In this presentation , i provide in depth information about the how MapReduce works. It contains many details about the execution steps , Fault tolerance , master / worker responsibilities.
Hadoop MapReduce is an open source framework for distributed processing of large datasets across clusters of computers. It allows parallel processing of large datasets by dividing the work across nodes. The framework handles scheduling, fault tolerance, and distribution of work. MapReduce consists of two main phases - the map phase where the data is processed key-value pairs and the reduce phase where the outputs of the map phase are aggregated together. It provides an easy programming model for developers to write distributed applications for large scale processing of structured and unstructured data.
Mapreduce - Simplified Data Processing on Large ClustersAbhishek Singh
The document describes MapReduce, a programming model and implementation for processing large datasets across clusters of computers. It allows users to write map and reduce functions to parallelize tasks. The MapReduce library automatically parallelizes jobs, distributes data and tasks, handles failures and coordinates communication between machines. It is scalable, processing terabytes of data on thousands of machines, and easy for programmers without parallel experience to use.
This document describes the MapReduce programming model for processing large datasets in a distributed manner. MapReduce allows users to write map and reduce functions that are automatically parallelized and run across large clusters. The input data is split and the map tasks run in parallel, producing intermediate key-value pairs. These are shuffled and input to the reduce tasks, which produce the final output. The system handles failures, scheduling and parallelization transparently, making it easy for programmers to write distributed applications.
MapReduce is a programming model for processing large datasets in a distributed manner across clusters of machines. It involves two functions - Map and Reduce. The Map function processes input key-value pairs to generate intermediate key-value pairs, and the Reduce function merges all intermediate values associated with the same intermediate key. This allows for distributed processing that hides complexity and provides fault tolerance. An example is counting word frequencies, where the Map function emits word counts and the Reduce function sums the counts for each word.
The MapReduce job begins when a client program uploads configuration files to HDFS and notifies the JobTracker. The JobTracker assigns map tasks to idle TaskTrackers and the tasks extract input data, invoke the user-provided map function, and output intermediate key-value pairs. When the map tasks complete, reduce tasks are assigned to TaskTrackers to download intermediate data and invoke the reduce function to generate the final output. The framework is resilient to failures and can re-execute failed tasks as needed.
MapReduce provides a programming model for processing large datasets in a distributed, parallel manner. It involves two main steps - the map step where the input data is converted into intermediate key-value pairs, and the reduce step where the intermediate outputs are aggregated based on keys to produce the final results. Hadoop is an open-source software framework that allows distributed processing of large datasets across clusters of computers using MapReduce.
Mapreduce examples starting from the basic WordCount to a more complex K-means algorithm. The code contained in these slides is available at https://github.com/andreaiacono/MapReduce
The document provides an introduction to MapReduce, describing its motivation as a framework for simplifying large-scale data processing across distributed systems. It outlines MapReduce's programming model and main features, including automatic parallelization, fault tolerance, and locality. The document also provides a detailed example of counting letter frequencies in a large file to illustrate how MapReduce works.
Map reduce - simplified data processing on large clustersCleverence Kombe
The document describes MapReduce, a programming model and software framework for processing large datasets in a distributed computing environment. It discusses how MapReduce allows users to specify map and reduce functions to parallelize tasks across large clusters of machines. It also covers how MapReduce handles parallelization, fault tolerance, and load balancing transparently through an easy-to-use programming interface.
This document provides a high-level overview of MapReduce and Hadoop. It begins with an introduction to MapReduce, describing it as a distributed computing framework that decomposes work into parallelized map and reduce tasks. Key concepts like mappers, reducers, and job tracking are defined. The structure of a MapReduce job is then outlined, showing how input is divided and processed by mappers, then shuffled and sorted before being combined by reducers. Example map and reduce functions for a word counting problem are presented to demonstrate how a full MapReduce job works.
MapReduce : Simplified Data Processing on Large ClustersAbolfazl Asudeh
MapReduce is a programming model and associated implementation for processing large datasets in a distributed system. It allows users to specify a map function that processes input key-value pairs to generate intermediate output pairs, and a reduce function that merges all intermediate values associated with the same key. The MapReduce system automatically parallelizes the computation across large clusters and handles tasks like scheduling, parallelization, and failure recovery. An example of word counting demonstrates how text documents are broken into words as input pairs, mapped to count occurrences, and reduced to output word frequencies.
And introdution to MR and Hadoop and an view on the opportunities to use MR with databases i.e., SQL-MapReduce by Teradata and In-database MR by Oracle.
The presentation was used during a class of Datenbanken Implementierungstechniken in 2013.
International Journal of Computational Engineering Research(IJCER) is an intentional online Journal in English monthly publishing journal. This Journal publish original research work that contributes significantly to further the scientific knowledge in engineering and Technology.
"MapReduce: Simplified Data Processing on Large Clusters" Paper Presentation ...Adrian Florea
MapReduce is a programming model and implementation for processing large datasets across clusters of computers. It allows users to specify map and reduce functions. The map function processes input key-value pairs to generate intermediate pairs, while the reduce function combines intermediate values into final output. Google developed MapReduce to simplify distributed computing on large datasets, addressing issues like parallelization, fault tolerance, and load balancing. It works by splitting input data into blocks and assigning them to worker nodes that apply the user-defined map and reduce functions to process the data in parallel.
Here is how you can solve this problem using MapReduce and Unix commands:
Map step:
grep -o 'Blue\|Green' input.txt | wc -l > output
This uses grep to search the input file for the strings "Blue" or "Green" and print only the matches. The matches are piped to wc which counts the lines (matches).
Reduce step:
cat output
This isn't really needed as there is only one mapper. Cat prints the contents of the output file which has the count of Blue and Green.
So MapReduce has been simulated using grep for the map and cat for the reduce functionality. The key aspects are - grep extracts the relevant data (map
Optimal Execution Of MapReduce Jobs In Cloud - Voices 2015Deanna Kosaraju
Optimal Execution Of MapReduce Jobs In Cloud
Anshul Aggarwal, Software Engineer, Cisco Systems
Session Length: 1 Hour
Tue March 10 21:30 PST
Wed March 11 0:30 EST
Wed March 11 4:30:00 UTC
Wed March 11 10:00 IST
Wed March 11 15:30 Sydney
Voices 2015 www.globaltechwomen.com
We use MapReduce programming paradigm because it lends itself well to most data-intensive analytics jobs run on cloud these days, given its ability to scale-out and leverage several machines to parallel process data. Research has demonstrates that existing approaches to provisioning other applications in the cloud are not immediately relevant to MapReduce -based applications. Provisioning a MapReduce job entails requesting optimum number of resource sets (RS) and configuring MapReduce parameters such that each resource set is maximally utilized.
Each application has a different bottleneck resource (CPU :Disk :Network), and different bottleneck resource utilization, and thus needs to pick a different combination of these parameters based on the job profile such that the bottleneck resource is maximally utilized.
The problem at hand is thus defining a resource provisioning framework for MapReduce jobs running in a cloud keeping in mind performance goals such as Optimal resource utilization with Minimum incurred cost, Lower execution time, Energy Awareness, Automatic handling of node failure and Highly scalable solution.
MapReduce: Simplified Data Processing On Large Clusterskazuma_sato
The document discusses the MapReduce programming model which has been successfully used at Google. It describes how MapReduce simplifies data processing on large clusters by hiding the complexities of parallelization, fault tolerance, locality optimization, and load balancing. Computation is expressed as two functions - Map and Reduce. The Map function produces intermediate key-value pairs from input pairs, and the Reduce function merges all intermediate values associated with the same key.
The document provides information about MapReduce jobs including:
- The number of maps is determined by input size and partitioning. The number of reducers is set by the user.
- Reducers receive sorted, grouped data from maps via shuffle and sort. They apply the reduce function to grouped keys/values.
- The optimal number of reducers depends on nodes and tasks. More reducers improve load balancing but increase overhead.
This document provides an overview of MapReduce, a programming model developed by Google for processing and generating large datasets in a distributed computing environment. It describes how MapReduce abstracts away the complexities of parallelization, fault tolerance, and load balancing to allow developers to focus on the problem logic. Examples are given showing how MapReduce can be used for tasks like word counting in documents and joining datasets. Implementation details and usage statistics from Google demonstrate how MapReduce has scaled to process exabytes of data across thousands of machines.
This document provides a summary of MapReduce algorithms. It begins with background on the author's experience blogging about MapReduce algorithms in academic papers. It then provides an overview of MapReduce concepts including the mapper and reducer functions. Several examples of recently published MapReduce algorithms are described for tasks like machine learning, finance, and software engineering. One algorithm is examined in depth for building a low-latency key-value store. Finally, recommendations are provided for designing MapReduce algorithms including patterns, performance, and cost/maintainability considerations. An appendix lists additional MapReduce algorithms from academic papers in areas such as AI, biology, machine learning, and mathematics.
This document provides an overview of the Hadoop MapReduce Fundamentals course. It discusses what Hadoop is, why it is used, common business problems it can address, and companies that use Hadoop. It also outlines the core parts of Hadoop distributions and the Hadoop ecosystem. Additionally, it covers common MapReduce concepts like HDFS, the MapReduce programming model, and Hadoop distributions. The document includes several code examples and screenshots related to Hadoop and MapReduce.
The document describes MapReduce, a programming model and implementation for processing large datasets across clusters of computers. The model uses map and reduce functions to parallelize computations. Map processes key-value pairs to generate intermediate pairs, and reduce merges values with the same intermediate key. The implementation handles parallelization, distribution, and fault tolerance transparently. Hundreds of programs have been implemented using MapReduce at Google, processing terabytes of data on thousands of machines daily.
Cloud computing is much more than x86 and virtual machines - it's about dealing with complex problems at scale.
"Algorithms for Cloud Computing" is an introductory talk, presenting high-level overview of selected algorithms and
data structures used in cloud computing.
The document discusses the Hadoop ecosystem and its key components. It describes how MapReduce works by mapping and reducing large datasets in parallel across clusters of commodity hardware. The major components are HDFS for storage, Hive for SQL-like queries, and other tools like HBase, Zookeeper, and Sqoop. MapReduce jobs are executed in phases like map, shuffle, sort, and reduce to process large amounts of data efficiently in a distributed manner. The ecosystem allows users to solve big data problems by breaking work into parallelizable tasks and processing data where it resides.
This document provides an overview of big data and Hadoop. It discusses the scale of big data, noting that Facebook handles 180PB per year and Twitter handles 1.2 million tweets per second. It also covers the volume, variety, and velocity challenges of big data. Hadoop and MapReduce are introduced as the leading solutions for distributed storage and processing of big data using a scale-out architecture. Key ideas of Hadoop include storing large data across multiple machines in HDFS and processing that data in parallel using MapReduce jobs.
This document summarizes Hadoop MapReduce, including its goals of distribution and reliability. It describes the roles of mappers, reducers, and other system components like the JobTracker and TaskTracker. Mappers process input splits in parallel and generate intermediate key-value pairs. Reducers sort and group the outputs by key before processing each unique key. The JobTracker coordinates jobs while the TaskTracker manages tasks on each node.
Pyshark is a wrapper around tshark comand line utility to capture a live Network packet or from a
capture file. Pyshark is useful in parsing capture data for analysis.
Ali Fahad presented on cyberterrorism. He defined cyberterrorism as using the internet for terrorist activities like disrupting computer networks through viruses or hacking. He discussed strategies terrorists use like hacking, virus writing, and spying. Cyber warfare is appealing to terrorists because computers are widely used and attacks have low barriers to entry compared to physical attacks. Forms of cyberterrorism include bank threats, data theft, and assassinations. Terrorists target the commercial sector, power grids, and personal data. Cyberterrorism can harm economies and cause terror through computer-based attacks. Pakistan has experienced cyberterrorism and taken steps to pass laws against it, including making it punishable by death.
Human: [SUMMARY
Hadoop - Introduction to map reduce programming - Reunião 12/04/2014soujavajug
Aaron Myers introduces MapReduce and Hadoop. MapReduce is a distributed programming paradigm that allows processing of large datasets across clusters. It works by splitting data, distributing it across nodes, processing it in parallel using map and reduce functions, and collecting the results. Hadoop is an open source software framework for distributed storage and processing of big data using MapReduce. It includes HDFS for storage and Hadoop MapReduce for distributed computing. Developers write MapReduce jobs in Java by implementing map and reduce functions.
Modern Algorithms and Data Structures - 1. Bloom Filters, Merkle TreesLorenzo Alberton
The first part of a series of talks about modern algorithms and data structures, used by nosql databases like HBase and Cassandra. An explanation of Bloom Filters and several derivates, and Merkle Trees.
The document presents an introduction to MapReduce. It discusses how MapReduce provides an easy framework for distributed computing by allowing programmers to write simple map and reduce functions without worrying about complex distributed systems issues. It outlines Google's implementation of MapReduce and how it uses the Google File System for fault tolerance. Alternative open-source implementations like Apache Hadoop are also covered. The document discusses how MapReduce has been widely adopted by companies to process massive amounts of data and analyzes some criticism of MapReduce from database experts. It concludes by noting trends in using MapReduce as a parallel database and for multi-core processing.
An Introduction to Map/Reduce with MongoDBRainforest QA
This document provides an introduction to using MapReduce with MongoDB. It explains what MapReduce is, how it works, and provides examples of mapping and reducing sample data to calculate applications by state, applications by status and state, and average wages by visa class and status. It also discusses some limitations and considerations when using MapReduce with MongoDB.
El documento describe las diferencias entre IPv4 e IPv6, la nueva versión del Protocolo de Internet. IPv6 fue creado para reemplazar a IPv4 debido a la falta de direcciones IPv4. IPv6 ofrece un espacio de direcciones de 128 bits, proporcionando muchas más direcciones que IPv4 de 32 bits. IPv6 también incluye mejoras como cabeceras más flexibles, características de seguridad y movilidad integrada.
This document describes the basic structure of an IPv6 packet header, which includes fields for version, traffic class, flow label, payload length, next header, hop limit, source address, destination address, and optional fields. The header provides routing and quality of service information for the packet as it travels across networks.
Introduction to MapReduce | MapReduce Architecture | MapReduce FundamentalsSkillspeed
This Hadoop MapReduce tutorial will unravel MapReduce Programming, MapReduce Commands, MapReduce Fundamentals, Driver Class, Mapper Class, Reducer Class, Job Tracker & Task Tracker.
At the end, you'll have a strong knowledge regarding Hadoop MapReduce Basics.
PPT Agenda:
✓ Introduction to BIG Data & Hadoop
✓ What is MapReduce?
✓ MapReduce Data Flows
✓ MapReduce Programming
----------
What is MapReduce?
MapReduce is a programming framework for distributed processing of large data-sets via commodity computing clusters. It is based on the principal of parallel data processing, wherein data is broken into smaller blocks rather than processed as a single block. This ensures a faster, secure & scalable solution. Mapreduce commands are based in Java.
----------
What are MapReduce Components?
It has the following components:
1. Combiner: The combiner collates all the data from the sample set based on your desired filters. For example, you can collate data based on day, week, month and year. After this, the data is prepared and sent for parallel processing.
2. Job Tracker: This allocates the data across multiple servers.
3. Task Tracker: This executes the program across various servers.
4. Reducer: It will isolate the desired output from across the multiple servers.
----------
Applications of MapReduce
1. Data Mining
2. Document Indexing
3. Business Intelligence
4. Predictive Modelling
5. Hypothesis Testing
----------
Skillspeed is a live e-learning company focusing on high-technology courses. We provide live instructor led training in BIG Data & Hadoop featuring Realtime Projects, 24/7 Lifetime Support & 100% Placement Assistance.
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This document discusses the topic of cyber terrorism, including:
- Defining cyber terrorism as using computer technology and the internet to cause fear and disruption.
- Describing three types of cyber terrorism: simple unstructured attacks, advanced structured attacks, and complex coordinated attacks.
- Explaining some of the effects of cyber terrorism, such as financial damage, weakening a nation's security and economy, and potentially causing death by disrupting critical systems.
- Providing some statistics on cyber attacks and losses due to various types of attacks.
- Listing some examples of cyber attacks that have taken place worldwide, targeting things like banking systems, air traffic control, and power grids.
Cyber terrorism involves using computers and technology to intimidate or harm people for political or religious goals. It differs from physical terrorism in that there is less risk of being caught and tracked online. Potential cyber terrorists include crackers, white hat hackers, and script kiddies. Factors contributing to cyber terrorism are dependence on technology, lack of security understanding, lack funding for security, and difficulty tracking online criminals. Common cyber attacks include information theft, credit card theft, hacking, and threatening infrastructure. Preventing cyber terrorism requires cooperation between agencies, prioritizing security, and reporting cyber crimes.
This document provides an overview of IPv6, including:
- The need for IPv6 due to the depletion of IPv4 addresses and limitations of IPv4's classful addressing.
- Techniques used to extend IPv4 like subnetting, CIDR, and NAT.
- Key features of IPv6 like its larger 128-bit address space, stateless autoconfiguration, and security improvements.
- Differences between IPv4 and IPv6 headers and IPv6's use of extension headers.
- The presentation concludes that IPv6 builds upon IPv4's foundations but addresses its limitations.
MapReduce is a programming model for processing large datasets in parallel across clusters of machines. It involves splitting the input data into independent chunks which are processed by the "map" step, and then grouping the outputs of the maps together and inputting them to the "reduce" step to produce the final results. The MapReduce paper presented Google's implementation which ran on a large cluster of commodity machines and used the Google File System for fault tolerance. It demonstrated that MapReduce can efficiently process very large amounts of data for applications like search, sorting and counting word frequencies.
This document discusses MapReduce, a programming model for processing large datasets in a distributed computing environment. It describes the key concepts of MapReduce including mapping input data to intermediate key-value pairs, shuffling, and reducing to output results. The document also covers MapReduce implementation details such as execution flow with a master and workers, fault tolerance, backup tasks, partitioning and combiner functions, skipping bad records, and counters.
This document provides an overview of MapReduce and Apache Hadoop. It discusses the history and components of Hadoop, including HDFS and MapReduce. It then walks through an example MapReduce job, the WordCount algorithm, to illustrate how MapReduce works. The WordCount example counts the frequency of words in documents by having mappers emit <word, 1> pairs and reducers sum the counts for each word.
Hadoop interview questions for freshers and experienced people. This is the best place for all beginners and Experts who are eager to learn Hadoop Tutorial from the scratch.
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The document discusses MapReduce and the Hadoop framework. It provides an overview of how MapReduce works, examples of problems it can solve, and how Hadoop implements MapReduce at scale across large clusters in a fault-tolerant manner using the HDFS distributed file system and YARN resource management.
The document summarizes the MapReduce programming model and associated implementation developed by Google for processing and generating large datasets in a distributed computing environment. It describes how users specify computations using map and reduce functions, and the underlying system automatically parallelizes execution across large clusters, handles failures, and coordinates inter-machine communication. The authors note over 10,000 distinct programs have been implemented using MapReduce internally at Google to process over 20 petabytes of data daily across its clusters.
This document summarizes a proposal to improve fault tolerance in Hadoop clusters. It proposes adding a "Backup" state to store intermediate MapReduce data, so reducers can continue working even if mappers fail. It also proposes a "supernode" protocol where neighboring slave nodes communicate task information. If one node fails, a neighbor can take over its tasks without involving the JobTracker. This would improve fault tolerance by allowing computation to continue locally between nodes after failures.
Big data refers to large volumes of unstructured or semi-structured data that is difficult to process using traditional databases and analysis tools. The amount of data generated daily is growing exponentially due to factors like increased internet usage and data collection by organizations. Hadoop is an open-source framework for distributed storage and processing of large datasets across clusters of commodity hardware. It uses HDFS for reliable storage and MapReduce as a programming model to process data in parallel across nodes.
Hadoop/MapReduce is an open source software framework for distributed storage and processing of large datasets across clusters of computers. It uses MapReduce, a programming model where input data is processed by "map" functions in parallel, and results are combined by "reduce" functions, to process and generate outputs from large amounts of data and nodes. The core components are the Hadoop Distributed File System for data storage, and the MapReduce programming model and framework. MapReduce jobs involve mapping data to intermediate key-value pairs, shuffling and sorting the data, and reducing to output results.
1. The document discusses concepts related to managing big data using Hadoop including data formats, analyzing data with MapReduce, scaling out, data flow, Hadoop streaming, and Hadoop pipes.
2. Hadoop allows for distributed processing of large datasets across clusters of computers using a simple programming model. It scales out to large clusters of commodity hardware and manages data processing and storage automatically.
3. Hadoop streaming and Hadoop pipes provide interfaces for running MapReduce jobs using any programming language, such as Python or C++, instead of just Java. This allows developers to use the language of their choice.
Hadoop Mapreduce Performance Enhancement Using In-Node Combinersijcsit
This document summarizes a research paper that proposes using in-node combiners to improve the performance of Hadoop MapReduce jobs. It discusses how MapReduce jobs are I/O intensive and describes two common bottlenecks: during the map phase when data is loaded from disks, and during the shuffle phase when intermediate results are transferred over the network. The paper introduces an in-node combiner approach to optimize I/O by locally aggregating intermediate results within nodes to reduce network traffic between mappers and reducers. It evaluates this approach through an experiment counting word occurrences in Twitter messages.
MapReduce: Ordering and Large-Scale Indexing on Large ClustersIRJET Journal
This document discusses MapReduce, a programming model for processing large datasets across large clusters. It describes how MapReduce works, with a map function that processes input key-value pairs to generate intermediate pairs, and a reduce function that combines values for the same intermediate key. The document provides examples of applications like distributed grep, counting URL access frequencies, and building an inverted index. It then describes the implementation of MapReduce across thousands of machines, how it provides fault tolerance, optimizes for data locality, and handles failures. Performance is evaluated for searching a terabyte of data and sorting a terabyte.
MapReduce provides an easy way to process large datasets in a distributed manner. It uses mappers to process input data and generate intermediate key-value pairs, and reducers to combine those intermediate pairs into the final output. Key aspects include job tracking, splitting data into tasks, and storing intermediate output locally rather than on HDFS for efficiency, since it is discarded after reducing.
This document describes MapReduce, a programming model and software framework for processing large datasets in a distributed manner. It introduces the key concepts of MapReduce including the map and reduce functions, distributed execution across clusters of machines, and fault tolerance. The document outlines how MapReduce abstracts away complexities like parallelization, data distribution, and failure handling. It has been used successfully at Google for large-scale tasks like search indexing and machine learning.
This is a deck of slides from a recent meetup of AWS Usergroup Greece, presented by Ioannis Konstantinou from the National Technical University of Athens.
The presentation gives an overview of the Map Reduce framework and a description of its open source implementation (Hadoop). Amazon's own Elastic Map Reduce (EMR) service is also mentioned. With the growing interest on Big Data this is a good introduction to the subject.
This document provides an overview of Hadoop frameworks and concepts. It discusses distributed file systems like HDFS and how they are organized at large scale. It then explains the MapReduce execution model and how it allows distributed processing of large datasets in a fault-tolerant manner. Specific algorithms like matrix-vector multiplication are discussed as examples of how MapReduce can be used. Finally, it introduces Hadoop YARN, which separates resource management from job execution, allowing more flexible processing of different types of applications on Hadoop clusters.
The document discusses big data and distributed computing. It explains that big data refers to large, unstructured datasets that are too large for traditional databases. Distributed computing uses multiple computers connected via a network to process large datasets in parallel. Hadoop is an open-source framework for distributed computing that uses MapReduce and HDFS for parallel processing and storage across clusters. HDFS stores data redundantly across nodes for fault tolerance.
MapReduce is a programming model for processing large datasets in a distributed environment. It consists of a map function that processes input key-value pairs to generate intermediate key-value pairs, and a reduce function that merges all intermediate values associated with the same key. It allows for parallelization of computations across large clusters. Example applications include word count, sorting, and indexing web links. Hadoop is an open source implementation of MapReduce that runs on commodity hardware.
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Big Data and Analytics Shaping the future of PaymentsRuchiRathor2
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Solution Manual for First Course in Abstract Algebra A, 8th Edition by John B...rightmanforbloodline
Solution Manual for First Course in Abstract Algebra A, 8th Edition by John B. Fraleigh, Verified Chapters 1 - 56,.pdf
Solution Manual for First Course in Abstract Algebra A, 8th Edition by John B. Fraleigh, Verified Chapters 1 - 56,.pdf
2. Contents
Computation Models for Distributed Computing
MPI
MapReduce
Why MapReduce?
How MapReduce works
Simple example
References
2
3. Distributed Computing
Why ?
Booming of big data generation (social media , e-commerce , banks , etc …)
Big data and machine learning , data mining AI became like bread and butter : better results
comes from analyzing bigger set of data.
How it works ?
Data-Partitioning : divide data into multiple tasks , each implementing the same procedure
(computations) at specific phase on its data segment
Task-Partitioning : assign different tasks to different computation units
Hardware for distributed Computing
Multiple processors (Multi-core processors) 3
4. Metrics
How to judge computational model suitability ?
Simplicity : level of developer experience
Scalability : adding more computational node , increase throughput / response time
fault tolerance : support recovering computed results when node is down
Maintainability : How easy fix bugs , add features
Cost : need for special hardware (Multi-core processors , large RAM , infiniband or can use
common ethernet cluster of commodity machines)
No one size fits all
sometimes it is better to use hybrid computational models 4
5. MPI (Message Passing Interface)
● Workload is divided among different
processes (each process may have
multiple threads)
● Communication is via Message passing
● Data exchange is via shared memory
(Physical / Virtual)
● Pros
○ Flexibility : programmer can customize message and communication between nodes
○ Speed : rely on sharing data via memory
Source :
https://computing.llnl.gov/tutorials/mpi/
5
6. MapReduce
Objective : Design scalable parallel programming
framework to be deployed on
large cluster of commodity machines
Data divided into splits , each processed
by map functions , whose output are
processed by reduce functions.
Originated and first practical
implementation in Google Inc. 2004
MapReduce implementations
Apache Hadoop (Computation)
6
8. MapReduce - Execution (2)
Platform :
Nodes communicating over ethernet network over TCP/IP
Two main type of processes :
Master : orchestrates the work
Worker : process data
Units of work :
Jobs :A MapReduce job is a unit of work that the client wants to be performed: it consists of the
input data, the MapReduce program, and configuration.
task : can be map task (process input to intermediate data), reduce task (process intermediate
data to output). Job is divided into several map / reduce tasks. 8
9. MapReduce Execution (3)
1. A copy of the master process is created
2. Input data is divided into M splits , each of 16 to 64 MB (user configured)
3. M map tasks are created and given unique IDs , each parses key/value pairs
of each split ,start processing , output is written into a memory buffer.
4. Map output is partitioned to R partitions. When buffers are full , they are
spilled into local hard disks , and master is notified by saved buffered
locations. All records with the same key are put in same partition.
Note : Map output stored in local worker file system , not distributed file system , as it is intermediate
and to avoid complexity.
5. Shuffling : when a reduce receives a notification from master that one of 9
10. MapReduce Execution (4)
6. When the reduce worker receives all its intermediate output , it sorts them by
key (sorting is need as reduce task may have several keys). (1)
7. When sorting finished , the reduce worker iterates over each key , passing the
key and list of values to the reduce function.
8. The output of reduce function is appended to the file corresponding to this
reduce worker.
9. For each HDFS block of the output of reduce task , one block is stored locally
in the reduce worker and the other 2 (assuming replication factor of 3) is
replicated on two other off-rack node for reliability.
■ Notes : 10
11. Master responsibilities
Find idle nodes (workers) to assign
map and reduce tasks.
monitor each task status
(idle , in-progress, finished).
Keep track of locations of R map
intermediate output on each map
worker machine.
Keep record of worker IDs and
other info (CPU , memory , disk size)
Continuously push information about
intermediate map output to reduce 11
12. Fault tolerance (1)
Objective : handle machine failures gracefully ,
i.e. programmer don’t need to handle
it or be aware of details.
Two type of failures :
Master failure
Worker failure
Two main activities
Failure detection
Recover lost (computed) data with least 12
13. Fault tolerance (2)
Worker failure
Detection : Timeout for master ping , mark worker as failed.
Remove worker from list of available worker.
For all map tasks assigned to that worker :
mark these tasks as idle
these tasks will be eligible for re-scheduling on other workers
map tasks are re-executed as output is stored in local file system in failed machine
all reduce workers are notified with re-execution so they can get intermediate data they
haven’t yet.
No need to re-execute reduce tasks as their output is stored in distributed file system and
13
14. Semantics in the Presence of Failures
Deterministic and Nondeterministic Functions
Deterministic functions always return the same result any time they are called with a specific set
of input values.
Nondeterministic functions may return different results each time they are called with a specific
set of input values.
If map and reduce function are “deterministic” , distributed implementation of
mapreduce framework must produce the same output of a non-faulting
sequential execution of the program.
Several copies of the same map/reduce task might run on different nodes for
sake of reliability and fault tolerance 14
15. Semantics in the Presence of Failures (2)
Mapper always write their output to tmp files (atomic commits).
When a map task finishes :
Renames the tmp file to final output.
Sends message to master informing it with the filename.
If another copy of the same map finished before , master ignores it , else store the filename.
Reducers do the same , and if multiple copies of the same reduce task finished ,
MapReduce framework rely on the atomic rename of the file system.
If map task are non-deterministic , and multiple copies of the map task run on
different machines , weak semantic condition can happen : two reducers read15
16. Semantics in the Presence of Failures (3)
16
● Workers #1 and #2 run the
same copy of map task M1
● Reducer task R1 reads its
input for M1 from worker #1
● Reducer task R2 reads its
input for M1 from worker #2 ,
as worker#1 has failed by the
time R2 has started.
● If M1’s function is deterministic
, we have complete
consistency.
● If M1’s function is not
deterministic , R1 and R2 may
receive different results from
M1.
17. Task granularity
Load balancing: fine grained is better , faster machines tend to take more tasks than slower machined
over time , leads to less overall job execution time.
Failure recovery : less time to re-execute failed tasks.
Very fine-grained tasks may not be desirable : overhead to manage and too much data shuffling
(consume valuable bandwidth).
Optimal Granularity : Split size = HDFS block size (128 MB by default)
HDFS block is guaranteed to be in the same node.
We need to maximize work done by one mapper locally
if split size < block : not fully utilizing possibility of local data processing
if split size > block : may need data transfer to make map function complete
17
18. Data locality
Network bandwidth is a valuable resource.
We assume a rack server hardware.
MapReduce scheduler works as follows :
1. Try to assign map task to the node where the
corresponding split block(s) reside , if it is free
assign , else go to step 2
2. try to find a free nod in the same rack to assign
the map task , if can’t find a free off-rack node
to assign.
● More complex implementation uses network cost
model.
18
19. Backup tasks
Stragglers : a set machines that run a set
of assigned tasks (MapReduce) very slow.
Slow running can be due to many reasons;
bad disk , slow network , low speed CPU.
Other tasks scheduled on stragglers cause
more
load and longer execution time.
Solution Mechanism:
When MapReduce job is close to finish
for all the “in-progress” tasks , issue backup tasks. 19
20. Refinements
Partitioning function :
Partition the output of mapping tasks into R partitions (each for a reduce task).
Good function should try as possible to make the partitions equal.
Default : hash(key) mod R
Works fine usually
problem arises when specific keys have many records
than the others.
Need design custom hash functions or change the key.
Combiner function
Reduce size of map intermediate output.
20
21. Refinements (2)
Skipping bad record
Bug in third party library that can’t be fixed , causes code crash at specific records
Terminating a job running for hours / days more expensive than sacrificing small percentage of
accuracy (If context allows , for ex. statistical analysis of large data).
How MapReduce handle that ?
1. Each worker process installs a signal handler that catches segmentation violations ,bus
errors and other possible fatal errors.
2. Before a map / reduce task runs , the MapReduce library stores the key value in global
variable.
3. When a map / reduce task function code generates a signal , the worker sends UDP 21
In google paper , sorting is mentioned as reducer responsibility , however in Hadoop Definitive Guide , sorting is mentioned as mapper responsibility and reducer is responsible for merging sorting intermediate output.