Eli5: what happens on the server when one selects to watch a movie on Netflix and how the architecture allows 500k other people to watch the same movie almost concurrently?
In: Technology
First of all, not everyone stream simultaneously in perfect sync. You would watch and I start a few minutes late and we would be watching different parts of the same video.
Content in stored close to you using a technology called CDN, also close to anyone watch it. If you are lucky you may be in the same city. You just download from a close point.
Also since video streaming is a one way thing unlike video chat, they can use faster protocols.
One nice fact is Netflix stores multiple copies of the same movie in different qualities and switches between them according to your streaming speed. According to their research a %7 change in quality is not noticed, so quality can change you without even noticing.
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