title: Twitter fanout design
body: <p>I am reading book 'Designing Data-Intensive Applications' by Martin Kleppmann. At one point author has explained different approaches Twitter takes to publish tweets(fanout) to users. As per one of the approach, when someone posts a tweet, Twitter adds this tweet to a home timeline cache for all of the followers of the poster, as shown in image below.
  <a href="https://i.stack.imgur.com/r4zsw.png" rel="noreferrer"><img src="https://i.stack.imgur.com/r4zsw.png" alt="enter image description here" /></a></p>
  <p>Does it mean Twitter maintains a live cache for all of its users who are followers of someone? Won't it be too many caches to maintain?</p>