Chapter 10: MapReduce

Question 1

Origins cont’d

Answer 1

• Task for processing large amounts of data
  
  • Split data
  • Forward data and code to participant nodes
  • Check node state to react to errors
  • Retrieve and reorganize results
• Need: Simple large-scale data processing
• Inspiration from functional programming
• Google published MapReduce paper in 2004

Question 2

From Lisp to MapReduce - important features

Answer 2

• Two important concepts in functional programming
  
  • Map: do something to everything in a list
  • Fold: combine results of a list in some way

Question 3

Map

Answer 3

• Map is a higher-order function (= takes one or more functions as arguments)
• How map works:
  
  • Function is applied to every element in a list
  • Result is a new list

Question 4

Fold

Answer 4

• Fold is also a higher-order function
• How fold works:
  
  • Accumulator (generalization of a counter) set to initial value
  • Function applied to list element and the accumulator
  • Result stored in the accumulator
  • Repeated for every item in the list
  • Result is the final value in the accumulator

Question 5

Lisp -> MapReduce

Answer 5

• Let’s assume a long list of records: imagine if…
  
  • We can distribute the execution of map operations to multiple nodes
  • We have a mechanism for bringing map results back together in the fold operation
• That’s MapReduce! (and Hadoop)
• Implicit parallelism:
  
  • We can parallelize execution of map operations since they are isolated
  • We can reorder folding if the fold function is commutative and associative
    
    • Commutative -> change order of operands: x*y = y*x
    • Associative -> change order of operations: (2+3)+4 = 2+(3+4)=9

Question 6

MapReduce - basics

Answer 6

• Programming model & runtime system for processing large data-sets
  
  • E.g., Google’s search algorithms (PageRank: 2005 – 200TB indexed)
  • Goal: make it easy to use 1000s of CPUs and TBs of data
• Inspiration: Functional programming languages
  
  • Programmer specifies only “what”
  • System determines “how”
    
    • Schedule, parallelism, locality, communication..
• Ingredients:
  • Automatic parallelization and distribution ( # nodes ,
  • Fault-tolerance
  • I/O scheduling
  • Status and monitoring

Question 7

Simplified View of Architecture

Answer 7

• Input data is distributed to workers (i.e., available nodes)
• Workers perform Data computation
• Master coordinates worker selection & fail-over
• Results stored in output data files

Question 8

MapReduce Programming Model

Answer 8

• Input & output: each a set of key/value pairs
• Programmer specifies two functions:
  
  • map (in_key, in_value) -> list(out_key, intermediate_value)
    
    • Processes input key/ value pair
    • Produces set of intermediate pairs
  • reduce (out_key, list(intermediate_value)) -> list(out_value)
    
    • Combines all intermediate values for a particular key
    • Produces a set of merged output values (usually just one)
  • User also specifies I/O locations and tuning parameters
    
    • Partition (out_key,number of partitions) -> partition for out_key

Question 9

Map() function

Answer 9

• Records from the data source (lines out of files, rows of a database, etc.) are fed into the map function as key-value pairs: e.g., (filename, line)
• Map() produces one or more intermediate values along with an output key from the input

Question 10

Sort and schuffle

Answer 10

• Map reduce framework
  
  • Shuffles and sorts intermediate pairs based on key
  • Assigns resulting streams to reducers

Question 11

Reduce()

Answer 11

• After the map phase is over, all the intermediate values for a given output key are combined together into a list
• Reduce() combines those intermediate values into one or more final values for that same output key
• In practice, usually only one final value per key)

Question 12

MapReduce execution stages

Answer 12

• Scheduling: assigns workers to map and reduce tasks
• Data distribution: moves processes to data (Map)
• Synchronization: gathers, sorts, and shuffles intermediate data (Reduce)
• Errors and faults: detects worker failures and restarts

Question 13

MapReduce example: Word count

Answer 13

• Word count: Count the frequency of word appearances in set of documents
• MAP: Each map gets a document. Mapper generates key/value pairs each time the word appears (word,”1”)
  
  • INPUT: (filename, fileContent), where FileName is the key and FileContent is the value
  • OUTPUT: (word wordApparence), where word is key and wordApparence is value
• REDUCE: Combines the values per key and computes the sum
  
  • INPUT: (word,List) where word is the key and Listis the value
  • OUTPUT: (word, sum) where word is the key and sumis the value

Question 14

Combiners

Answer 14

• Often a map task will produce many pairs of the form (k,v1), (k,v2), … for the same key k
  
  • E.g., popular words in Word Count
• Can save network time by pre-aggregating at mapper
• For associative ops. like sum, count, max
• Decreases size of intermediate data
• Example: local counting for word count:
• def combiner(key, values): output(key, sum(values))

Question 15

Partition Function

Answer 15

• Inputs to map tasks are created by contiguous splits of input file
• For reduce, we need to ensure that records with the same
• intermediate key end up at the same worker
• System uses a default partition function e.g., hash(key) mod R
• Sometimes useful to override
  
  • Balance the loads
  • Specific requirement on which key value pairs should be in the same output files

Question 16

Parallelism

Answer 16

• map() functions run in parallel, creating different intermediate values from different input data sets
• reduce() functions also run in parallel, each working on a different output key
• All values are processed independently
• Bottleneck: reduce phase can’t start until map phase is completely finished.

Question 17

Fault Tolerance & Optimizations

Answer 17

• In a machine with 1000s of nodes, failures are common
• On worker failure:
  
  • Detect failure via periodic heartbeats
  • Re-execute completed and in-progress map tasks
  • Re-execute in progress reduce tasks
  • Task completion committed through master
• Not dealing with master failures so far…
• Optimization for fault-tolerance and load-balancing
  
  • Slow workers significantly lengthen completion time
    
    • Due to other jobs on machines, disk with errors, caching issues, …
    • Other jobs consuming resources on machine
  • Solution: Near end of phase, spawn backup copies of tasks
  • Which ever one finishes first “wins”

Question 18

Criticism

Answer 18

• Too low level
  
  • Manual programming of per record manipulation
  • As opposed to declarative model
• Nothing new
  
  • Map and reduce are classical Lisp or higher order functions
• Low per node performance
  
  • Due to replication and data transfer

Question 19

MapReducable?

Answer 19

• One-iteration algorithms are perfect fits
• Multiple-iteration algorithms are good fits
  
  • But small shared data have to be synchronized across iterations (typically through filesystem)
• Some algorithms are not good for MapReduce framework
  
  • Those algorithms typically require large shared data with a lot of synchronization (many machine learning algorithms)
  • Many alternatives: Iterative MapReduce Frameworks or Bulk Synchronous
• • Processing Frameworks

Question 20

One-iteration: Inverted index NN

Answer 20

• List of documents that contain a certain term – Basic structure of modern search engines
• Map process (parser)
  
  • Tokenize documents
  • Create (term, document) pairs
• Combine process
  
  • Locally combine pairs with same term to postings lists
• Reduce process (inverter)
  
  • Combine all postings lists for a term

Question 21

Iterative MapReduce

Answer 21

• Iterative algorithms: Repeat map and reduce jobs
• Examples: PageRank, K-means, SVM and many more
• In MapReduce, the only way to share data across jobs is stable storage -> slow !
• Some proposed solutions include: Twister and Spark

Question 22

Spark basics NN

Answer 22

• Up to 100x faster than Hadoop MapReduce
• Several programming interfaces (Java, Scala, Python, R)
• Powers a stack of useful libraries (MLlib, GprahX, Spark Streaming )
• Fault tolerance (for crashes & stragglers)
• Extremely well documented
• Based on the concept of a resilient distributed dataset (RDD):
  • Read-only collection of objects partitioned across a set of machines that can be rebuilt if a partition is lost
  • Created by transforming data in stable storage using data flow operators (map, filter, group by, …)
  • A handle to an RDD contains enough information to compute the RDD starting from data in reliable storage
  • Can be cached across parallel operations

Question 23

Bulk Synchronous Parallel (BSP) abstract computer

Answer 23

• BSP is a parallel programming model: Distributed-memory parallel computer
• Global vision of a number of processor/memory pairs interconnected by a communication network
• Offers several advantages over MapReduce and MPI
  
  • Supports message passing paradigm style for application development
  • Provides a flexible, simple, and easy-to-use small API
  • Enables to perform better than MPI for communication-intensive applications: All communication actions considered as a unit
  • Guarantees impossibility of deadlocks or collisions in the communication mechanisms: Synchronization is part of the programming model
  • Predictable performance

Question 24

BSP model NN

Answer 24

• Processor-memory pairs
• Communications network that delivers messages in a point- to-point manner
• Mechanism for the efficient barrier synchronization for all or a subset of the processes
• There are no special combining, replicating, or broadcasting facilities

Question 25

BSP programs NN

Answer 25

• BSP programs composed of supersteps
• In each superstep, processors execute computation
• Steps using locally stored data, and steps can send and receive messages
• Processors synchronize at end of superstep (at which time all messages have been received)

Question 26

What is MapReduce?

Answer 26

Programming model and runtime for processing large data-sets (in clusters).

Question 27

How does MapReduce differ from message-passing interface (MPI) and remote procedure calls (RPC)?

Answer 27

It offers a higher layer of abstraction at the cost of generality.

Question 28

What does MapReduce offer? Provide at least three bullet points

Answer 28

• Automatic parallelization and distribution
• Fault-tolerance
• I/O scheduling
• Status and monitoring

Question 29

Describe the different stages in the execution framework of MapReduce (4 bullet points).

Answer 29

• Scheduling: assigns workers to map and reduce tasks
• Data distribution: moves processes to data (Map)
• Synchronization: gathers, sorts, and shuffles intermediate data (Reduce)
• Errors and faults: detects worker failures and restarts

Question 30

What type of algorithms are not good for the MapReduce framework ?

Answer 30

Algorithms that typically require large shared info with a lot of synchronization, e.g., SVM.

Question 31

Word count: Count the frequency of word apperances in set of ducuments.

Answer 31

MAP: Each map gets a document. The mapper generates many key/value pairs for the apperance of a word, i.e., (word, “1”)

• Input: (fileName, fileContent) , where fileName is the key and fileContent is the value
• Output: (word, wordApparence)-pairs, where word is the key, and wordApparence is the value

REDUCE: Combines thevalues for a key and computes the sum

• Input: (word, List)
• Output: (word, sum(wordApparence))

Question 32

Search for a pattern: Data is a set of files containing lines of text. Output the file names that contain this pattern.

Answer 32

MAP: Given (filename, some text) and “pattern”, if “text” matches “pattern” output (filename, _)

• Input: The whole file, line by line (filename, some text) and pattern
• Output: (filename, _)-pairs, where filename is the key and value is notrelevant

REDUCE: Identity function. Data is not changed

• Input: (filename, _)
• Output: (filename, _)

Question 33

The MapReduce formulation of K-means usually needs a driver or wrapper around the normal execution framework. What is the reason for this?

Answer 33

Because of the iterations required in the K-means procedure, we need to loop MapReduce. The MapReduce K-means algorithm can be formulated using the following components:

• Driver or wrapper
• Mapper
• Combiner
• Reducer

Question 34

Consider that a single file contains the predefined cluster centers K and the data points are distributed in several files. Provide a short description defining the task performed by each component of MapReduce K-means and define their input and output.

Answer 34

• Driver or wrapper
  
  • Runs multiple iteration jobs using mapper+combiner+reducer
• Mapper
  
  • Task: Assign each point to closest centroids
  • Configure: A single file containing cluster centers
  • Input: Input data points
  • Output: (data id, cluster id)
• Combiner
  
  • Input:(data id, cluster id)
  • Output: (cluster id, (partial sum, number of points))
• Reduces
  
  • Task: Update each centroid with its new location
  • Input:(data id, cluster id)
  • Output: (cluster id, cluster centroid)

Question 35

What characteristic of K-means could cause a large computation overhead?

Answer 35

Since we are looping MapReduce, this means constant access to the IO (Hard-drive), if the K-means requires a large amount of iterations this could be a major source for time delay.