What Is a Load Balancer? Definition and How It Works

A load balancer distributes incoming traffic across multiple servers to deliver high availability, performance, and scalability. It is essential for web applications, databases, and SaaS platforms. Technologies include L4 (TCP/UDP), L7 (HTTP/HTTPS), and GSLB (global).

Layers

• L4 (TCP/UDP): distribution based on IP address/port
• L7 (application): distribution based on URL, cookie, header, or content
• GSLB (Global Server Load Balancing): multi-datacenter distribution via DNS

Algorithms

• Round-robin: sequential distribution
• Least connections
• Weighted round-robin
• IP hash
• Response time
• URL hash (caching)

Health Checks

• TCP connect
• HTTP GET with 200 status code validation
• Custom scripts
• Automatic removal of unavailable backends
• Monitoring alerts

Vendors

• F5 BIG-IP: enterprise leader
• Citrix NetScaler: enterprise solution
• HAProxy: powerful open-source solution
• NGINX +: commercial web proxy + LB
• AWS ALB/NLB, Azure App Gateway, GCP Cloud LB: cloud-native solutions
• Kemp LoadMaster: mid-market solution

Additional Features

• SSL termination
• WAF (Web Application Firewall)
• Bot protection
• DDoS mitigation
• API gateway
• Caching

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