Load Balancer

A component that distributes traffic across multiple backend instances.

Load balancing diagram

Where it fits

A load balancer distributes traffic across multiple backend instances. It is a core system design concept and appears in production operations through concrete cloud or proxy implementations.

Mental model

The load balancer is a traffic director. It hides multiple backends behind one entry point and helps with scale, failover, and maintenance.

Subconcepts

Round robin

Send each request to the next backend in order.

Least connections

Send traffic to the backend with the fewest active connections.

Health checks

Continuously test whether a backend should receive traffic.

Sticky sessions

Route a user to the same backend when stateful behavior requires it.

Common mistakes

Assuming load balancing alone makes a system reliable.
Forgetting health checks and connection draining.
Using sticky sessions when stateless design would be simpler.

Horizontal scaling, failover, reverse proxy, server, availability.