筆記文：Why are Facebook, Digg, and Twitter so hard to scale?

傳統的 web site 很好 scale 的原因在於：

  • They usually access only their own data and common cached data.
  • Only 1-2% of users are active on the site at one time.

但 facebook、dig 與 twitter 就不是如此了。

Facebook takes a Pull on Demand approach. 所以 facebook 有好友數上限，因為 pull on demand 可能會來不及抓完所有資料。Another approach to find out what's new is the Push on Change model. 代價就是 de-normalize 導致儲存需求大增。

如何在兩者之間達到平衡，就是關鍵了。