1 - For the last 10 years, servers have been beasts. You have a lot of cores, plenty of HD and RAM. Servers are less expensives than devs. Scaling vertically can go VERY far.
2 - Caching is life. We have 3 layers of caching: cloudflare, varnish, and redis. Most things don't need to be real time. A lot of things can be a month old and the user doesn't care. User need immediate feedback to be happy, but not necessary fresh data.
3 - if you compile nginx manually, you get to use a lot of plugins that can do stuff super fast, including serving videos. You can script stuff in lua that will just skip the backend completly.
4 - mind your encoding. We carefully chose how we encode videos. The ffmpeg parameters are pretty insane, but the space / quality ratio is amazing, espacially on mobile. It takes a lot of time to experiment with those, nobody share them :)
5 - we offload everything we can to cron tasks or task queues. Including, obviously, encoding, screenshooting, etc.
6 - don't hold data you can't lose. E.G: billing. This way you can have a relaxed attitute toward data. If we ever loose a day of business, users will be in a bad mood for a week, but that won't be the end of the world. We don't need a bullet proof system if bullets can't kill us.
7 - give money to ffmpeg and opencv, because damn those things are fast. And good.
8 - servers are hosted accross 2 providers. This way, if one goes down, or decide to stop doing business with us Google style, we have a second one. Happened recently with leaseweb: they shutdown a whole room without offering an alternative.
E.G: votes.
They don't hit the backend on write. We pile them from nginx to redis, then once a day, we aggregate and store on postgres, which the backends will consumme. We just store each vote on localstorage as well so that the user feels like it's real time when they vote, but in reality it's updated once a day. But votes don't affect the money side of our business, so if we lose them one day, it does not mean death.
P.S: yes, posgres/redis/elasticsearch only hold metadata. Videos are stored on disk. There is no docker images, no mircoservices, FS is ext4. Which means with a lot of RAM, the OS FS cache will have most popular videos already loaded and ready to be streamed. Everything is raid 0, so if we get one disk corrupted, you lose the server. But we upload each videos on severeal servers, so when a disk get corrupted, we just replace the whole server. In fact, anything goes wrong on a server, we replace it. It's not worth it to find the root cause, unless 2 servers die in the same way successively.
