Load balancing is the process of distributing incoming network or application traffic across multiple servers to ensure no single server becomes overwhelmed, thereby improving performance, reliability, and availability of services. It acts as a traffic manager that ensures users get routed to optimal resources.
Why It’s Used
- To scale horizontally by adding more servers
- To increase fault tolerance: if one server fails, others continue to serve traffic
- To reduce latency by routing requests based on proximity or performance
- To optimize resource usage
Types of Load Balancing
Layer 4 Load Balancing
Operates at the Transport Layer (TCP/UDP). Makes routing decisions based on IP address and port information.
- Faster, more efficient
- Cannot inspect application-level content
Layer 7 Load Balancing
Operates at the Application Layer. Makes decisions based on HTTP headers, cookies, URL paths, etc.
- Allows for intelligent routing (e.g. A/B testing, URL-based routing)
- Supports content switching and user session awareness
Load Balancing Algorithms
| Algorithm | Description |
|---|---|
| Round Robin | Requests are distributed evenly across all servers |
| Least Connections | Traffic is sent to the server with the fewest active connections |
| IP Hash | Uses client IP to determine which server handles the request |
| Weighted Round Robin | Servers are assigned weights based on capacity |
| Random | Requests are distributed randomly |
| Response Time-Based | Sends traffic to the server with the lowest response time |
Health Checks
Load balancers perform regular health checks on backend servers to ensure they’re available. If a server fails, the balancer stops sending traffic to it until it recovers.