Backend & systems·Data modelling and storage engines
you check a small probabilistic structure first so you can skip opening a file that definitely does not contain the key.
Bloom filter
A compact structure that answers "definitely not present" or "possibly present" for set membership. Storage engines put one in front of each on-disk file to avoid pointless reads, and caches use them to avoid pointless origin fetches. False positives cost you a wasted lookup; false negatives cannot happen, which is what makes them safe.