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Almost every product counts something: views on a video, likes on a post, clicks on an ad. On one server, a count is just adding one. Across many servers and millions of events per second, it gets much harder.

This lesson covers how large systems count things like views, likes, and clicks across many servers, why something as simple as adding one gets hard at scale, and the main techniques used for different kinds of counts.

It also covers how to approach counting in a system design interview, since many common interview problems need some kind of counter.