responding server

Layer 4 (Transport) load balancers route based on IP address and TCP/UDP port without inspecting packet contents. They are faster but less flexible.

Layer 7 (Application) load balancers inspect HTTP headers, cookies, and URLs to make smarter routing decisions. They can do content-based routing, SSL termination, and request modification.

Example: HAProxy supports both; AWS NLB is L4, ALB is L7.