Definition
A load balancer is a network component that distributes incoming requests across a pool of backend servers so that no single machine is overwhelmed. Functionally it acts as a reverse proxy: clients connect to one front-end address, and the balancer forwards each connection to a healthy backend according to a distribution policy. By spreading work and continuously checking server health, it lets a self-hosted service scale horizontally and survive the failure of any individual node.
Layer 4 versus Layer 7
Load balancers operate at one of two layers. A Layer 4 balancer routes purely on network information such as IP address and TCP or UDP port, which is fast and protocol-agnostic. A Layer 7 balancer understands the application protocol and can route on HTTP details like the requested URL, host header, or cookies, enabling smarter rules such as sending API traffic to one pool and static assets to another. Common distribution methods include round robin, weighted round robin for servers of different sizes, and IP-hash for basic session stickiness.
Health checks and availability
The defining feature is the health check. The balancer periodically probes each backend and automatically stops sending traffic to any node that becomes slow or unresponsive, rerouting users to healthy servers so the outage is invisible. This is what turns a cluster of ordinary machines into a highly available service. Load balancing complements wider-scale techniques such as anycast for cross-region distribution, and a balancer is frequently the only host that needs to sit exposed in a DMZ while the real backends stay private.
In Simple Terms
A load balancer is a network component that distributes incoming requests across a pool of backend servers so that no single machine is overwhelmed. Functionally…