1. Large-Scale Data Processing with MapReduce AAAI 2011 Tutorial Jimmy Lin University of Maryland Sunday, August 7, 2011 This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States See http://creativecommons.org/licenses/by-nc-sa/3.0/us/ for details These slides are available on my homepage at http://www.umiacs.umd.edu/~jimmylin/

2. First things first… About me Course history Audience survey

3. Agenda Setting the stage: Why large data? Why is this different? Introduction to MapReduce MapReduce algorithm design Text retrieval Managing relational data Graph algorithms Beyond MapReduce

4. Expectations Focus on “thinking at scale” Deconstruction into “design patterns” Basic intuitions, not fancy math Mapping well-known algorithms to MapReduce Not a tutorial on programming Hadoop Entry point to book

5. Setting the Stage: Why large data? Setting the stage Introduction to MapReduce MapReduce algorithm design Text retrieval Managing relational data Graph algorithms Beyond MapReduce

6. Source: Wikipedia (Everest)

7. How much data? 6.5 PB of user data + 50 TB/day (5/2009) processes 20 PB a day (2008) 36 PB of user data + 80-90 TB/day (6/2010) Wayback Machine: 3 PB + 100 TB/month (3/2009) LHC: 15 PB a year (any day now) LSST: 6-10 PB a year (~2015) 640K ought to be enough for anybody.

8. No data like more data! (Banko and Brill, ACL 2001) (Brants et al., EMNLP 2007) s/knowledge/data/g; How do we get here if we’re not Google?

9. + simple, distributed programming models cheap commodity clusters = data-intensive computing for the masses!

10. Setting the Stage: Why is this different? Setting the stage Introduction to MapReduce MapReduce algorithm design Text retrieval Managing relational data Graph algorithms Beyond MapReduce

11. Parallel computing is hard! Message Passing P1P1 P2P2 P3P3 P4P4 P5P5 Shared Memory P1P1 P2P2 P3P3 P4P4 P5P5 Memory Different programming models Different programming constructs mutexes, conditional variables, barriers, … masters/slaves, producers/consumers, work queues, … Fundamental issues scheduling, data distribution, synchronization, inter-process communication, robustness, fault tolerance, … Common problems livelock, deadlock, data starvation, priority inversion… dining philosophers, sleeping barbers, cigarette smokers, … Architectural issues Flynn’s taxonomy (SIMD, MIMD, etc.), network typology, bisection bandwidth UMA vs. NUMA, cache coherence The reality: programmer shoulders the burden of managing concurrency… (I want my students developing new algorithms, not debugging race conditions) master slaves producerconsumer producerconsumer work queue

12. Where the rubber meets the road Concurrency is difficult to reason about At the scale of datacenters (even across datacenters) In the presence of failures In terms of multiple interacting services The reality: Lots of one-off solutions, custom code Write you own dedicated library, then program with it Burden on the programmer to explicitly manage everything

13. Source: Ricardo Guimarães Herrmann

14. Source: NY Times (6/14/2006) The datacenter is the computer! I think there is a world market for about five computers.

15. What’s the point? It’s all about the right level of abstraction Hide system-level details from the developers No more race conditions, lock contention, etc. Separating the what from how Developer specifies the computation that needs to be performed Execution framework (“runtime”) handles actual execution The datacenter is the computer!

16. “Big Ideas” Scale “out”, not “up” Limits of SMP and large shared-memory machines Move processing to the data Cluster have limited bandwidth Process data sequentially, avoid random access Seeks are expensive, disk throughput is reasonable Seamless scalability From the mythical man-month to the tradable machine-hour

17. Introduction to MapReduce Setting the stage Introduction to MapReduce MapReduce algorithm design Text retrieval Managing relational data Graph algorithms Beyond MapReduce

18. Typical Large-Data Problem Iterate over a large number of records Extract something of interest from each Shuffle and sort intermediate results Aggregate intermediate results Generate final output Key idea: provide a functional abstraction for these two operations Map Reduce (Dean and Ghemawat, OSDI 2004)

19. ggggg fffff Map Fold Roots in Functional Programming

20. MapReduce Programmers specify two functions: map (k, v) → * reduce (k’, v’) → * All values with the same key are sent to the same reducer The execution framework handles everything else…

21. map Shuffle and Sort: aggregate values by keys reduce k1k1 k2k2 k3k3 k4k4 k5k5 k6k6 v1v1 v2v2 v3v3 v4v4 v5v5 v6v6 ba12cc36ac52bc78 a15b27c2368 r1r1 s1s1 r2r2 s2s2 r3r3 s3s3

22. MapReduce Programmers specify two functions: map (k, v) → * reduce (k’, v’) → * All values with the same key are sent to the same reducer The execution framework handles everything else… What’s “everything else”?

23. MapReduce “Runtime” Handles scheduling Assigns workers to map and reduce tasks Handles “data distribution” Moves processes to data Handles synchronization Gathers, sorts, and shuffles intermediate data Handles errors and faults Detects worker failures and restarts Everything happens on top of a distributed FS

24. MapReduce Programmers specify two functions: map (k, v) → * reduce (k’, v’) → * All values with the same key are reduced together The execution framework handles everything else… Not quite…usually, programmers also specify: partition (k’, number of partitions) → partition for k’ Often a simple hash of the key, e.g., hash(k’) mod n Divides up key space for parallel reduce operations combine (k’, v’) → * Mini-reducers that run in memory after the map phase Used as an optimization to reduce network traffic

25. combine ba12c9ac52bc78 partition map k1k1 k2k2 k3k3 k4k4 k5k5 k6k6 v1v1 v2v2 v3v3 v4v4 v5v5 v6v6 ba12cc36ac52bc78 Shuffle and Sort: aggregate values by keys reduce a15b27c298 r1r1 s1s1 r2r2 s2s2 r3r3 s3s3 c2368

26. Two more details… Barrier between map and reduce phases But we can begin copying intermediate data earlier Keys arrive at each reducer in sorted order No enforced ordering across reducers

27. “Hello World”: Word Count

28. MapReduce can refer to… The programming model The execution framework (aka “runtime”) The specific implementation Usage is usually clear from context!

29. MapReduce Implementations Google has a proprietary implementation in C++ Bindings in Java, Python Hadoop is an open-source implementation in Java Original development led by Yahoo Now an Apache open source project Emerging as the de facto big data stack Rapidly expanding software ecosystem Lots of custom research implementations For GPUs, cell processors, etc. Includes variations of the basic programming model Most of these slides are focused on Hadoop

30. split 0 split 1 split 2 split 3 split 4 worker Master User Program output file 0 output file 1 (1) submit (2) schedule map (2) schedule reduce (3) read (4) local write (5) remote read (6) write Input files Map phase Intermediate files (on local disk) Reduce phase Output files Adapted from (Dean and Ghemawat, OSDI 2004)

31. How do we get data to the workers? Compute Nodes NAS SAN What’s the problem here?

32. Distributed File System Don’t move data to workers… move workers to the data! Store data on the local disks of nodes in the cluster Start up the workers on the node that has the data local A distributed file system is the answer GFS (Google File System) for Google’s MapReduce HDFS (Hadoop Distributed File System) for Hadoop

33. GFS: Assumptions Commodity hardware over “exotic” hardware Scale “out”, not “up” High component failure rates Inexpensive commodity components fail all the time “Modest” number of huge files Multi-gigabyte files are common, if not encouraged Files are write-once, mostly appended to Perhaps concurrently Large streaming reads over random access High sustained throughput over low latency GFS slides adapted from material by (Ghemawat et al., SOSP 2003)

34. GFS: Design Decisions Files stored as chunks Fixed size (64MB) Reliability through replication Each chunk replicated across 3+ chunkservers Single master to coordinate access, keep metadata Simple centralized management No data caching Little benefit due to large datasets, streaming reads Simplify the API Push some of the issues onto the client (e.g., data layout) HDFS = GFS clone (same basic ideas)

35. From GFS to HDFS Terminology differences: GFS master = Hadoop namenode GFS chunkservers = Hadoop datanodes Functional differences: File appends in HDFS is relatively new HDFS performance is (likely) slower For the most part, we’ll use the Hadoop terminology…

36. Adapted from (Ghemawat et al., SOSP 2003) (file name, block id) (block id, block location) instructions to datanode datanode state (block id, byte range) block data HDFS namenode HDFS datanode Linux file system … HDFS datanode Linux file system … File namespace /foo/bar block 3df2 Application HDFS Client HDFS Architecture

37. Namenode Responsibilities Managing the file system namespace: Holds file/directory structure, metadata, file-to-block mapping, access permissions, etc. Coordinating file operations: Directs clients to datanodes for reads and writes No data is moved through the namenode Maintaining overall health: Periodic communication with the datanodes Block re-replication and rebalancing Garbage collection

38. Putting everything together… datanode daemon Linux file system … tasktracker slave node datanode daemon Linux file system … tasktracker slave node datanode daemon Linux file system … tasktracker slave node namenode namenode daemon job submission node jobtracker

39. MapReduce Algorithm Design Setting the stage Introduction to MapReduce MapReduce algorithm design Text retrieval Managing relational data Graph algorithms Beyond MapReduce

40. MapReduce: Recap Programmers must specify: map (k, v) → * reduce (k’, v’) → * All values with the same key are reduced together Optionally, also: partition (k’, number of partitions) → partition for k’ Often a simple hash of the key, e.g., hash(k’) mod n Divides up key space for parallel reduce operations combine (k’, v’) → * Mini-reducers that run in memory after the map phase Used as an optimization to reduce network traffic The execution framework handles everything else…

41. combine ba12c9ac52bc78 partition map k1k1 k2k2 k3k3 k4k4 k5k5 k6k6 v1v1 v2v2 v3v3 v4v4 v5v5 v6v6 ba12cc36ac52bc78 Shuffle and Sort: aggregate values by keys reduce a15b27c298 r1r1 s1s1 r2r2 s2s2 r3r3 s3s3

42. “Everything Else” The execution framework handles everything else… Scheduling: assigns workers to map and reduce tasks “Data distribution”: moves processes to data Synchronization: gathers, sorts, and shuffles intermediate data Errors and faults: detects worker failures and restarts Limited control over data and execution flow All algorithms must expressed in m, r, c, p You don’t know: Where mappers and reducers run When a mapper or reducer begins or finishes Which input a particular mapper is processing Which intermediate key a particular reducer is processing

43. Tools for Synchronization Cleverly-constructed data structures Bring partial results together Sort order of intermediate keys Control order in which reducers process keys Partitioner Control which reducer processes which keys Preserving state in mappers and reducers Capture dependencies across multiple keys and values

44. Preserving State Mapper object configure map close state one object per task Reducer object configure reduce close state one call per input key-value pair one call per intermediate key API initialization hook API cleanup hook

45. Scalable Hadoop Algorithms: Themes Avoid object creation Inherently costly operation Garbage collection Avoid buffering Limited heap size Works for small datasets, but won’t scale!

46. Importance of Local Aggregation Ideal scaling characteristics: Twice the data, twice the running time Twice the resources, half the running time Why can’t we achieve this? Synchronization requires communication Communication kills performance Thus… avoid communication! Reduce intermediate data via local aggregation Combiners can help

47. Shuffle and Sort Mapper Reducer other mappers other reducers circular buffer (in memory) spills (on disk) merged spills (on disk) intermediate files (on disk) Combiner

48. Word Count: Baseline What’s the impact of combiners?

49. Word Count: Version 1 Are combiners still needed?

50. Word Count: Version 2 Are combiners still needed? Key: preserve state across input key-value pairs!

51. Design Pattern for Local Aggregation “In-mapper combining” Fold the functionality of the combiner into the mapper by preserving state across multiple map calls Advantages Speed Why is this faster than actual combiners? Disadvantages Explicit memory management required Potential for order-dependent bugs

52. Combiner Design Combiners and reducers share same method signature Sometimes, reducers can serve as combiners Often, not… Remember: combiner are optional optimizations Should not affect algorithm correctness May be run 0, 1, or multiple times Example: find average of all integers associated with the same key

53. Computing the Mean: Version 1 Why can’t we use reducer as combiner?

54. Computing the Mean: Version 2 Why doesn’t this work?

55. Computing the Mean: Version 3 Fixed?

56. Computing the Mean: Version 4 Are combiners still needed?

57. “Count and Normalize” Many algorithms reduce to estimating relative frequencies: In the case of EM, pseudo-counts instead of actual counts For a large class of algorithms: intuition is the same, just varying complexity in terms of bookkeeping Let’s start with the intuition…

58. Algorithm Design: Running Example Term co-occurrence matrix for a text collection M = N x N matrix (N = vocabulary size) M ij : number of times i and j co-occur in some context (for concreteness, let’s say context = sentence) Why? Distributional profiles as a way of measuring semantic distance Semantic distance useful for many language processing tasks

59. MapReduce: Large Counting Problems Term co-occurrence matrix for a text collection = specific instance of a large counting problem A large event space (number of terms) A large number of observations (the collection itself) Goal: keep track of interesting statistics about the events Basic approach Mappers generate partial counts Reducers aggregate partial counts How do we aggregate partial counts efficiently?

60. First Try: “Pairs” Each mapper takes a sentence: Generate all co-occurring term pairs For all pairs, emit (a, b) → count Reducers sum up counts associated with these pairs Use combiners!

61. Pairs: Pseudo-Code

62. “Pairs” Analysis Advantages Easy to implement, easy to understand Disadvantages Lots of pairs to sort and shuffle around (upper bound?) Not many opportunities for combiners to work

63. Another Try: “Stripes” Idea: group together pairs into an associative array Each mapper takes a sentence: Generate all co-occurring term pairs For each term, emit a → { b: count b, c: count c, d: count d … } Reducers perform element-wise sum of associative arrays (a, b) → 1 (a, c) → 2 (a, d) → 5 (a, e) → 3 (a, f) → 2 a → { b: 1, c: 2, d: 5, e: 3, f: 2 } a → { b: 1, d: 5, e: 3 } a → { b: 1, c: 2, d: 2, f: 2 } a → { b: 2, c: 2, d: 7, e: 3, f: 2 } + Key: cleverly-constructed data structure brings together partial results

64. Stripes: Pseudo-Code

65. “Stripes” Analysis Advantages Far less sorting and shuffling of key-value pairs Can make better use of combiners Disadvantages More difficult to implement Underlying object more heavyweight Fundamental limitation in terms of size of event space

66. Cluster size: 38 cores Data Source: Associated Press Worldstream (APW) of the English Gigaword Corpus (v3), which contains 2.27 million documents (1.8 GB compressed, 5.7 GB uncompressed)

68. Relative Frequencies How do we estimate relative frequencies from counts? Why do we want to do this? How do we do this with MapReduce?

69. f(B|A): “Stripes” Easy! One pass to compute (a, *) Another pass to directly compute f(B|A) a → {b 1 :3, b 2 :12, b 3 :7, b 4 :1, … }

70. f(B|A): “Pairs” For this to work: Must emit extra (a, *) for every b n in mapper Must make sure all a’s get sent to same reducer (use partitioner) Must make sure (a, *) comes first (define sort order) Must hold state in reducer across different key-value pairs (a, b 1 ) → 3 (a, b 2 ) → 12 (a, b 3 ) → 7 (a, b 4 ) → 1 … (a, *) → 32 (a, b 1 ) → 3 / 32 (a, b 2 ) → 12 / 32 (a, b 3 ) → 7 / 32 (a, b 4 ) → 1 / 32 … Reducer holds this value in memory

71. “Order Inversion” Common design pattern Computing relative frequencies requires marginal counts But marginal cannot be computed until you see all counts Buffering is a bad idea! Trick: getting the marginal counts to arrive at the reducer before the joint counts Optimizations Apply in-memory combining pattern to accumulate marginal counts Should we apply combiners?

72. Synchronization: Pairs vs. Stripes Approach 1: turn synchronization into an ordering problem Sort keys into correct order of computation Partition key space so that each reducer gets the appropriate set of partial results Hold state in reducer across multiple key-value pairs to perform computation Illustrated by the “pairs” approach Approach 2: construct data structures that bring partial results together Each reducer receives all the data it needs to complete the computation Illustrated by the “stripes” approach

73. Secondary Sorting MapReduce sorts input to reducers by key Values may be arbitrarily ordered What if want to sort value also? E.g., k → (v 1, r), (v 3, r), (v 4, r), (v 8, r)…

74. Secondary Sorting: Solutions Solution 1: Buffer values in memory, then sort Why is this a bad idea? Solution 2: “Value-to-key conversion” design pattern: form composite intermediate key, (k, v 1 ) Let execution framework do the sorting Preserve state across multiple key-value pairs to handle processing Anything else we need to do?

75. Recap: Tools for Synchronization Cleverly-constructed data structures Bring data together Sort order of intermediate keys Control order in which reducers process keys Partitioner Control which reducer processes which keys Preserving state in mappers and reducers Capture dependencies across multiple keys and values

76. Issues and Tradeoffs Number of key-value pairs Object creation overhead Time for sorting and shuffling pairs across the network Size of each key-value pair De/serialization overhead Local aggregation Opportunities to perform local aggregation varies Combiners make a big difference Combiners vs. in-mapper combining RAM vs. disk vs. network

77. Text Retrieval Setting the stage Introduction to MapReduce MapReduce algorithm design Text retrieval Managing relational data Graph algorithms Beyond MapReduce

78. Abstract IR Architecture Documents Query Hits Representation Function Representation Function Query RepresentationDocument Representation Comparison Function Index offlineonline document acquisition (e.g., web crawling)

79. “Bag of Words” Terms weights computed as functions of: Term frequency Collection frequency Document frequency Average document length … Well-known weighting functions TF.IDF BM25 Dirichlet scores (LM framework) Similarity boils down to inner products of feature vectors:

80. 2 1 1 2 1 1 1 1 1 1 1 Inverted Index 2 2 1 1 2 2 1 1 1 1 1 1 123 1 1 1 1 1 1 4 1 1 1 1 1 1 1 1 1 1 1 1 2 2 1 1 tf df blue cat egg fish green ham hat one 1 1 1 1 1 1 2 1 blue cat egg fish green ham hat one 1 1 1 1 red 1 1 1 1 two 1 red 1 two one fish, two fish Doc 1 red fish, blue fish Doc 2 cat in the hat Doc 3 green eggs and ham Doc 4 3 4 1 4 4 3 2 1 2 2 1

81. [2,4] [3] [2,4] [2] [1] [3] [2] [1] [3] 2 1 1 2 1 1 1 1 1 1 1 Inverted Index: Positional Information 2 2 1 1 2 2 1 1 1 1 1 1 123 1 1 1 1 1 1 4 1 1 1 1 1 1 1 1 1 1 1 1 2 2 1 1 tf df blue cat egg fish green ham hat one 1 1 1 1 1 1 2 1 blue cat egg fish green ham hat one 1 1 1 1 red 1 1 1 1 two 1 red 1 two one fish, two fish Doc 1 red fish, blue fish Doc 2 cat in the hat Doc 3 green eggs and ham Doc 4 3 4 1 4 4 3 2 1 2 2 1

82. Retrieval in a Nutshell Look up postings lists corresponding to query terms Traverse postings for each query term Store partial query-document scores in accumulators Select top k results to return

83. Retrieval: Document-at-a-Time Evaluate documents one at a time (score all query terms) Tradeoffs Small memory footprint (good) Must read through all postings (bad), but skipping possible More disk seeks (bad), but blocking possible fish 2131231921343580 … blue 21192135 … Accumulators (e.g. priority queue) Document score in top k? Yes: Insert document score, extract-min if queue too large No: Do nothing

84. Retrieval: Query-at-a-Time Evaluate documents one query term at a time Usually, starting from most rare term (often with tf-sorted postings) Tradeoffs Early termination heuristics (good) Large memory footprint (bad), but filtering heuristics possible fish 2131231921343580 … blue 21192135 … Accumulators (e.g., hash) Score {q=x} (doc n) = s

85. MapReduce it? The indexing problem Scalability is critical Must be relatively fast, but need not be real time Fundamentally a batch operation Incremental updates may or may not be important For the web, crawling is a challenge in itself The retrieval problem Must have sub-second response time For the web, only need relatively few results Perfect for MapReduce! Uh… not so good…

86. Indexing: Performance Analysis Fundamentally, a large sorting problem Terms usually fit in memory Postings usually don’t How is it done on a single machine? How can it be done with MapReduce? First, let’s characterize the problem size: Size of vocabulary Size of postings

87. Vocabulary Size: Heaps’ Law Heaps’ Law: linear in log-log space Vocabulary size grows unbounded! M is vocabulary size T is collection size (number of documents) k and b are constants Typically, k is between 30 and 100, b is between 0.4 and 0.6

88. Heaps’ Law for RCV1 Reuters-RCV1 collection: 806,791 newswire documents (Aug 20, 1996-August 19, 1997) k = 44 b = 0.49 First 1,000,020 terms: Predicted = 38,323 Actual = 38,365 Manning, Raghavan, Schütze, Introduction to Information Retrieval (2008)

89. Postings Size: Zipf’s Law Zipf’s Law: (also) linear in log-log space Specific case of Power Law distributions In other words: A few elements occur very frequently Many elements occur very infrequently cf is the collection frequency of i-th common term c is a constant

90. Zipf’s Law for RCV1 Reuters-RCV1 collection: 806,791 newswire documents (Aug 20, 1996-August 19, 1997) Fit isn’t that good… but good enough! Manning, Raghavan, Schütze, Introduction to Information Retrieval (2008)

91. MapReduce: Index Construction Map over all documents Emit term as key, (docno, tf) as value Emit other information as necessary (e.g., term position) Sort/shuffle: group postings by term Reduce Gather and sort the postings (e.g., by docno or tf) Write postings to disk MapReduce does all the heavy lifting!

92. 1 1 2 1 1 22 1 1 1 1 1 1 1 1 2 Inverted Indexing with MapReduce 1 one 1 two 1 fish one fish, two fish Doc 1 2 red 2 blue 2 fish red fish, blue fish Doc 2 3 cat 3 hat cat in the hat Doc 3 1 fish 2 1 one 1 two 2 red 3 cat 2 blue 3 hat Shuffle and Sort: aggregate values by keys Map Reduce

93. Inverted Indexing: Pseudo-Code

94. [2,4] [1] [3] [1] [2] [1] [3] [2] [3] [2,4] [1] [2,4] [1] [3] 1 1 2 1 1 2 1 1 22 1 1 1 1 1 1 Positional Indexes 1 one 1 two 1 fish 2 red 2 blue 2 fish 3 cat 3 hat 1 fish 2 1 one 1 two 2 red 3 cat 2 blue 3 hat Shuffle and Sort: aggregate values by keys Map Reduce one fish, two fish Doc 1 red fish, blue fish Doc 2 cat in the hat Doc 3

95. Inverted Indexing: Pseudo-Code What’s the problem?

96. Scalability Bottleneck Initial implementation: terms as keys, postings as values Reducers must buffer all postings associated with key (to sort) What if we run out of memory to buffer postings? Uh oh!

97. [2,4] [9] [1,8,22] [23] [8,41] [2,9,76] [2,4] [9] [1,8,22] [23] [8,41] [2,9,76] 2 1 3 1 2 3 Another Try… 1 fish 9 21 (values)(key) 34 35 80 1 fish 9 21 (values)(keys) 34 35 80 fish How is this different? Let the framework do the sorting Term frequency implicitly stored Directly write compressed postings Where have we seen this before?

98. 213123 213123 Postings Encoding 1 fish 921343580 … 1 fish 81213145 … Conceptually: In Practice: Don’t encode docnos, encode gaps (or d-gaps) But it’s not obvious that this save space…

99. Overview of Index Compression Byte-aligned vs. bit-aligned Non-parameterized bit-aligned Unary codes  codes  codes Parameterized bit-aligned Golomb codes (local Bernoulli model) Block-based methods Simple-9 PForDelta Want more detail? Start with Managing Gigabytes by Witten, Moffat, and Bell!

100. Index Compression: Performance Witten, Moffat, Bell, Managing Gigabytes (1999) Unary2621918 Binary1520  6.516.63  6.236.38 Golomb6.095.84 BibleTREC Bible: King James version of the Bible; 31,101 verses (4.3 MB) TREC: TREC disks 1+2; 741,856 docs (2070 MB) One common approach Comparison of Index Size (bits per pointer) Issue: For Golomb compression, optimal b ~ 0.69 (N/df) Which means different b for every term!

101. Chicken and Egg? 1 fish 9 [2,4] [9] 21 [1,8,22] (value)(key) 34 [23] 35 [8,41] 80 [2,9,76] fish Write directly to disk But wait! How do we set the Golomb parameter b? We need the df to set b… But we don’t know the df until we’ve seen all postings! … Optimal b ~ 0.69 (N/df) Sound familiar?

102. Getting the df In the mapper: Emit “special” key-value pairs to keep track of df In the reducer: Make sure “special” key-value pairs come first: process them to determine df Remember: proper partitioning!

103. Getting the df: Modified Mapper one fish, two fish Doc 1 1 fish [2,4] (value)(key) 1 one [1] 1 two [3]  fish [1]  one [1]  two [1] Input document… Emit normal key-value pairs… Emit “special” key-value pairs to keep track of df…

104. Getting the df: Modified Reducer 1 fish 9 [2,4] [9] 21 [1,8,22] (value)(key) 34 [23] 35 [8,41] 80 [2,9,76] fish Write compressed postings  fish [63][82][27] … … First, compute the df by summing contributions from all “special” key-value pair… Compute Golomb parameter b… Important: properly define sort order to make sure “special” key-value pairs come first! Where have we seen this before?

105. MapReduce it? The indexing problem Scalability is paramount Must be relatively fast, but need not be real time Fundamentally a batch operation Incremental updates may or may not be important For the web, crawling is a challenge in itself The retrieval problem Must have sub-second response time For the web, only need relatively few results

106. Retrieval with MapReduce? MapReduce is fundamentally batch-oriented Optimized for throughput, not latency Startup of mappers and reducers is expensive MapReduce is not suitable for real-time queries! Use separate infrastructure for retrieval…

107. Important Ideas Partitioning (for scalability) Replication (for redundancy) Caching (for speed) Routing (for load balancing) The rest is just details!

108. Term vs. Document Partitioning … T D T1T1 T2T2 T3T3 D T … D1D1 D2D2 D3D3 Term Partitioning Document Partitioning

109. partitions …… … … … … … … replicas brokers Typical Search Architecture

110. Managing Relational Data Setting the stage Introduction to MapReduce MapReduce algorithm design Text retrieval Managing relational data Graph algorithms Beyond MapReduce

111. Managing Relational Data In the “good old days”, organizations used relational databases to manage big data Then along came Hadoop… Where does MapReduce fit in? BTW, Hadoop is “hot” in the SIGMOD community…

112. Relational Databases vs. MapReduce Relational databases: Multipurpose: analysis and transactions; batch and interactive Data integrity via ACID transactions Lots of tools in software ecosystem (for ingesting, reporting, etc.) Supports SQL (and SQL integration, e.g., JDBC) Automatic SQL query optimization MapReduce (Hadoop): Designed for large clusters, fault tolerant Data is accessed in “native format” Supports many query languages Programmers retain control over performance Open source Source: O’Reilly Blog post by Joseph Hellerstein (11/19/2008)

113. Database Workloads OLTP (online transaction processing) Typical applications: e-commerce, banking, airline reservations User facing: real-time, low latency, highly-concurrent Tasks: relatively small set of “standard” transactional queries Data access pattern: random reads, updates, writes (involving relatively small amounts of data) OLAP (online analytical processing) Typical applications: business intelligence, data mining Back-end processing: batch workloads, less concurrency Tasks: complex analytical queries, often ad hoc Data access pattern: table scans, large amounts of data involved per query

114. One Database or Two? Downsides of co-existing OLTP and OLAP workloads Poor memory management Conflicting data access patterns Variable latency Solution: separate databases User-facing OLTP database for high-volume transactions Data warehouse for OLAP workloads How do we connect the two?

115. OLTP/OLAP Architecture OLTPOLAP ETL (Extract, Transform, and Load)

116. OLTP/OLAP Integration OLTP database for user-facing transactions Retain records of all activity Periodic ETL (e.g., nightly) Extract-Transform-Load (ETL) Extract records from source Transform: clean data, check integrity, aggregate, etc. Load into OLAP database OLAP database for data warehousing Business intelligence: reporting, ad hoc queries, data mining, etc. Feedback to improve OLTP services

117. Business Intelligence Premise: more data leads to better business decisions Periodic reporting as well as ad hoc queries Analysts, not programmers (importance of tools and dashboards) Examples: Slicing-and-dicing activity by different dimensions to better understand the marketplace Analyzing log data to improve OLTP experience Analyzing log data to better optimize ad placement Analyzing purchasing trends for better supply-chain management Mining for correlations between otherwise unrelated activities

118. OLTP/OLAP Architecture: Hadoop? OLTPOLAP ETL (Extract, Transform, and Load) Hadoop here? What about here?

119. OLTP/OLAP/Hadoop Architecture OLTPOLAP ETL (Extract, Transform, and Load) Hadoop Why does this make sense?

120. ETL Bottleneck Reporting is often a nightly task: ETL is often slow: why? What happens if processing 24 hours of data takes longer than 24 hours? Hadoop is perfect: Most likely, you already have some data warehousing solution Ingest is limited by speed of HDFS Scales out with more nodes Massively parallel Ability to use any processing tool Much cheaper than parallel databases ETL is a batch process anyway!

121. Working Scenario Two tables: User demographics (gender, age, income, etc.) User page visits (URL, time spent, etc.) Analyses we might want to perform: Statistics on demographic characteristics Statistics on page visits Statistics on page visits by URL Statistics on page visits by demographic characteristic … How to perform common relational operations in MapReduce… Except, don’t! (later)

122. Relational Algebra Primitives Projection (  ) Selection (  ) Cartesian product (  ) Set union (  ) Set difference (  ) Rename (  ) Other operations Join ( ⋈ ) Group by… aggregation …

123. Projection R1R1 R2R2 R3R3 R4R4 R5R5 R1R1 R2R2 R3R3 R4R4 R5R5

124. Projection in MapReduce Easy! Map over tuples, emit new tuples with appropriate attributes No reducers, unless for regrouping or resorting tuples Alternatively: perform in reducer, after some other processing Basically limited by HDFS streaming speeds Speed of encoding/decoding tuples becomes important Relational databases take advantage of compression Semistructured data? No problem!

125. Selection R1R1 R2R2 R3R3 R4R4 R5R5 R1R1 R3R3

126. Selection in MapReduce Easy! Map over tuples, emit only tuples that meet criteria No reducers, unless for regrouping or resorting tuples Alternatively: perform in reducer, after some other processing Basically limited by HDFS streaming speeds Speed of encoding/decoding tuples becomes important Relational databases take advantage of compression Semistructured data? No problem!

127. Group by… Aggregation Example: What is the average time spent per URL? In SQL: SELECT url, AVG(time) FROM visits GROUP BY url In MapReduce: Map over tuples, emit time, keyed by url Framework automatically groups values by keys Compute average in reducer Optimize with combiners

128. Relational Joins R1R1 R2R2 R3R3 R4R4 S1S1 S2S2 S3S3 S4S4 R1R1 S2S2 R2R2 S4S4 R3R3 S1S1 R4R4 S3S3

129. Types of Relationships One-to-OneOne-to-Many Many-to-Many

130. Join Algorithms in MapReduce Reduce-side join Map-side join In-memory join Striped variant Memcached variant

131. Reduce-side Join Basic idea: group by join key Map over both sets of tuples Emit tuple as value with join key as the intermediate key Execution framework brings together tuples sharing the same key Perform actual join in reducer Similar to a “sort-merge join” in database terminology Two variants 1-to-1 joins 1-to-many and many-to-many joins

132. Reduce-side Join: 1-to-1 R1R1 R4R4 S2S2 S3S3 R1R1 R4R4 S2S2 S3S3 keysvalues Map R1R1 R4R4 S2S2 S3S3 keysvalues Reduce Note: no guarantee if R is going to come first or S

133. Reduce-side Join: 1-to-many R1R1 S2S2 S3S3 R1R1 S2S2 S3S3 S9S9 keysvalues Map R1R1 S2S2 keysvalues Reduce S9S9 S3S3 … What’s the problem?

134. Reduce-side Join: V-to-K Conversion R1R1 keysvalues In reducer… S2S2 S3S3 S9S9 R4R4 S3S3 S7S7 New key encountered: hold in memory Cross with records from other set New key encountered: hold in memory Cross with records from other set

135. Reduce-side Join: many-to-many R1R1 keysvalues In reducer… S2S2 S3S3 S9S9 Hold in memory Cross with records from other set R5R5 R8R8 What’s the problem?

136. Map-side Join: Basic Idea Assume two datasets are sorted by the join key: R1R1 R2R2 R3R3 R4R4 S1S1 S2S2 S3S3 S4S4 A sequential scan through both datasets to join (called a “merge join” in database terminology)

137. Map-side Join: Parallel Scans If datasets are sorted by join key, join can be accomplished by a scan over both datasets How can we accomplish this in parallel? Partition and sort both datasets in the same manner In MapReduce: Map over one dataset, read from other corresponding partition No reducers necessary (unless to repartition or resort) Consistently partitioned datasets: realistic to expect?

138. In-Memory Join Basic idea: load one dataset into memory, stream over other dataset Works if R << S and R fits into memory Called a “hash join” in database terminology MapReduce implementation Distribute R to all nodes Map over S, each mapper loads R in memory, hashed by join key For every tuple in S, look up join key in R No reducers, unless for regrouping or resorting tuples

139. In-Memory Join: Variants Striped variant: R too big to fit into memory? Divide R into R 1, R 2, R 3, … s.t. each R n fits into memory Perform in-memory join:  n, R n ⋈ S Take the union of all join results Memcached join: Load R into memcached Replace in-memory hash lookup with memcached lookup

140. Which join to use? In-memory join > map-side join > reduce-side join Why? Limitations of each? In-memory join: memory Map-side join: sort order and partitioning Reduce-side join: general purpose

141. Key Features in Databases Common optimizations in relational databases Reducing the amount of data to read Reducing the amount of tuples to decode Data placement Query planning and cost estimation Same ideas can be applied to MapReduce For example, column stores in Google Dremel A few commercialized products Many research prototypes

142. One size does not fit all… Databases when: You know what the question is: query optimizers work well Well-specified schema, clean data MapReduce when: You don’t necessarily know what the question is: go brute force Exploratory data analysis Semi-structured, noisy, diverse data ETL is the insight-generation process

143. Graph Algorithms Setting the stage Introduction to MapReduce MapReduce algorithm design Text retrieval Managing relational data Graph algorithms Beyond MapReduce

144. What’s a graph? G = (V,E), where V represents the set of vertices (nodes) E represents the set of edges (links) Both vertices and edges may contain additional information Different types of graphs: Directed vs. undirected edges Presence or absence of cycles Graphs are everywhere: Hyperlink structure of the Web Physical structure of computers on the Internet Interstate highway system Social networks

145. Source: Wikipedia (Königsberg)

146. Some Graph Problems Finding shortest paths Routing Internet traffic and UPS trucks Finding minimum spanning trees Telco laying down fiber Finding Max Flow Airline scheduling Identify “special” nodes and communities Breaking up terrorist cells, spread of avian flu Bipartite matching Monster.com, Match.com And of course... PageRank

147. Graphs and MapReduce Graph algorithms typically involve: Performing computations at each node: based on node features, edge features, and local link structure Propagating computations: “traversing” the graph Key questions: How do you represent graph data in MapReduce? How do you traverse a graph in MapReduce?

148. Representing Graphs G = (V, E) Two common representations Adjacency matrix Adjacency list

149. Adjacency Matrices Represent a graph as an n x n square matrix M n = |V| M ij = 1 means a link from node i to j 1234 10101 21011 31000 41010 1 1 2 2 3 3 4 4

150. Adjacency Matrices: Critique Advantages: Amenable to mathematical manipulation Iteration over rows and columns corresponds to computations on outlinks and inlinks Disadvantages: Lots of zeros for sparse matrices Lots of wasted space

151. Adjacency Lists Take adjacency matrices… and throw away all the zeros 1: 2, 4 2: 1, 3, 4 3: 1 4: 1, 3 1234 10101 21011 31000 41010

152. Adjacency Lists: Critique Advantages: Much more compact representation Easy to compute over outlinks Disadvantages: Much more difficult to compute over inlinks

153. Single Source Shortest Path Problem: find shortest path from a source node to one or more target nodes Shortest might also mean lowest weight or cost First, a refresher: Dijkstra’s Algorithm

154. Dijkstra’s Algorithm Example 0 0     10 5 23 2 1 9 7 46 Example from CLR

155. Dijkstra’s Algorithm Example 0 0 10 5 5   Example from CLR 10 5 23 2 1 9 7 46

156. Dijkstra’s Algorithm Example 0 0 8 5 5 14 7 7 Example from CLR 10 5 23 2 1 9 7 46

157. Dijkstra’s Algorithm Example 0 0 8 8 5 5 13 7 7 Example from CLR 10 5 23 2 1 9 7 46

158. Dijkstra’s Algorithm Example 0 0 8 8 5 5 9 9 7 7 1 Example from CLR 10 5 23 2 1 9 7 46

159. Dijkstra’s Algorithm Example 0 0 8 8 5 5 9 9 7 7 Example from CLR 10 5 23 2 1 9 7 46

160. Single Source Shortest Path Problem: find shortest path from a source node to one or more target nodes Shortest might also mean lowest weight or cost Single processor machine: Dijkstra’s Algorithm MapReduce: parallel Breadth-First Search (BFS)

161. Finding the Shortest Path Consider simple case of equal edge weights Solution to the problem can be defined inductively Here’s the intuition: Define: b is reachable from a if b is on adjacency list of a D ISTANCE T O (s) = 0 For all nodes p reachable from s, D ISTANCE T O (p) = 1 For all nodes n reachable from some other set of nodes M, D ISTANCE T O (n) = 1 + min(D ISTANCE T O (m), m  M) s s m3m3 m3m3 m2m2 m2m2 m1m1 m1m1 n n … … … d1d1 d2d2 d3d3

162. Source: Wikipedia (Wave)

163. Visualizing Parallel BFS n0n0 n0n0 n3n3 n3n3 n2n2 n2n2 n1n1 n1n1 n7n7 n7n7 n6n6 n6n6 n5n5 n5n5 n4n4 n4n4 n9n9 n9n9 n8n8 n8n8

164. From Intuition to Algorithm Data representation: Key: node n Value: d (distance from start), adjacency list (list of nodes reachable from n) Initialization: for all nodes except for start node, d =  Mapper:  m  adjacency list: emit (m, d + 1) Sort/Shuffle Groups distances by reachable nodes Reducer: Selects minimum distance path for each reachable node Additional bookkeeping needed to keep track of actual path

165. Multiple Iterations Needed Each MapReduce iteration advances the “known frontier” by one hop Subsequent iterations include more and more reachable nodes as frontier expands Multiple iterations are needed to explore entire graph Preserving graph structure: Problem: Where did the adjacency list go? Solution: mapper emits (n, adjacency list) as well

166. BFS Pseudo-Code

167. Stopping Criterion How many iterations are needed in parallel BFS (equal edge weight case)? When a node is first “discovered”, we’re guaranteed to have found the shortest path

168. Comparison to Dijkstra Dijkstra’s algorithm is more efficient At any step it only pursues edges from the minimum-cost path inside the frontier MapReduce explores all paths in parallel Lots of “waste” Useful work is only done at the “frontier” Why can’t we do better using MapReduce?

169. Weighted Edges Now add positive weights to the edges Simple change: adjacency list now includes a weight w for each edge In mapper, emit (m, d + w p ) instead of (m, d + 1) for each node m That’s it?

170. Stopping Criterion How many iterations are needed in parallel BFS (positive edge weight case)? When a node is first “discovered”, we’re guaranteed to have found the shortest path Not true!

171. Additional Complexities s p q r search frontier 10 n1n1 n2n2 n3n3 n4n4 n5n5 n6n6 n7n7 n8n8 n9n9 1 1 1 1 1 1 1 1

172. Stopping Criterion How many iterations are needed in parallel BFS (positive edge weight case)? Practicalities of implementation in MapReduce

173. Graphs and MapReduce Graph algorithms typically involve: Performing computations at each node: based on node features, edge features, and local link structure Propagating computations: “traversing” the graph Generic recipe: Represent graphs as adjacency lists Perform local computations in mapper Pass along partial results via outlinks, keyed by destination node Perform aggregation in reducer on inlinks to a node Iterate until convergence: controlled by external “driver” Don’t forget to pass the graph structure between iterations

174. Random Walks Over the Web Random surfer model: User starts at a random Web page User randomly clicks on links, surfing from page to page PageRank Characterizes the amount of time spent on any given page Mathematically, a probability distribution over pages PageRank captures notions of page importance Correspondence to human intuition? One of thousands of features used in web search Note: query-independent

175. Given page x with inlinks t 1 …t n, where C(t) is the out-degree of t  is probability of random jump N is the total number of nodes in the graph PageRank: Defined X X t1t1 t1t1 t2t2 t2t2 tntn tntn …

176. Computing PageRank Properties of PageRank Can be computed iteratively Effects at each iteration are local Sketch of algorithm: Start with seed PR i values Each page distributes PR i “credit” to all pages it links to Each target page adds up “credit” from multiple in-bound links to compute PR i+1 Iterate until values converge

177. Simplified PageRank First, tackle the simple case: No random jump factor No dangling links Then, factor in these complexities… Why do we need the random jump? Where do dangling links come from?

178. Sample PageRank Iteration (1) n 1 (0.2) n 4 (0.2) n 3 (0.2) n 5 (0.2) n 2 (0.2) 0.1 0.2 0.1 0.066 n 1 (0.066) n 4 (0.3) n 3 (0.166) n 5 (0.3) n 2 (0.166) Iteration 1

179. Sample PageRank Iteration (2) n 1 (0.066) n 4 (0.3) n 3 (0.166) n 5 (0.3) n 2 (0.166) 0.033 0.3 0.166 0.083 0.1 n 1 (0.1) n 4 (0.2) n 3 (0.183) n 5 (0.383) n 2 (0.133) Iteration 2

180. PageRank in MapReduce n 5 [n 1, n 2, n 3 ]n 1 [n 2, n 4 ]n 2 [n 3, n 5 ]n 3 [n 4 ]n 4 [n 5 ] n2n2 n4n4 n3n3 n5n5 n1n1 n2n2 n3n3 n4n4 n5n5 n2n2 n4n4 n3n3 n5n5 n1n1 n2n2 n3n3 n4n4 n5n5 n 5 [n 1, n 2, n 3 ]n 1 [n 2, n 4 ]n 2 [n 3, n 5 ]n 3 [n 4 ]n 4 [n 5 ] Map Reduce

181. PageRank Pseudo-Code

182. Complete PageRank Two additional complexities What is the proper treatment of dangling nodes? How do we factor in the random jump factor? Solution: Second pass to redistribute “missing PageRank mass” and account for random jumps p is PageRank value from before, p' is updated PageRank value |G| is the number of nodes in the graph m is the missing PageRank mass

183. PageRank Convergence Alternative convergence criteria Iterate until PageRank values don’t change Iterate until PageRank rankings don’t change Fixed number of iterations Convergence for web graphs?

184. Beyond PageRank Link structure is important for web search PageRank is one of many link-based features: HITS, SALSA, etc. One of many thousands of features used in ranking… Adversarial nature of web search Link spamming Spider traps Keyword stuffing …

185. Efficient Graph Algorithms: Tricks In-mapper combining: efficient local aggregation Smarter partitioning: create more opportunities for local aggregation Schimmy: avoid shuffling the graph Jimmy Lin and Michael Schatz. Design Patterns for Efficient Graph Algorithms in MapReduce. Proceedings of the Eighth Workshop on Mining and Learning with Graphs Workshop (MLG-2010), pages 78-85, July 2010, Washington, D.C.

186. In-Mapper Combining Use combiners Perform local aggregation on map output Downside: intermediate data is still materialized Better: in-mapper combining Preserve state across multiple map calls, aggregate messages in buffer, emit buffer contents at end Downside: requires memory management configure map close buffer

187. Better Partitioning Default: hash partitioning Randomly assign nodes to partitions Observation: many graphs exhibit local structure E.g., communities in social networks Better partitioning creates more opportunities for local aggregation Unfortunately, partitioning is hard! Sometimes, chick-and-egg… But cheap heuristics sometimes available For webgraphs: range partition on domain-sorted URLs

188. Schimmy Design Pattern Basic implementation contains two dataflows: Messages (actual computations) Graph structure (“bookkeeping”) Schimmy: separate the two data flows, shuffle only the messages Basic idea: merge join between graph structure and messages ST both relations sorted by join key S1S1 T1T1 S2S2 T2T2 S3S3 T3T3 both relations consistently partitioned and sorted by join key

189. S1S1 T1T1 Do the Schimmy! Schimmy = reduce side parallel merge join between graph structure and messages Consistent partitioning between input and intermediate data Mappers emit only messages (actual computation) Reducers read graph structure directly from HDFS S2S2 T2T2 S3S3 T3T3 Reducer intermediate data (messages) intermediate data (messages) intermediate data (messages) from HDFS (graph structure) from HDFS (graph structure) from HDFS (graph structure)

190. Experiments Cluster setup: 10 workers, each 2 cores (3.2 GHz Xeon), 4GB RAM, 367 GB disk Hadoop 0.20.0 on RHELS 5.3 Dataset: First English segment of ClueWeb09 collection 50.2m web pages (1.53 TB uncompressed, 247 GB compressed) Extracted webgraph: 1.4 billion links, 7.0 GB Dataset arranged in crawl order Setup: Measured per-iteration running time (5 iterations) 100 partitions

191. Results “Best Practices”

192. Results +18% 1.4b 674m

193. Results +18% -15% 1.4b 674m

194. Results +18% -15% -60% 1.4b 674m 86m

195. Results +18% -15% -60% -69% 1.4b 674m 86m

196. Beyond MapReduce Setting the stage Introduction to MapReduce MapReduce algorithm design Text retrieval Managing relational data Graph algorithms Beyond MapReduce

197. From GFS to Bigtable Google’s GFS is a distributed file system Bigtable is a storage system for structured data Built on top of GFS Solves many GFS issues: real-time access, short files, short reads Serves as a source and a sink for MapReduce jobs

198. Bigtable: Data Model A table is a sparse, distributed, persistent multidimensional sorted map Map indexed by a row key, column key, and a timestamp (row:string, column:string, time:int64)  uninterpreted byte array Supports lookups, inserts, deletes Single row transactions only Image Source: Chang et al., OSDI 2006

199. HBase Image Source: http://www.larsgeorge.com/2009/10/hbase-architecture-101-storage.html

200. Source: flickr (60in3/2338247189)

201. Source: NY Times (6/14/2006) The datacenter is the computer! It’s all about the right level of abstraction

202. Need for High-Level Languages Hadoop is great for large-data processing! But writing Java programs for everything is verbose and slow Analysts don’t want to (or can’t) write Java Solution: develop higher-level data processing languages Hive: HQL is like SQL Pig: Pig Latin is a dataflow language

203. Hive and Pig Hive: data warehousing application in Hadoop Query language is HQL, variant of SQL Tables stored on HDFS as flat files Developed by Facebook, now open source Pig: large-scale data processing system Scripts are written in Pig Latin, a dataflow language Developed by Yahoo!, now open source Roughly 1/3 of all Yahoo! internal jobs Common idea: Provide higher-level language to facilitate large-data processing Higher-level language “compiles down” to Hadoop jobs

204. Hive: Example Hive looks similar to an SQL database Relational join on two tables: Table of word counts from Shakespeare collection Table of word counts from the bible Source: Material drawn from Cloudera training VM SELECT s.word, s.freq, k.freq FROM shakespeare s JOIN bible k ON (s.word = k.word) WHERE s.freq >= 1 AND k.freq >= 1 ORDER BY s.freq DESC LIMIT 10; the2584862394 I230318854 and1967138985 to1803813526 of1670034654 a141708057 you127022720 my112974135 in1079712445 is88826884

205. Hive: Behind the Scenes SELECT s.word, s.freq, k.freq FROM shakespeare s JOIN bible k ON (s.word = k.word) WHERE s.freq >= 1 AND k.freq >= 1 ORDER BY s.freq DESC LIMIT 10; (TOK_QUERY (TOK_FROM (TOK_JOIN (TOK_TABREF shakespeare s) (TOK_TABREF bible k) (= (. (TOK_TABLE_OR_COL s) word) (. (TOK_TABLE_OR_COL k) word)))) (TOK_INSERT (TOK_DESTINATION (TOK_DIR TOK_TMP_FILE)) (TOK_SELECT (TOK_SELEXPR (. (TOK_TABLE_OR_COL s) word)) (TOK_SELEXPR (. (TOK_TABLE_OR_COL s) freq)) (TOK_SELEXPR (. (TOK_TABLE_OR_COL k) freq))) (TOK_WHERE (AND (>= (. (TOK_TABLE_OR_COL s) freq) 1) (>= (. (TOK_TABLE_OR_COL k) freq) 1))) (TOK_ORDERBY (TOK_TABSORTCOLNAMEDESC (. (TOK_TABLE_OR_COL s) freq))) (TOK_LIMIT 10))) (one or more of MapReduce jobs) (Abstract Syntax Tree)

206. Hive: Behind the Scenes STAGE DEPENDENCIES: Stage-1 is a root stage Stage-2 depends on stages: Stage-1 Stage-0 is a root stage STAGE PLANS: Stage: Stage-1 Map Reduce Alias -> Map Operator Tree: s TableScan alias: s Filter Operator predicate: expr: (freq >= 1) type: boolean Reduce Output Operator key expressions: expr: word type: string sort order: + Map-reduce partition columns: expr: word type: string tag: 0 value expressions: expr: freq type: int expr: word type: string k TableScan alias: k Filter Operator predicate: expr: (freq >= 1) type: boolean Reduce Output Operator key expressions: expr: word type: string sort order: + Map-reduce partition columns: expr: word type: string tag: 1 value expressions: expr: freq type: int Reduce Operator Tree: Join Operator condition map: Inner Join 0 to 1 condition expressions: 0 {VALUE._col0} {VALUE._col1} 1 {VALUE._col0} outputColumnNames: _col0, _col1, _col2 Filter Operator predicate: expr: ((_col0 >= 1) and (_col2 >= 1)) type: boolean Select Operator expressions: expr: _col1 type: string expr: _col0 type: int expr: _col2 type: int outputColumnNames: _col0, _col1, _col2 File Output Operator compressed: false GlobalTableId: 0 table: input format: org.apache.hadoop.mapred.SequenceFileInputFormat output format: org.apache.hadoop.hive.ql.io.HiveSequenceFileOutputFormat Stage: Stage-2 Map Reduce Alias -> Map Operator Tree: hdfs://localhost:8022/tmp/hive-training/364214370/10002 Reduce Output Operator key expressions: expr: _col1 type: int sort order: - tag: -1 value expressions: expr: _col0 type: string expr: _col1 type: int expr: _col2 type: int Reduce Operator Tree: Extract Limit File Output Operator compressed: false GlobalTableId: 0 table: input format: org.apache.hadoop.mapred.TextInputFormat output format: org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat Stage: Stage-0 Fetch Operator limit: 10

207. Pig: Example UserUrlTime Amycnn.com8:00 Amybbc.com10:00 Amyflickr.com10:05 Fredcnn.com12:00 UrlCategoryPageRank cnn.comNews0.9 bbc.comNews0.8 flickr.comPhotos0.7 espn.comSports0.9 VisitsUrl Info Task: Find the top 10 most visited pages in each category Pig Slides adapted from Olston et al. (SIGMOD 2008)

208. Pig Query Plan Load Visits Group by url Foreach url generate count Foreach url generate count Load Url Info Join on url Group by category Foreach category generate top10(urls) Foreach category generate top10(urls) Pig Slides adapted from Olston et al. (SIGMOD 2008)

209. Pig Script visits = load ‘/data/visits’ as (user, url, time); gVisits = group visits by url; visitCounts = foreach gVisits generate url, count(visits); urlInfo = load ‘/data/urlInfo’ as (url, category, pRank); visitCounts = join visitCounts by url, urlInfo by url; gCategories = group visitCounts by category; topUrls = foreach gCategories generate top(visitCounts,10); store topUrls into ‘/data/topUrls’; Pig Slides adapted from Olston et al. (SIGMOD 2008)

210. Load Visits Group by url Foreach url generate count Foreach url generate count Load Url Info Join on url Group by category Foreach category generate top10(urls) Foreach category generate top10(urls) Pig Script in Hadoop Map 1 Reduce 1 Map 2 Reduce 2 Map 3 Reduce 3 Pig Slides adapted from Olston et al. (SIGMOD 2008)

211. Different Programming Models Multitude of MapReduce hybrids, variants, etc. Mostly research prototypes A few commercial companies Dryad/DryadLINQ (Microsoft)

212. Emerging Themes Continuing quest for alternative programming models Batch vs. real-time data processing Continuing quest for better implementations MapReduce as yet another tool Growth of the Hadoop ecosystem Evolving role of MapReduce and parallel databases

213. Questions?