Can someone enlighten me as to why 6000 tweets a second is something to make a big deal about? At 140 characters per message that comes out to 840,000 bytes/s < 1 Megabytes per second. In 2014 is a service that can handle 1 Megabytes/s impressive?
By comparison, many complex machine-generated data sources (e.g. real-time entity tracking) that are sometimes fused with the Twitter firehose operate at millions of complex records every second (often tens of gigabytes per second) that need to be processed, indexed, and analyzed in real-time. You can't deal with this kind of data model using something like Twitter's current architecture because the several order of magnitude difference in velocity and volume exposes the limitations of most database platform designs people typically use.
Not that I took it vary far, but my first stab to see what the scaling issues would be was actually plenty fast to run there feed process at the time.
PS: The 'trick' is to keep two lists one everything a user follows, and another is everything that follows each account. For showing the messages to someone when they log in you keep the last 10 message Id's with timestamp or sequence ID so if someone follows 5k accounts you can avoid looking at the vast majority of those messages then sign them up the device to listen to new messages. (Sure, sometimes you will need to look past that top 10 but it's rather effective.)
(And for the downvoters I can upload some code if you want to see it.)
PS: I did not keep old messages just there ID because that was not going to fit in RAM. My assumption was using Redis or other key value store would be fine what they needed was an internal index so you would only need to look up messages that would be displayed.
Note: there current setup once they worked the bugs out handled a peak of over 100,000 tweets per second in 2013. https://blog.twitter.com/2013/new-tweets-per-second-record-a... Which is well beyond the target I was shooting for.
That's the difference between "getting the basic process running" and operating at scale.
Back when they where growing from 500,000 users to 7 million total users they where having major issues and that's when I was looking into things.
Anyway, not I was suggesting 1GB would be fine today. Still, I was saturating a 1GB connection so 15M (over twice the total users back then) * ~200bytes * 8 / ~1000^3 = ~24 seconds did you have an extra 60x multiplier in there somewhere?
Things add up eventually. Suddenly your 6000 tweets/s has turned into a million op/s fanout.
Problems become a lot less straightforward over network links too.