- Probabilistic data structures
- Often used for real-time analytics and large datasets
- Bloom filters for compact existence check:
- Always tells if item is in set, slight change of false positives
- bitmap
- Check if a website is malicious (bloom filter of giant set of malicious sites)

- hyperloglog
- t-digest
- Ted Dunning talk
- on-line accumuluation of rank-based statistics
- sort of like a histogram with many quintiles, more accuracy in very high and very low quintiles, less in middle quintiles
- variable accuracy, constant relative accuracy
- computes clusters, large in middle, small towards end of distribution
- continuously compute median (50th percentile) of a large number of integers from distributed nodes
- order points, collect points together given scale of quintile, continually merging points, short buffer for new points, sort and merge
- <100 nanosecond overhead per measurement
- under the hood uses avl trees

- semigroups and monoids