We know when we might have a use case for a space and time efficient membership test where storing things explicitly would too expensive.
I did see the Elixir water park presentation where they used a bloom filter to record health messages on a per patient basis where every patient was represented as an in memory Elixir process mirrored across many datacenters in 5 regions with no backend persistent storage, just memory only, and doing hot code upgrades over years. Using a bloom filter enabled memory efficient and fast testing for any combination of messages those patient processes had “witnessed” to quickly evaluate business rules and co-ordinate patient care. It’s a brilliant talk.
