Quoting Jeremy Edberg

from blog Simon Willison's Weblog, | ↗ original
[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...