# LECTURE 2 Map-Reduce for large scale similarity computation.

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LECTURE 2 Map-Reduce for large scale similarity computation

…from last lecture How to convert entities into high-dimensional numerical vectors How to compute similarity between two vectors. For example, is x and y are two vectors then

..from last lecture Example: X = (1,2,3) ; Y= (3,2,1) ||X|| = (1+4+9) = 14 0.5 = 3.74 ||Y|| = ||X|| Sim(X,Y) = (1.3 + 2.2 + 3.1)/(3.74 2 ) = 10/14 = 5/7 We also learnt that for large data sets computing pair-wise similarity can be very time consuming.

Map-Reduce Map-Reduce has become a popular framework for speeding up computations like pair-wise similarity Map-Reduce was popularized by Google and then Yahoo! (through the Hadoop open-source implementation) Map-Reduce is a programming model built on top of “cluster computing”

Cluster Computing Put simple (commodity) machines together, each with their own CPU, RAM and DISK, for parallel computing CPURAMDISK CPURAMDISK CPURAMDISK CPURAMDISK CPURAMDISK CPURAMDISK Switch rack

Map-Reduce Map-Reduce consists of two distinct entities  Distributed File System (DFS)  Library to implement Mapper and Reducer functions A DFS seamlessly manages files on the “cluster computer.”  A file is broken into “chunks” and these chunks are replicated across the nodes of a cluster.  If a node which contains chunk A fails, the system will re-start the computation on a node which contains a copy of the chunk.

Distributed File System A DFS will “chunk” files and replicated them across several nodes and then keep track of the chunks. Only practical when data is mostly read only (e.g., historical data; not for live data –like airline reservation system). File Chunk Node 3,2,18 Node 2,6,7

Node failure When several nodes are in play the chances that a single node goes down at any time goes up significantly... Suppose they are n nodes and let p be the probability that a single node will fail..  (1-p) that single node will not fail  (1-p) n that none of the nodes will fail  1 – (1-p) n that at least one will fail.

Node failure The probability that at least one node failing is: f= 1 – (1-p) n When n =1; then f =p Suppose p=0.0001 but n=10000, then: f = 1 – (1 -0.0001) 10000 = 0.63 [why/how ?] This is one of the most important formulas to know (in general).

Example: “Hello World” of MR DocidContent 1Silent mind, holy mind 2road kill in Java 3Java programming is fun 4My mind in Java 5Where the fun rolls 6Silent road to Cairns Task: Produce an output which, for each word in the file, counts the number of times it appears in the file. Answer: (Java, 3); (Silent, 2), (mind,3)……

Example For example  {doc1, doc2}  machine 1  {doc3,doc4}  machine 2  {doc5,doc6}  machine 3 Each chunk is also duplicated to other machines.

Example Now apply the MAP operation to each node and emit the pair (key, 1). Thus doc1 emits:  (silent,1); (mind,1); (holy, 1); (mind,1) Similarly doc6 emits:  (silent,1);(road,1); (to,1); (Cairns,1)

Example Note in the first chunk which contains (doc1, doc2)..each doc emits (key,value) pairs. We can think that each computer node emits a list of (key, value) pairs. Now this list is “grouped” so that the REDUCE function can be applied.

Example Note now that the (key,value) pairs have no connection with the docs…  (silent,1),(mind,1), (holy, 1), (mind,1), (road,1),(to,1),(Cairns,1); (Java,1),(programming,1),(is,1),(fun,1),……. Now we have a hash function h:{a..z}  {0,1}  Basically two REDUCE nodes  And (key,value) effectively become (key, list)

Example For example suppose the hash functions maps {to, Java, road} to one node. Then  (to,1) remains (to,1)  (Java,1);(Java,1);(Java,1)  (Java, [1,1,1])  (road,1);(road,1)  (road,[1,1]); Now REDUCE function converts  (Java,[1,1,1])  (Java,3) etc. Remember this is a very simple example…the challenge is to take complex tasks and express them as Map and Reduce!

Schema of Map-Reduce Tasks [MMDS] chunks Map Task (key,value) pairs Group By Keys (k,v) [k,(v,u,w,x,z)] Reduce Task Output

The similarity join problem Last time we discussed about computing the pair- wise similarity of all articles/documents in Wikipedia. As we discussed it was time consuming problem because if N is the number of documents, and d is the length of each vector, then the running time proportional to O(N 2 d). How can this problem be attacked using the Map Reduce framework.

Similarity Join Assume we are given two documents (vectors) d1 and d2. Then (ignoring the denominator) Example:  d1 = {silent mind to holy mind}; d2 = {silent road to cairns}  sim(d1,d2) = 1 silent,d1 1 silent,d2 + 1 to,d11 1 to,d2 = 2 Exploit the fact that a term (word) only contributes if it belongs to at least two documents.

Similarity Example [2] Notice, it requires some ingenuity to come up with key-value pairs. This is key to suing map-reduce effectively

Amazon Map Reduce For this class we have received an educational grant from Amazon to run exercises on their Map Reduce servers. Terminology  EC2 – is the name of Amazon’s cluster  S3 – is the name of their storage machines  Elastic Map Reduce – is the name Amazon’s Hadoop implementation of Map-Reduce Lets watch this video.this

References 1. Massive Mining of Data Sets (Rajaram, Leskovec, Ullman) 2. Computing Pairwise Similarity in Large Document Collection: A Map Reduce Perspective (El Sayed, Lin, Oard)