Introduction

Software platform to easily process vast amounts of data. The main features are:

• Scalable: It can reliably store and process petabytes.
• Economical: It distributes the data and processing across clusters of commonly available computers (in thousands).
• Efficient: By distributing the data, it can process it in parallel on the nodes where the data is located.
• Reliable: It automatically maintains multiple copies of data and automatically redeploys computing tasks based on failures.

Hadoop implements Google’s MapReduce, using HDFS.

Components overview

• Apache Hadoop
• Apache Hive
• Apache Pig
• Apache HBase
• Apache Zookeeper
• Flume, Hue, Oozie, and Sqoop

HDFS

It is a file system responsible for storing data on the cluster. Data files are split into blocks and distributed across the nodes in the cluster where each block is replicated multiple times.

• written in Java based on the Google’s GFS
• Provides redundant storage for massive amounts of data
• HDFS works best with a smaller number of large files
• Files in HDFS are write once read many
• Optimized for streaming reads of large files and not random reads

File storing

• Files are split into blocks and these blocks are split across many machines at load time
• Different blocks from the same file will be stored on different machines
• Blocks are replicated across multiple machines
• The NameNode keeps track of which blocks make up a file and where they are stored
• Single Namespace for entire cluster
• The default replication strategy is 3-fold.

Hadoop Properties

1. Fault Tolerance:
   • Worker failure – handled via re-execution
   • Master failure rare and can be recovered from checkpoints
2. Disk Locality:
   • Leveraging HDFS
   • Map tasks are scheduled close to data
   • Conserves network bandwidth
3. Task Granularity:
   • No. of map tasks > no. of worker nodes
   • Increases load on Master
   • Map could be chosen w.r.t to block size of the file system
   • Reduce is usually specified by users