Load Balancer

Load balancers are critical infrastructure components that prevent server overload, eliminate single points of failure, and enable horizontal scaling.

Why it matters

• Ensures application availability even when individual servers fail.
• Enables horizontal scaling by adding servers behind the load balancer.
• Improves response times by routing requests to the least-busy server.
• Required for high-availability architectures and disaster recovery.
• Essential for meeting SLA commitments for uptime and performance.

Load balancing algorithms

• Round Robin: Distributes requests sequentially across servers.
• Least Connections: Routes to the server with fewest active connections.
• Weighted: Assigns proportional traffic based on server capacity.
• IP Hash: Routes requests from the same client IP to the same server (session persistence).
• Least Response Time: Chooses the server with fastest response and fewest connections.

Types of load balancers

• Layer 4 (Transport): Routes based on IP address and TCP/UDP port; fast but less flexible.
• Layer 7 (Application): Routes based on HTTP content (URL, headers, cookies); more intelligent but higher overhead.
• Global (GSLB): Distributes traffic across geographically distributed data centers.
• Internal: Balances traffic between services within a private network.

Health checks

• Active checks: Load balancer periodically probes servers for availability.
• Passive checks: Monitors actual traffic for errors and response times.
• Graceful degradation: Remove unhealthy servers from rotation without dropping connections.

High availability patterns

• Active-Passive: Standby load balancer takes over if primary fails.
• Active-Active: Multiple load balancers share traffic with automatic failover.
• DNS failover: GSLB redirects traffic to healthy data centers.

Cloud implementations

• AWS: Application Load Balancer (ALB), Network Load Balancer (NLB), Classic Load Balancer.
• Azure: Azure Load Balancer, Application Gateway.
• GCP: Cloud Load Balancing (HTTP(S), TCP/UDP, Internal).