Welcome!

Open Source Cloud Authors: Stackify Blog, Vaibhaw Pandey, Automic Blog, John Walsh, Mehdi Daoudi

Related Topics: @DXWorldExpo, Open Source Cloud

@DXWorldExpo: Blog Post

Hadoop Deep Dive: HDFS and MapReduce

How does Hadoop process data? What is MapReduce?

Following my initial introduction to Hadoop and overview of Hadoop components, I studied the Yahoo Hadoop tutorial,  and have a deeper understanding of Hadoop. I  would like to share my learning and help others understand Hadoop.

Why does Hadoop require HDFS, what's wrong with NFS?

Nothing! NFS been around for years and is incredibly stable. A distributed file system, such as NFS, provides the functionality required for servers to share files. It does this by exposing a volume to the network, which users connect to over the TCP/IP protocol.

For Hadoop applications that require highly available distributed file systems with unlimited capacity HDFS is preferred. The following table outlines where NFS is deficient for Hadoop and where HDFS solves the limitation.

NFS limitation Hadoop optimization
What happens when the volume is full? No more files can be stored. HDFS stores information (terabytes or petabytes) across many servers. HDFS supports larger file sizes than NFS
What happens if a volume or server fails? There is no built-in redundancy. HDFS stores data reliably. If a server in the cluster fails, the data is still available on other servers.
NTFS & EXT3 file systems typically used by NFS have 4-8KB block sizes that result in large metadata about each file and multiple reads from the file system. The default block size in HDFS is 64MB, which results in fewer files to store and decreased metadata information stored for each file. HDFS is optimized to provide streaming read performance, rather than random seek to arbitrary positions in files
Most NFS administration is command line or included with overall system management tools. HDFS provides a web server to perform status monitoring and file browsing operations (by default port 50070).

Hadoop can exploit HDFS and thus:

  • Hadoop is aware of physical racks that span more than one network switch. Replicas of the data exist across racks, thus if a physical switch fails, the data is available from another server rack.
  • There is no local caching since files are large and sequentially read
  • HDFS reads a block start-to-finish for the Hadoop MapReduce application. MapReduce? Read on....

What is MapReduce?

MapReduce is a programming model to process large volumes of data, usually tuple (pair) lists, for example:

  • city, temperature
  • student, grade
  • car, maximum speed

MapReduce accomplishes this in parallel by dividing the work into independent tasks, spread across many nodes (servers). This model would not scale to large clusters (hundreds or thousands of nodes) if the components shared data arbitrarily. The communication overhead required to keep the data on the nodes synchronized would be inefficient. Rather, the data elements in MapReduce are immutable, meaning that they cannot be updated. Example, if during a MapReduce job, input data is changed eg. (modifying a student grade or car's speed) the change does not get reflected in the input files; instead new output (key, value) pairs are generated which are then forwarded by Hadoop into the next phase of execution.

Driver

The driver initializes the job and instructs Hadoop to execute the job on a set of input files, and controls where the output files are placed. Pretty simple.

Map

The map portion of MapReduce provides a list of data elements, one at a time, to a function called the Mapper. The mapper transforms each input element to an output data element.

Reduce

Reducing aggregates the values together.  A reducer function receives the input values from an input list, then combines these values together, returning a single output value; example: average car speed, student grade or city temperature.

Sounds simple? At a high level yes, details details...

The mapper produces one output element for each input element, and a reducer produces one output element for each input list. A map job may input into zero or more outputs; a reducer may compute over an input list and create one or more  outputs.

What if the amount of data to reduce is enormous? Keys divide the reduce space: A reducing function turns a large list of values into one (or a few) output values but may not be reduced together. All of the values with the same key are presented to a single reducing function together. This is performed independently of any reduce operations occurring on other lists of values, with different keys attached. So in our example, perhaps all the Toyota vehicles are reduced together, Ford vehicles in another group and Chevrolet in a third set; similarly grade 10 kids' scores are reduced in one set, grade 11 in another etc.

So why is HDFS needed for MapReduce?

The map jobs: MapReduce inputs come from input files loaded which are evenly distributed across all servers. Executing a MapReduce program involves running mapping tasks on many or all of the nodes. Each of these mapping tasks is identical, therefore, any mapper can process any input file. Each mapping job loads the files on that server and processes them. Thus data is processed at the node where it exists and not copied to a central server (see my introductory article for more details). Individual map tasks do not communicate with one another, nor are they aware of one another's existence.

The reduce jobs:
When the mapping phase has completed, the intermediate (key, value) pairs must be exchanged between servers to send all values with the same key to a single reduce job. To illustrate the example above, all the values from the Toyota vehicles must be sent to the reduce job responsible for Toyota; similarly for other vehicles, city temperatures and class grades must be sent to their unique reduce function. The reduce tasks are spread across the same nodes in the Hadoop cluster as the map jobs. Similarly, different reduce tasks do not communicate with one another.

If servers in the Hadoop cluster fail, the map or reduce tasks must be able to be restarted. Thus a highly scalable and fault tolerant file system like HDFS is required for successful MapReduce jobs.

Fault tolerance HDFS is key

Hadoop achieves fault tolerance by restarting tasks when a server (node) fails. Individual task nodes (TaskTrackers) communicate with the head node of the system (JobTracker). If a TaskTracker fails to communicate with the JobTracker  (by default, 1 minute), the JobTracker will assume that the specific TaskTracker has failed. The JobTracker knows which map and reduce tasks were assigned to each TaskTracker; other TaskTrackers will re-execute a failed map or reduce job.

Completed reduce tasks, are written back to HDFS. Thus, if a TaskTracker has already completed two out of three reduce tasks assigned to it, only the third task must be executed on another node/server.
Hadoop code should have no 'side effects'
If Map and Reduce jobs had individual identities and communicated with one another (rather than the TaskTracker) , then restarting a task would require the other nodes to communicate with the new instances of the map and reduce tasks, and the re-executed tasks would need to reestablish their prior state.  An individual task sees only its own direct inputs and knows only its own outputs, to make this failure and restart process clean and dependable.

Sounds complicated? Try a tutorial download from HortonWorks, Cloudera or Yahoo.

Image below show two nodes, multiple mappers and reducers; courtesy of Yahoo Hadoop tutorial, module 4

More Stories By Jonathan Gershater

Jonathan Gershater has lived and worked in Silicon Valley since 1996, primarily doing system and sales engineering specializing in: Web Applications, Identity and Security. At Red Hat, he provides Technical Marketing for Virtualization and Cloud. Prior to joining Red Hat, Jonathan worked at 3Com, Entrust (by acquisition) two startups, Sun Microsystems and Trend Micro.

(The views expressed in this blog are entirely mine and do not represent my employer - Jonathan).

Comments (0)

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


@ThingsExpo Stories
"Cloud Academy is an enterprise training platform for the cloud, specifically public clouds. We offer guided learning experiences on AWS, Azure, Google Cloud and all the surrounding methodologies and technologies that you need to know and your teams need to know in order to leverage the full benefits of the cloud," explained Alex Brower, VP of Marketing at Cloud Academy, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clar...
In his session at 21st Cloud Expo, Carl J. Levine, Senior Technical Evangelist for NS1, will objectively discuss how DNS is used to solve Digital Transformation challenges in large SaaS applications, CDNs, AdTech platforms, and other demanding use cases. Carl J. Levine is the Senior Technical Evangelist for NS1. A veteran of the Internet Infrastructure space, he has over a decade of experience with startups, networking protocols and Internet infrastructure, combined with the unique ability to it...
"Akvelon is a software development company and we also provide consultancy services to folks who are looking to scale or accelerate their engineering roadmaps," explained Jeremiah Mothersell, Marketing Manager at Akvelon, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
"Space Monkey by Vivent Smart Home is a product that is a distributed cloud-based edge storage network. Vivent Smart Home, our parent company, is a smart home provider that places a lot of hard drives across homes in North America," explained JT Olds, Director of Engineering, and Brandon Crowfeather, Product Manager, at Vivint Smart Home, in this SYS-CON.tv interview at @ThingsExpo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
It is of utmost importance for the future success of WebRTC to ensure that interoperability is operational between web browsers and any WebRTC-compliant client. To be guaranteed as operational and effective, interoperability must be tested extensively by establishing WebRTC data and media connections between different web browsers running on different devices and operating systems. In his session at WebRTC Summit at @ThingsExpo, Dr. Alex Gouaillard, CEO and Founder of CoSMo Software, presented ...
"There's plenty of bandwidth out there but it's never in the right place. So what Cedexis does is uses data to work out the best pathways to get data from the origin to the person who wants to get it," explained Simon Jones, Evangelist and Head of Marketing at Cedexis, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
WebRTC is great technology to build your own communication tools. It will be even more exciting experience it with advanced devices, such as a 360 Camera, 360 microphone, and a depth sensor camera. In his session at @ThingsExpo, Masashi Ganeko, a manager at INFOCOM Corporation, introduced two experimental projects from his team and what they learned from them. "Shotoku Tamago" uses the robot audition software HARK to track speakers in 360 video of a remote party. "Virtual Teleport" uses a multip...
"IBM is really all in on blockchain. We take a look at sort of the history of blockchain ledger technologies. It started out with bitcoin, Ethereum, and IBM evaluated these particular blockchain technologies and found they were anonymous and permissionless and that many companies were looking for permissioned blockchain," stated René Bostic, Technical VP of the IBM Cloud Unit in North America, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Conventi...
Gemini is Yahoo’s native and search advertising platform. To ensure the quality of a complex distributed system that spans multiple products and components and across various desktop websites and mobile app and web experiences – both Yahoo owned and operated and third-party syndication (supply), with complex interaction with more than a billion users and numerous advertisers globally (demand) – it becomes imperative to automate a set of end-to-end tests 24x7 to detect bugs and regression. In th...
SYS-CON Events announced today that Telecom Reseller has been named “Media Sponsor” of SYS-CON's 22nd International Cloud Expo, which will take place on June 5-7, 2018, at the Javits Center in New York, NY. Telecom Reseller reports on Unified Communications, UCaaS, BPaaS for enterprise and SMBs. They report extensively on both customer premises based solutions such as IP-PBX as well as cloud based and hosted platforms.
SYS-CON Events announced today that CrowdReviews.com has been named “Media Sponsor” of SYS-CON's 22nd International Cloud Expo, which will take place on June 5–7, 2018, at the Javits Center in New York City, NY. CrowdReviews.com is a transparent online platform for determining which products and services are the best based on the opinion of the crowd. The crowd consists of Internet users that have experienced products and services first-hand and have an interest in letting other potential buye...
"MobiDev is a software development company and we do complex, custom software development for everybody from entrepreneurs to large enterprises," explained Alan Winters, U.S. Head of Business Development at MobiDev, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
Coca-Cola’s Google powered digital signage system lays the groundwork for a more valuable connection between Coke and its customers. Digital signs pair software with high-resolution displays so that a message can be changed instantly based on what the operator wants to communicate or sell. In their Day 3 Keynote at 21st Cloud Expo, Greg Chambers, Global Group Director, Digital Innovation, Coca-Cola, and Vidya Nagarajan, a Senior Product Manager at Google, discussed how from store operations and ...
A strange thing is happening along the way to the Internet of Things, namely far too many devices to work with and manage. It has become clear that we'll need much higher efficiency user experiences that can allow us to more easily and scalably work with the thousands of devices that will soon be in each of our lives. Enter the conversational interface revolution, combining bots we can literally talk with, gesture to, and even direct with our thoughts, with embedded artificial intelligence, whic...
SYS-CON Events announced today that Evatronix will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Evatronix SA offers comprehensive solutions in the design and implementation of electronic systems, in CAD / CAM deployment, and also is a designer and manufacturer of advanced 3D scanners for professional applications.
Leading companies, from the Global Fortune 500 to the smallest companies, are adopting hybrid cloud as the path to business advantage. Hybrid cloud depends on cloud services and on-premises infrastructure working in unison. Successful implementations require new levels of data mobility, enabled by an automated and seamless flow across on-premises and cloud resources. In his general session at 21st Cloud Expo, Greg Tevis, an IBM Storage Software Technical Strategist and Customer Solution Architec...
To get the most out of their data, successful companies are not focusing on queries and data lakes, they are actively integrating analytics into their operations with a data-first application development approach. Real-time adjustments to improve revenues, reduce costs, or mitigate risk rely on applications that minimize latency on a variety of data sources. In his session at @BigDataExpo, Jack Norris, Senior Vice President, Data and Applications at MapR Technologies, reviewed best practices to ...
An increasing number of companies are creating products that combine data with analytical capabilities. Running interactive queries on Big Data requires complex architectures to store and query data effectively, typically involving data streams, an choosing efficient file format/database and multiple independent systems that are tied together through custom-engineered pipelines. In his session at @BigDataExpo at @ThingsExpo, Tomer Levi, a senior software engineer at Intel’s Advanced Analytics gr...
When talking IoT we often focus on the devices, the sensors, the hardware itself. The new smart appliances, the new smart or self-driving cars (which are amalgamations of many ‘things’). When we are looking at the world of IoT, we should take a step back, look at the big picture. What value are these devices providing? IoT is not about the devices, it’s about the data consumed and generated. The devices are tools, mechanisms, conduits. In his session at Internet of Things at Cloud Expo | DXWor...
Everything run by electricity will eventually be connected to the Internet. Get ahead of the Internet of Things revolution. In his session at @ThingsExpo, Akvelon expert and IoT industry leader Sergey Grebnov provided an educational dive into the world of managing your home, workplace and all the devices they contain with the power of machine-based AI and intelligent Bot services for a completely streamlined experience.