MapReduce - Ahmet Sayar

Computer Engineering Department
Distributed Systems Course
Asst. Prof. Dr. Ahmet Sayar
Kocaeli University - Fall 2014
What does Scalable Mean?
• Operationally
– In the past: works even if data does not fit in main
– Now: can make use of 1000s of cheap computers
• Algorithmically
– In the past: if you have N data items, you must do no
more than Nm operations – polynomial time
– Now: if you have N data items, you should do no more
than Nm / k operations, for some large k
• Polynomial time algorithms must be parallelized
– Soon: if you have N data items, you should do no
more than N logN operations
Example: Find matching DNA
• Given a set of sequences
• Find all sequences equal to
Sequential (Linear) search
• Time = 0
• Match? NO
Sequential (Linear) search
• 40 Records, 40 comparison
• N Records, N comparison
• The algorithmic complexity is order N: O(N)
What if Sorted Sequences?
• No Match – keep searching in other half…
• O(log N)
New Task: Read Trimming
• Given a set of DNA sequences
• Trim the final n bps of each sequence
• Generate a new dataset
• Can we use an index?
– No we have to touch every record no matter what.
– O(N)
• Can we do any better?
New Task: Convert 405K TIFF Images to PNG
Another Example: Computing Word Frequency
of Every Word in a Single document
There is a pattern here …
• A function that maps a read to a trimmed read
• A function that maps a TIFF image to a PNG image
• A function that maps a document to its most
common word
• A function that maps a document to a histogram of
word frequencies.
Compute Word Frequency Across all Documents
(word, count)
• How to split things into pieces
– How to write map and reduce
Map Reduce
• Map-reduce: high-level programming model and
implementation for large-scale data processing.
• Programming model is based on functional
– Every record is assumed to be in form of <key, value>
• Google: paper published 2004
• Free variant: Hadoop – java – Apache
Example: Upper-case Mapper in ML
Example: Explode Mapper
Example: Filter Mapper
Example: Chaining Keyspaces
• Output key is int
Data Model
• Files
• A File = a bag of (key, value) pairs
• A map-reduce program:
– Input: a bag of (inputkey, value) pairs
– Output: a bag of (outputkey, value) pairs
Step 1: Map Phase
• User provides the Map function:
– Input: (input key, value)
– Output: bag of (intermediate key, value)
• System applies the map function in parallel to
all (input key, value) pairs in the input file
Step 2: Reduce Phase
• User provides Reduce function
– Input: (intermediate key, bag of values)
– Output: bag of output (values)
• The system will group all pairs with the same
intermediate key, and passes the bag of values
to the reduce function
• 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
Example: Sum Reducer
In summary
• Input and output : Each a set of key/value pairs
• Programmer specifies two function
• Map(in_key, in_value) -> list(out_key, intermediate_value)
– Process input key/value pair
– Produces set of intermediate pairs
• Reduce (out_key, list(intermediate_value)) ->
– Combines all intermediate values for a particular key
– Produces a set of merged output values (usually just one)
• Inspired by primitives from functional programming
languages such as Lisp, Scheme, and Haskell
Example: What does this do?
• Word count application of map reduce
Example: Word Length Histogram
Example: Word Length Histogram
• Big = Yellow =
• Medium = Red
= 5..9 letters
• Small = Blue =
2..4 letters
• Tiny = Pink = 1
More Examples: Building an Inverted Index
• Input
• Desired output
• Tweet1, (“I love pancakes
for breakfast”)
• Tweet2, (“I dislike
• Tweet3, (“what should I eat
for breakfast”)
• Tweet4, (“I love to eat”)
• “pancakes”, (tweet1,
• “breakfast”, (tweet1,
• “eat”, (tweet3, tweet4)
• “love”, (tweet1, tweet4)
More Examples: Relational Joins
Relational Join MapReduce: Before Map Phase
Relational Join MapReduce: Map Phase
Relational Join MapReduce: Reduce Phase
Relational Join in MapReduce,
Another Example
Simple Social Network Analysis:
Count Friends
Taxonomy of Parallel Architectures
Cluster Computing
• Large number of commodity servers, connected
by high speed, commodity network
• Rack holds a small number of servers
• Data center: holds many racks
• Massive parallelism
– 100s, or 1000s servers
– Many hours
• Failure
– If medium time between failure is 1 year
– Then, 1000 servers have one failure / hour
Distributed File System (DFS)
• For very large files: TBs, PBs
• Each file is partitioned into chunks, typically
• Each chunk is replicated several times (>2), on
different racks, for fault tolerance
• Implementations:
– Google’s DFS: GFS, proprietary
– Hadoop’s DFS: HDFS, open source
HDFS: Motivation
• Based on Google’s GFS
• Redundant storage of massive amounts of
data on cheap and unreliable computers
• Why not use an existing file system?
– Different workload and design priorities
– Handles much bigger dataset sizes than other file
• High component failure rates
– Inexpensive commodity components fail all the time
• Modest number of HUGE files
– Just a few million
– Each is 100MB or larger; multi-GB files typical
• Files are write-once, mostly appended to
– Perhaps concurrently
• Large streaming reads
• High sustained throughput favored over low
Hdfs Design Decisions
• Files are stored as blocks
– Much larger size than most filesystems (default is 64MB)
• Reliability through replication
– Each block replicated across 3+ DataNodes
• Single master (NameNode) coordinates access,
– Simple centralized management
• No data caching
– Little benefit due to large data sets, streaming reads
• Familiar interface, but customize API
– Simplify the problem; focus on distributed apps
Based on GFS Architecture

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