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A load balancer is a device that acts as a reverse proxy and is responsible for distributing network traffic across multiple servers. Load balancers smooth out the concurrent user experience of the application and improve reliability. An evenly distributed load means that each server will be better able to service application/network requests.
The requests received by a load balancer are distributed among multiple servers using a configured algorithm that could be based on:
Least response time
Load balancers ensure reliability and availability of servers around the clock by constantly monitoring the load that each server is under and only sending requests to servers and applications that can respond in a timely manner.