What is a Load Balancer?
A load balancer distributes incoming traffic across multiple servers — solving scalability and availability simultaneously.
Load Balancing Algorithms
Round Robin
Request 1 → Server 1
Request 2 → Server 2
Request 3 → Server 3
Request 4 → Server 1 (repeats)
Best for: identical servers, equal-cost requests
Least Connections
Server 1: 100 active connections
Server 2: 20 active connections ← next request goes here
Server 3: 80 active connections
Best for: long-lived connections, variable request cost
Layer 4 vs Layer 7
Health Checks
Session Persistence
Key Takeaway
- Round robin for simple cases, least connections for variable workloads
- Layer 7 for most web apps — content-based routing, SSL termination
- Always implement health checks — automatic failure detection is critical
- Use shared session store (Redis), never sticky sessions