[dapz]

"Firstly, we expect some users to have a large number of documents in their Dropbox, making it non-trivial to update their corresponding index “instantly”."

If the alternative is maintaining a single index, won't the time it takes to update it at least be the time it takes to update per-user indexes? The former naively sounds like updating a single, gigantic binary search tree, the latter seems like updating a hashmap of UserId/BST pairs.

"Secondly, this approach requires the system to maintain as many indices as there are users with each stored in a separate file. With over 300 million users, keeping track of so many indices in production would be an operational nightmare."

..Why?

Anyway the stuff about shared documents is enough to make per-user indexing probably a bad idea, but I don't understand the reasons they provided above.