Simon Willison’s Weblog

[On Reddit] we had to look up every single comment on the page to see if you had voted on it [...]

But with a bloom filter, we could very quickly look up all the comments and get back a list of all the ones you voted on (with a couple of false positives in there). Then we could go to the cache and see if your actual vote was there (and if it was an upvote or a downvote). It was only after a failed cache hit did we have to actually go to the database.

But that bloom filter saved us from doing sometimes 1000s of cache lookups.

— Jeremy Edberg