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(Berkeley CS186 guest lecture) Big Data Analyti...

(Berkeley CS186 guest lecture) Big Data Analytics Systems: What Goes Around Comes Around

Reynold Xin

April 09, 2015
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  1. Big Data Analytics Systems: What Goes Around Comes Around Reynold

    Xin, CS186 guest lecture @ Berkeley Apr 9, 2015
  2. Who am I? Co-founder & architect @ Databricks On-leave from

    PhD @ Berkeley AMPLab Current world record holder in 100TB sorting (Daytona GraySort Benchmark)
  3. Agenda What is “Big Data” (BD)? GFS, MapReduce, Hadoop, Spark

    What’s different between BD and DB? Assumption: you learned about parallel DB already.
  4. Gartner’s Definition “Big data” is high-volume, -velocity and -variety information

    assets that demand cost-effective, innovative forms of information processing for enhanced insight and decision making.
  5. 3 Vs of Big Data Volume: data size Velocity: rate

    of data coming in Variety (most important V): data sources, formats, workloads
  6. Challenges Data size growing (volume & velocity) –  Processing has

    to scale out over large clusters Complexity of analysis increasing (variety) –  Massive ETL (web crawling) –  Machine learning, graph processing
  7. Examples Google web index: 10+ PB Types of data: HTML

    pages, PDFs, images, videos, … Cost of 1 TB of disk: $50 Time to read 1 TB from disk: 6 hours (50 MB/s)
  8. The Big Data Problem Semi-/Un-structured data doesn’t fit well with

    databases Single machine can no longer process or even store all the data! Only solution is to distribute general storage & processing over clusters.
  9. GFS Assumptions “Component failures are the norm rather than the

    exception” “Files are huge by traditional standards” “Most files are mutated by appending new data rather than overwriting existing data” - GFS paper
  10. File Splits 17 Large  File   1100101010011100101010011100101010011100101010011100110010101001110010101001110010101001110010101001110010101001   1100101010011100101010011100101010011100101010011100110010101001110010101001110010101001110010101001110010101001  

    1100101010011100101010011100101010011100101010011100110010101001110010101001110010101001110010101001110010101001   1100101010011100101010011100101010011100101010011100110010101001110010101001110010101001110010101001110010101001   1100101010011100101010011100101010011100101010011100110010101001110010101001110010101001110010101001110010101001   1100101010011100101010011100101010011100101010011100110010101001110010101001110010101001110010101001110010101001   1100101010011100101010011100101010011100101010011100110010101001110010101001110010101001110010101001110010101001   1100101010011100101010011100101010011100101010011100110010101001110010101001110010101001110010101001110010101001   …     6440MB   Block     1   Block     2   Block     3   Block     4   Block     5   Block     6   Block     100   Block     101   64MB   64MB   64MB   64MB   64MB   64MB   …   64MB   40MB   Block     1   Block     2   Let’s color-code them Block     3   Block     4   Block     5   Block     6   Block     100   Block     101   e.g., Block Size = 64MB Files are composed of set of blocks •  Typically 64MB in size •  Each block is stored as a separate file in the local file system (e.g. NTFS)
  11. Default placement policy: –  First copy is written to the

    node creating the file (write affinity) –  Second copy is written to a data node within the same rack (to minimize cross-rack network traffic) –  Third copy is written to a data node in a different rack (to tolerate switch failures) Node  5   Node  4   Node  3   Node  2   Node  1   Block Placement 18 Block     1   Block     3   Block     2   Block     1   Block     3   Block     2   Block     3   Block     2   Block     1   e.g., Replication factor = 3 Objec<ves:  load  balancing,  fast  access,  fault  tolerance  
  12. GFS Architecture 19 NameNode   BackupNode   DataNode   DataNode

      DataNode   DataNode   DataNode   (heartbeat, balancing, replication, etc.) namespace backups
  13. Failure types: q  Disk errors and failures q  DataNode failures

    q  Switch/Rack failures q  NameNode failures q  Datacenter failures Failures, Failures, Failures GFS paper: “Component failures are the norm rather than the exception.” 20 NameNode   DataNode  
  14. Traditional Network Programming Message-passing between nodes (MPI, RPC, etc) Really

    hard to do at scale: –  How to split problem across nodes? •  Important to consider network and data locality –  How to deal with failures? •  If a typical server fails every 3 years, a 10,000-node cluster sees 10 faults/day! –  Even without failures: stragglers (a node is slow) Almost nobody does this!
  15. Data-Parallel Models Restrict the programming interface so that the system

    can do more automatically “Here’s an operation, run it on all of the data” –  I don’t care where it runs (you schedule that) –  In fact, feel free to run it twice on different nodes Does this sound familiar?
  16. MapReduce First widely popular programming model for data- intensive apps

    on clusters Published by Google in 2004 –  Processes 20 PB of data / day Popularized by open-source Hadoop project
  17. MapReduce Programming Model Data type: key-value records Map function: (Kin

    , Vin ) -> list(Kinter , Vinter ) Reduce function: (Kinter , list(Vinter )) -> list(Kout , Vout )
  18. Hello World of Big Data: Word Count the  quick  

    brown  fox   the  fox  ate   the  mouse   how  now   brown   cow   Map   Map   Map   Reduce   Reduce   brown,  2   fox,  2   how,  1   now,  1   the,  3   ate,  1   cow,  1   mouse,  1   quick,  1   the,  1   brown,  1   fox,  1   quick,  1   the,  1   fox,  1   the,  1   how,  1   now,  1   brown,  1   ate,  1   mouse,  1   cow,  1   Input   Map   Shuffle  &  Sort   Reduce   Output  
  19. MapReduce Execution Automatically split work into many small tasks Send

    map tasks to nodes based on data locality Load-balance dynamically as tasks finish
  20. MapReduce Fault Recovery If a task fails, re-run it and

    re-fetch its input –  Requirement: input is immutable If a node fails, re-run its map tasks on others –  Requirement: task result is deterministic & side effect is idempotent If a task is slow, launch 2nd copy on other node –  Requirement: same as above
  21. MapReduce Summary By providing a data-parallel model, MapReduce greatly simplified

    cluster computing: –  Automatic division of job into tasks –  Locality-aware scheduling –  Load balancing –  Recovery from failures & stragglers Also flexible enough to model a lot of workloads…
  22. Hadoop Open-sourced by Yahoo! –  modeled after the two Google

    papers Two components: –  Storage: Hadoop Distributed File System (HDFS) –  Compute: Hadoop MapReduce Sometimes synonymous with Big Data
  23. Why didn’t Google just use databases? Cost –  database vendors

    charge by $/TB or $/core Scale –  no database systems at the time had been demonstrated to work at that scale (# machines or data size) Data Model –  A lot of semi-/un-structured data: web pages, images, videos Compute Model –  SQL not expressive (or “simple”) enough for many Google tasks (e.g. crawl the web, build inverted index, log analysis on unstructured data) Not-invented-here
  24. MapReduce Programmability Most real applications require multiple MR steps – 

    Google indexing pipeline: 21 steps –  Analytics queries (e.g. count clicks & top K): 2 – 5 steps –  Iterative algorithms (e.g. PageRank): 10’s of steps Multi-step jobs create spaghetti code –  21 MR steps -> 21 mapper and reducer classes –  Lots of boilerplate code per step
  25. Higher Level Frameworks SELECT count(*) FROM users A = load

    'foo'; B = group A all; C = foreach B generate COUNT(A); In reality, 90+% of MR jobs are generated by Hive SQL
  26. SQL on Hadoop (Hive) Meta   store   HDFS  

         Client   Driver   SQL   Parser   Query   Op<mizer   Physical  Plan   Execu<on   CLI   JDBC   MapReduce  
  27. Problems with MapReduce 1.  Programmability –  We covered this earlier

    … 2.  Performance –  Each MR job writes all output to disk –  Lack of more primitives such as data broadcast
  28. Spark Started in Berkeley AMPLab in 2010; addresses MR problems.

    Programmability: DSL in Scala / Java / Python –  Functional transformations on collections –  5 – 10X less code than MR –  Interactive use from Scala / Python REPL –  You can unit test Spark programs! Performance: –  General DAG of tasks (i.e. multi-stage MR) –  Richer primitives: in-memory cache, torrent broadcast, etc –  Can run 10 – 100X faster than MR
  29. Programmability #include "mapreduce/mapreduce.h" // User’s map function class SplitWords: public

    Mapper { public: virtual void Map(const MapInput& input) { const string& text = input.value(); const int n = text.size(); for (int i = 0; i < n; ) { // Skip past leading whitespace while (i < n && isspace(text[i])) i++; // Find word end int start = i; while (i < n && !isspace(text[i])) i++; if (start < i) Emit(text.substr( start,i-start),"1"); } } }; REGISTER_MAPPER(SplitWords); // User’s reduce function class Sum: public Reducer { public: virtual void Reduce(ReduceInput* input) { // Iterate over all entries with the // same key and add the values int64 value = 0; while (!input->done()) { value += StringToInt( input->value()); input->NextValue(); } // Emit sum for input->key() Emit(IntToString(value)); } }; REGISTER_REDUCER(Sum); int main(int argc, char** argv) { ParseCommandLineFlags(argc, argv); MapReduceSpecification spec; for (int i = 1; i < argc; i++) { MapReduceInput* in= spec.add_input(); in->set_format("text"); in->set_filepattern(argv[i]); in->set_mapper_class("SplitWords"); } // Specify the output files MapReduceOutput* out = spec.output(); out->set_filebase("/gfs/test/freq"); out->set_num_tasks(100); out->set_format("text"); out->set_reducer_class("Sum"); // Do partial sums within map out->set_combiner_class("Sum"); // Tuning parameters spec.set_machines(2000); spec.set_map_megabytes(100); spec.set_reduce_megabytes(100); // Now run it MapReduceResult result; if (!MapReduce(spec, &result)) abort(); return 0; } Full Google WordCount:
  30. Programmability Spark WordCount: val file = spark.textFile(“hdfs://...”) val counts =

    file.flatMap(line => line.split(“ ”))
 .map(word => (word, 1))
 .reduceByKey(_ + _)
 
 counts.save(“out.txt”)
  31. Performance 0.96 110 0 25 50 75 100 125 Logistic

    Regression 4.1 155 0 30 60 90 120 150 180 K-Means Clustering Hadoop MR Spark Time per Iteration (s)
  32. 43 Performance Time to sort 100TB 2100 machines 2013 Record:

    Hadoop 2014 Record: Spark Source: Daytona GraySort benchmark, sortbenchmark.org 72 minutes 207 machines 23 minutes Also sorted 1PB in 4 hours
  33. Spark Summary Spark generalizes MapReduce to provide: –  High performance

    –  Better programmability –  (consequently) a unified engine The most active open source data project
  34. Beyond Hadoop Users 47 Spark early adopters Data Engineers Data

    Scientists Statisticians R users PyData … Users Understands MapReduce & functional APIs
  35. 48

  36. DataFrames in Spark Distributed collection of data grouped into named

    columns (i.e. RDD with schema) DSL designed for common tasks –  Metadata –  Sampling –  Project, filter, aggregation, join, … –  UDFs Available in Python, Scala, Java, and R (via SparkR) 49
  37. Plan Optimization & Execution 50 DataFrames and SQL share the

    same optimization/execution pipeline Maximize code reuse & share optimization efforts SQL  AST   DataFrame   Unresolved   Logical  Plan   Logical  Plan   Op<mized   Logical  Plan   Physical  Plans   Physical  Plans   RDDs   Selected   Physical  Plan   Analysis   Logical   Op<miza<on   Physical   Planning   Cost  Model   Physical  Plans   Code   Genera<on   Catalog  
  38. Our Experience So Far SQL is wildly popular and important

    –  100% of Databricks customers use some SQL Schema is very useful –  Most data pipelines, even the ones that start with unstructured data, end up having some implicit structure –  Key-value too limited –  That said, semi-/un-structured support is paramount Separation of logical vs physical plan –  Important for performance optimizations (e.g. join selection)
  39. Why SQL? Almost everybody knows SQL Easier to write than

    MR (even Spark) for analytic queries Lingua franca for data analysis tools (business intelligence, etc) Schema is useful (key-value is limited)
  40. What’s really different? SQL on BD (Hadoop/Spark) vs SQL in

    DB? Two perspectives: 1.  Flexibility in data and compute model 2.  Fault-tolerance
  41. Data-Parallel Engine (Spark, MR) SQL DataFrame M.L. Big Data Ecosystems

    (Layered) Decoupled storage, low vs high level compute Structured, semi-structured, unstructured data Schema on read, schema on write
  42. Evolution of Database Systems Decouple Storage from Compute Physical Execution

    Engine (Dataflow) SQL Applications Physical Execution Engine (Dataflow) SQL Applications Traditional 2014 - 2015 IBM Big Insight Oracle EMC Greenplum … support for nested data (e.g. JSON)
  43. Perspective 2: Fault Tolerance Database systems: coarse-grained fault tolerance – 

    If fault happens, fail the query (or rerun from the beginning) MapReduce: fine-grained fault tolerance –  Rerun failed tasks, not the entire query
  44. We were writing it to 48,000 hard drives (we did

    not use the full capacity of these disks, though), and every time we ran our sort, at least one of our disks managed to break (this is not surprising at all given the duration of the test, the number of disks involved, and the expected lifetime of hard disks).
  45. MapReduce Checkpointing-based Fault Tolerance Checkpoint all intermediate output –  Replicate

    them to multiple nodes –  Upon failure, recover from checkpoints –  High cost of fault-tolerance (disk and network I/O) Necessary for PBs of data on thousands of machines What if I have 20 nodes and my query takes only 1 min?
  46. Spark Unified Checkpointing and Rerun Simple idea: remember the lineage

    to create an RDD, and recompute from last checkpoint. When fault happens, query still continues. When faults are rare, no need to checkpoint, i.e. cost of fault-tolerance is low.
  47. What’s Really Different? Monolithic vs layered storage & compute – 

    DB becoming more layered –  Although “Big Data” still far more flexible than DB Fault-tolerance –  DB mostly coarse-grained fault-tolerance, assuming faults are rare –  Big Data mostly fine-grained fault-tolerance, with new strategies in Spark to mitigate faults at low cost
  48. Convergence DB evolving towards BD –  Decouple storage from compute

    –  Provide alternative programming models –  Semi-structured data (JSON, XML, etc) BD evolving towards DB –  Schema beyond key-value –  Separation of logical vs physical plan –  Query optimization –  More optimized storage formats
  49. Acknowledgement Some slides taken from: Zaharia. Processing Big Data with

    Small Programs Franklin. SQL, NoSQL, NewSQL? CS186 2013