Networking & Delivery
Load Balancer
A service that distributes incoming traffic across multiple servers, improving capacity and keeping the site up if one server fails.
A load balancer sits in front of several servers and spreads incoming requests across them. This does two jobs at once: it multiplies the traffic you can handle by adding backends, and it keeps your site up when a server dies, because the balancer just stops sending requests to the dead one. It's the backbone of any setup that needs to survive a single machine failing.
Managed load balancers are a cheap building block. DigitalOcean's runs around $12/mo, and Vultr, Linode, and the hyperscalers all offer them. They also handle health checks, deciding which backends are alive, and often terminate SSL so your app servers don't have to.
The moment you run more than one app server, you need something to distribute across them, and a load balancer is that something. It's also the piece that makes horizontal autoscaling possible, since new instances just register behind it.