A load balancer is more than a box you draw in your system design diagram. It's one of the most critical pieces in your whole system, and if it goes down, it takes everything behind it down with it.
The general idea is simple. It's the entry point to your servers. It sits in front of them and spreads the traffic between them. For most people that's where the understanding stops, and honestly it's enough to draw it in an interview.
But this one component carries a lot of complexity. Changing a single option can completely change how your servers behave. Switch the hashing method and adding one server can move most of your users to a different machine. Put retries in the wrong place and one struggling database can get hit with hundreds of times its normal load. Pick layer 4 when you needed layer 7 and you lose the ability to route by URL at all.
That's why understanding the different algorithms, what each network layer gives you, and how the balancer itself stays up matters so much. It's the difference between drawing a load balancer and actually knowing what it does and how to scale it with your system.
In this video I break down every concept of load balancing, starting with the simplest thing it does, distributing load, and building up to the bigger pieces like high availability, global load balancing across regions, and load balancing between services.
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