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A server that sits in front of your app servers and distributes incoming requests among them — so no single server gets overwhelmed.
Algorithms: round-robin, least connections, IP hash (sticky sessions). Layer 4 (TCP) balances by IP/port. Layer 7 (HTTP) balances by URL path, headers, or cookies — enables routing /api to backend and / to frontend.
Health checks detect unhealthy instances and remove them from rotation. Connection draining gives in-flight requests time to complete before removing an instance. AWS ALB, Nginx, HAProxy, Cloudflare are common implementations.