Extremely useful for anyone doing real-time analytics or filtering. Answering "has this person visited this page before" or "how many distinct people have visit

Extremely useful for anyone doing real-time analytics or filtering. Answering "has this person visited this page before" or "how many distinct people have visited this page/website" can be done exactly with map-reduce or streaming pipelines, but if you want an immediate answer with low latency and can tolerate false negatives/bounded inaccuracy, you can simply keep a Bloom filter or HyperLogLog in memory. For example, see the top comment here: https://news.ycombinator.com/item?id=12124722 https://news.ycombinator.com/item?id=12124722