[dmunoz]

Jeremy Edberg mentioned in his talk on scaling reddit [0] that they used a bloom filter to provide fast (negative) lookup of whether or not a story or comment had been up voted by the user. The negative case was important since it's often the case that the majority of stories or comments appearing on a page will not have been up voted by the user.

[0] http://www.infoq.com/presentations/scaling-reddit

[jedberg]

To be pedantic, what we did was take advantage of Cassandra's bloom filter.