Bloom Filters An Introduction and Really Most Of It CMSC 491 Hadoop-Based Distributed Computing Spring 2015 Adam Shook

Agenda Discuss what a set data structure is using math terms Discuss the concept of a Bloom filter Explore the mathematical magic behind Bloom filters

Set! A set is an unsorted data structure containing unique values Most common uses are: Error-free set membership tests Storing unique members of data (remove duplicates) Iterating through data in no particular order Other fun operations like unions, intersections, subsets, etceteras! Other sets support sorting and duplicate values But we aren’t here to talk about those

Set Insertion peter lois chris peter stewie chris stewie lois chris peter insert

is_member Set Membership Test peter stewie lois chris peter

is_member Set Membership Test adam stewie lois chris peter

Use Case! I’ve got a bunch of interesting keywords, A I’ve got a data set B I want to check if a record in B contains a word in A Make a new data set C for some cool data science for each record x in B for each word w in x if w in A emit x

Use Case, Solved! Stuff all the data in A into a set Get an A+ on your computer science project Impress the boss But what if A is stupid big? credit to mr. squarepants

Memory Footprint A contains 1 billion unique strings, average of 32 characters in length 8 bits per character 32 characters per string 1 billion of them 8 bits * 32 * 1,000,000,000 … Roughly 29.8 GB of raw storage required to hold these elements + overhead + even more if you are using Java For the sake of argument, let’s all agree that A doesn’t fit comfortably on a computer…

credit to xkcd and paint

Making a Set Smaller What two ‘features’ of a set can we relax to meet our requirements and have a reasonable memory footprint? Functionality Only want set membership operations Accuracy Don’t really need to be 100% accurate

Use Case, Revised! I’ve got a bunch of interesting keywords, A I’ve got a data set B I want to check if a record in B contains a word in A Make a new data set C for some cool data science I don’t really care if some stuff in C doesn’t contain words from A for each record x in B for each word w in x if w is likely in A with false positive p emit x

Let me paint you a story… We travel back to 1970… Burton Howard Bloom was investigating means to eliminate unnecessary disk accesses for particular algorithms Came up with the a probabilistic data structure for set membership Useful for programs with expensive operations where the operation is often unnecessary A structure only 15% of the size of the original can eliminate 85% of unnecessary disk accesses

Bloom Filter A space-efficient means to test if an element is a member of a set Elements can be added, but cannot be removed Storage cost for a single element is independent of the element size The members are not stored, so they cannot be retrieved There are no false negatives, but false positives are possible

How It’s Made – Training a Bloom Filter Given An array of bits size m, initialized to 0 k hash functions n elements foreach element n i in n foreach function k i in k m[k i (n i ) % m] = 1 Training a Bloom filter is O(n)

How It’s Made – Training a Bloom Filter peterloischris

How It’s Made – Membership Testing Given A trained Bloom filter of size m The same k hash functions An element x foreach function k i in k if m[k i (x) % m] is 0 return false return true Testing a Bloom filter is O(1)

How It’s Made – Membership Testing peter

How It’s Made – Membership Testing adam

I know what you’re thinking

The Catch What we make up for in space, we give up the accuracy… I give you… the false positive! cleveland

credit to xkcd and paint

Controlling the False Positive Rate Given Approximate number of elements in A, n A willingness to tolerate a percent p of false positives k is the optimal number of hash functions We can approximate m If you want the full details, read the paper or Wikipedia

Back to our use case… n = 1,000,000,000 p =.1 After dusting off the calculators…. m = x10 9 bits or GB An improvement of 29.8/0.558 = 53.4!

And now that we have m… We can use n and m to calculate k = m/n * ln(2) But I haven’t heard of 3.32 hash functions so let’s call it 4

References Wikipedia