Earlier quoted context omitted.
> You can also say what you think would be the number of followers (millions+), what would be the time needed for a write you a database (~millisecond each) and why that won't match 3 seconds, so you know how much time you have per follower. While we are on the topic I am really curious to know how they solve it actually.
I don't work at twitter, but one thing you can know for sure is that the tweet needs to be distributed to multiple machines to meet this performance requirement. A single machine probably could probably send three million trivial messages in three seconds, but if you mix in any business logic it's not gonna happen. What I'd do is have a shared highfanout queue. Once you have more than say 10k followers your tweets go…
Seems to me you’d need to do the latter right? That way you ensure you process each follower at least once, but each worker doesn’t need to be aware of others it’s just pulling messages from the queue.