# DISTRIBUTED COMPUTING & MAP REDUCE CS16: Introduction to Data Structures & Algorithms Thursday, April 17, 2014 1.

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DISTRIBUTED COMPUTING & MAP REDUCE CS16: Introduction to Data Structures & Algorithms Thursday, April 17, 2014 1

Outline 1) Distributed Computing Overview 2) MapReduce I. Application: Word Count II. Application: Mutual Friends Thursday, April 17, 2014 2

Distributed Computing: Motivation Throughout the course of the semester, we have talked about many ways to optimize algorithm speed, for example: Using efficient data structures Taking greedy “shortcuts” when we are sure they are correct Dynamic programming algorithms However, we left out a seemingly obvious one... Using more than one computer! Thursday, April 17, 2014 3

Distributed Computing: Explanation Distributed computing is the field of taking a computational problem and distributing it among many worker computers (nodes) Usually there is a master computer, which coordinates the distribution to and feedback from the nodes The distributed system can be represented as a graph where the vertices are nodes and the edges are the network connecting them Thursday, April 17, 2014 4

Distributed Computing: Primes Thursday, April 17, 2014 5

Distributed Computing: Primes (2) Wrong! The largest known prime number was discovered last year: 2 57,885,161 – 1. This number has 17,425,170 digits! A single machine cannot verify the primality of this number! This prime (named M48) was discovered through GIMPS, the distributed program “Great Internet Mersenne Prime Search” launched in 1996 to find large prime numbers searches for primes of the form 2 m -1 where m itself is prime GIMPS allows users to donate some of their computer’s rest time toward testing the primality of numbers of this form Thursday, April 17, 2014 6

Distributed Computing: Primes (3) Thursday, April 17, 2014 7 Is 101 prime? Node 3Node 2 Node 1 Master {5,6,7} | 101?{8,9,10} | 101?{2,3,4} | 101? No

More Applications There are many problems being solved by massive distributed systems like GIMPS: Stanford’s Folding@home project uses distributed computing to solve the problem of how proteins fold, helping scientists studying Alzheimer’s, Parkinson’s, and cancers Berkeley’s Milkyway@home project uses distributed computing to generate a highly accurate 3D model of the Milky Way galaxy using data collected over the past decade Thursday, April 17, 2014 8

Typical Large-Data Problem As the Internet has grown, so has the amount of data in existence! To make sense of all this data, we generally want to: Iterate over a large number of records Extract something of interest from each Shuffle and sort intermediate results Aggregate intermediate results Generate final output Thursday, April 17, 2014 9

Developing MapReduce In 2004, Google was “indexing” websites: crawling pages and keeping track of words and their locations on each page The size of the Web was huge! To handle all this information, Google researchers came up with the MapReduce Framework Thursday, April 17, 2014 10

MapReduce MapReduce is a programming abstraction seeking to hide system-level details from developers while providing the advantages of distributed computation Only focus on the “what” and not the “how” The developer specifies the computation that needs to be performed, and the framework (MapReduce) handles the actual execution Serves as a black box for distributed computing Thursday, April 17, 2014 11

Typical Large-Data Problem To make sense of all this data, we generally want to: Iterate over a large number of records Extract something of interest from each Shuffle and sort intermediate results Aggregate intermediate results Generate final output The MapReduce framework takes care of the rest Thursday, April 17, 2014 12 REDUCE MAP

MapReduce Thursday, April 17, 2014 13

Counting Words Suppose you want to know the most popular word on the Internet One method you could use would be to create a hashtable with words as keys and counts as values, but looping through every word on every page and hashing it to the table takes a long time… MapReduce allows you to speed up the entire process. You need to determine the “mapping” and “reducing” portions, and the framework does the rest! Thursday, April 17, 2014 14

MapReduce Example Suppose we have three documents and we would like to know the number of times each word occurs throughout all the documents Thursday, April 17, 2014 15 do not forget to do what you do send me a forget me not two plus two is not one doc1doc2doc3

MapReduce Example The MapReduce runtime takes care of calling our map routine three times, once for each document. What are the input arguments to map ? Thursday, April 17, 2014 16 do not forget to do what you do send me a forget me not two plus two is not one doc1doc2doc3

MapReduce Example: Mapping Thursday, April 17, 2014 17 do not forget to do what you do send me a forget me not two plus two is not one doc1doc2doc3 map (doc1, [do, not, forget, to do, what, you, do]) map (doc2, [send, me, a, forget, me, not]) map (doc3, [two, plus, two, is, not, one])

What should our map function return? Remember that the function must return a list of (key, value) tuples. The output from all of the map calls is grouped by key and then passed to the reducer. MapReduce Example: Mapping Thursday, April 17, 2014 18 map (doc1, [do, not, forget, to, do, what, you, do]) map (doc2, [send, me, a, forget, me, not]) map (doc3, [two, plus, two, is, not, one]) ? ? ?

MapReduce Example: Mapping Thursday, April 17, 2014 19 map (doc1, [do, not, forget, to, do, what, you, do])  [(do,1),(not,1),(forget,1),(to,1),(what,1),(you,1),(do,1)] map (doc2, [send, me, a, forget, me, not])  [(send,1),(me,1),(a,1),(forget,1),(me,1),(not,1)] map (doc3, [two, plus, two, is, not, one])  [(two,1),(plus,1),(two,1),(is,1),(not,1),(one,1)]

The framework takes care of grouping the returned tuples by key and passing the list of values to the reducers. All values for a certain key are sent to the same reducer MapReduce Example: Mapping Thursday, April 17, 2014 20 map(map_k, map_v): // In this example, map_k is a document name and // map_v is a list of strings in the document tuples = [] for v in map_v: tuples.add((v,1)) return tuples

Reduce is called for each of these (word, []) pairs, e.g. reduce(forget, [1,1]) MapReduce Example: Reducing Thursday, April 17, 2014 21 do [1, 1, 1] not [1, 1, 1] forget [1, 1] me [1, 1] two [1, 1] to [1] what [1] you [1] send [1] a [1] plus [1] is [1] one [1]

Once the reducers all return, we have our counts for all the words! MapReduce Example: Reducing Thursday, April 17, 2014 22 reduce(k, values): // In this example, k is a particular word and // values is a list of all 1’s sum = 0 for v in values: sum += 1 return (k, sum)

The MapReduce Process Thursday, April 17, 2014 23 framework

Note that all of the map operations can run entirely in parallel, as can all of the reduce operations (once the map operations terminate) With hundreds or thousands of computing nodes, this is a huge benefit! This example seemed trivial, but suppose we were counting billions of entries! MapReduce Benefits Thursday, April 17, 2014 24

One More Example For every person on Facebook, Facebook stores their friends in the format Person  [List of friends] How can we use MapReduce precompute for any two people on Facebook all of the friends they have in common? Hint: we’ll have to use the property that identical keys from the mapper all go to the same reducer! Thursday, April 17, 2014 25

MapReduce Benefits (2) The MapReduce framework Handles scheduling Assigns workers to map and reduce tasks Handles “data distribution” Handles synchronization Gathers, sorts, and shuffles intermediate data Handles errors and worker failures and restarts Everything happens on top of a distributed filesystem (for another lecture!) All you have to do (for the most part) is write two functions, map and reduce ! Thursday, April 17, 2014 26

Acknowledgements http://www.umiacs.umd.edu/~jimmylin/clou d-2010-Spring/session1-slides.pdf http://www.umiacs.umd.edu/~jimmylin/clou d-2010-Spring/session1-slides.pdf http://stevekrenzel.com/finding-friends-with- mapreduce http://stevekrenzel.com/finding-friends-with- mapreduce Thursday, April 17, 2014 27

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