Hadoop 101 Mohit Soni eBay
Inc.BarCamp Chennai - 5 Mohit Soni

About Me• I work as
a Software Engineer at eBay• Worked on large-scale data processing with eBay Research Labs BarCamp Chennai - 5 Mohit Soni

First Things FirstBarCamp Chennai -
5 Mohit Soni

MapReduce• Inspired from functional operations
– Map – Reduce• Functional operations do not modify data, they generate new data• Original data remains unmodified BarCamp Chennai - 5 Mohit Soni

Functional OperationsMap Reducedef sqr(n): def
add(i, j): return n * n return i + jlist = [1,2,3,4] list = [1,2,3,4]map(sqr, list) -> [1,4,9,16] reduce(add, list) -> 10 MapReduce def MapReduce(data, mapper, reducer): return reduce(reducer, map(mapper, data)) MapReduce(list, sqr, add) -> 30 Python code BarCamp Chennai - 5 Mohit Soni

BarCamp Chennai - 5 Mohit
Soni

What is Hadoop ?• Framework
for large-scale data processing• Based on Google’s MapReduce and GFS• An Apache Software Foundation project• Open Source!• Written in Java• Oh, btw BarCamp Chennai - 5 Mohit Soni

Why Hadoop ?• Need to
process lots of data (PetaByte scale)• Need to parallelize processing across multitude of CPUs• Achieves above while KeepIng Software Simple• Gives scalability with low-cost commodity hardware BarCamp Chennai - 5 Mohit Soni

Hadoop fansSource: Hadoop Wiki BarCamp
Chennai - 5 Mohit Soni

When to use and not-use
Hadoop ?Hadoop is a good choice for:• Indexing data• Log Analysis• Image manipulation• Sorting large-scale data• Data MiningHadoop is not a good choice:• For real-time processing• For processing intensive tasks with little data• If you have Jaguar or RoadRunner in your stock BarCamp Chennai - 5 Mohit Soni

HDFS – Overview• Hadoop Distributed
File System• Based on Google’s GFS (Google File System)• Write once read many access model• Fault tolerant• Efficient for batch-processing BarCamp Chennai - 5 Mohit Soni

HDFS – Blocks Block 1
Block 2 Input Data Block 3• HDFS splits input data into blocks• Block size in HDFS: 64/128MB (configurable)• Block size *nix: 4KB BarCamp Chennai - 5 Mohit Soni

HDFS – Replication Block 1
Block 1 Block 2 Block 3 Block 2 Block 3• Blocks are replicated across nodes to handle hardware failure• Node failure is handled gracefully, without loss of data BarCamp Chennai - 5 Mohit Soni

HDFS – Architecture NameNodeClient Cluster
DataNodes BarCamp Chennai - 5 Mohit Soni

HDFS – NameNode• NameNode (Master)
– Manages filesystem metadata – Manages replication of blocks – Manages read/write access to files• Metadata – List of files – List of blocks that constitutes a file – List of DataNodes on which blocks reside, etc• Single Point of Failure (candidate for spending $$) BarCamp Chennai - 5 Mohit Soni

HDFS – DataNode• DataNode (Slave)
– Contains actual data – Manages data blocks – Informs NameNode about block IDs stored – Client read/write data blocks from DataNode – Performs block replication as instructed by NameNode• Block Replication – Supports various pluggable replication strategies – Clients read blocks from nearest DataNode• Data Pipelining – Client write block to first DataNode – First DataNode forwards data to next DataNode in pipeline – When block is replicated across all replicas, next block is chosen BarCamp Chennai - 5 Mohit Soni

Hadoop - ArchitectureUser JobTracker TaskTracker
TaskTracker NameNode DataNode DataNode DataNode DataNode DataNode DataNode BarCamp Chennai - 5 Mohit Soni

Hadoop - Terminology• JobTracker (Master)
– 1 Job Tracker per cluster – Accepts job requests from users – Schedule Map and Reduce tasks for TaskTrackers – Monitors tasks and TaskTrackers status – Re-execute task on failure• TaskTracker (Slave) – Multiple TaskTrackers in a cluster – Run Map and Reduce tasks BarCamp Chennai - 5 Mohit Soni

MapReduce – FlowInput Map Shuffle
+ Sort Reduce Output Map ReduceInput Output Data Map Data Reduce Map BarCamp Chennai - 5 Mohit Soni

Word Count Hadoop’s HelloWorldBarCamp Chennai
- 5 Mohit Soni

Word Count Example• Input –
Text files• Output – Single file containing (Word <TAB> Count)• Map Phase – Generates (Word, Count) pairs – [{a,1}, {b,1}, {a,1}] [{a,2}, {b,3}, {c,5}] [{a,3}, {b,1}, {c,1}]• Reduce Phase – For each word, calculates aggregate – [{a,7}, {b,5}, {c,6}] BarCamp Chennai - 5 Mohit Soni

Word Count – Mapperpublic class
WordCountMapper extends MapReduceBase implements Mapper<LongWritable, Text, Text, IntWritable> { private final static IntWritable one = new IntWritable(1); private Text word = new Text(); public void map(LongWritable key, Text value, OutputCollector<Text, IntWritable> out, Reporter reporter) throws Exception { String l = value.toString(); StringTokenizer t = new StringTokenizer(l); while(t.hasMoreTokens()) { word.set(t.nextToken()); out.collect(word, one); } }} BarCamp Chennai - 5 Mohit Soni

Word Count – Reducerpublic class
WordCountReducer extends MapReduceBase implements Reducer<Text, IntWritable, Text, IntWritable> { public void reduce(Text key, Iterator<IntWriter> values, OutputCollector<Text, IntWritable> out, Reporter reporter) throws Exception { int sum = 0; while(values.hasNext()) { sum += values.next().get(); } out.collect(key, new IntWritable(sum)); }} BarCamp Chennai - 5 Mohit Soni

Word Count – Configpublic class
WordCountConfig { public static void main(String[] args) throws Exception { if (args.length() != 2) { System.exit(1); } JobConf conf = new JobConf(WordCountConfig.class); conf.setJobName(“Word Counter”); FileInputFormat.addInputPath(conf, new Path(args[0]); FileInputFormat.addOutputPath(conf, new Path(args[1])); conf.setMapperClass(WordCountMapper.class); conf.setCombinerClass(WordCountReducer.class); conf.setReducerClass(WordCountReducer.class); conf.setInputFormat(TextInputFormat.class); conf.setOutputFormat(TextOutputFormat.class); JobClient.runJob(conf); }} BarCamp Chennai - 5 Mohit Soni

Diving Deeper• http://hadoop.apache.org/• Jeffrey Dean
and Sanjay Ghemwat, MapReduce: Simplified Data Processing on Large Clusters• Tom White, Hadoop: The Definitive Guide, O’Reilly• Setting up a Single-Node Cluster: http://bit.ly/glNzs4• Setting up a Multi-Node Cluster: http://bit.ly/f5KqCP BarCamp Chennai - 5 Mohit Soni

Catching-Up• Follow me on twitter
@mohitsoni• http://mohitsoni.com/ BarCamp Chennai - 5 Mohit Soni