Distributed Computing with MapReduce
Lecture 2 of NoSQL Databases (PA195)
David Novak & Vlastislav Dohnal
Faculty of Informatics, Masaryk University, Brno
Agenda
● Distributed Data Processing
● Google MapReduce
○ Motivation and History
○ Google File System (GFS)
○ MapReduce: Schema, Example, MapReduce Framework
● Apache Hadoop
○ Hadoop Modules and Related Projects
○ Hadoop Distributed File System (HDFS)
○ Hadoop MapReduce
● MapReduce in Other Systems 2
Distributed Data Processing
What is the best way of doing distributed processing?
Centralized (and in memory)
Don't do it, if don't have to
3
Big Data Processing
● Big Data analytics (or data mining)
○ need to process large data volumes quickly
○ want to use computing cluster instead of a super-computer
● Communication (sending data) between compute
nodes is expensive
=> model of “moving the computing to data”
4
Big Data Processing II
● HW failures are rather a rule than an exception:
1. Files must be stored redundantly
■ over different racks to overcome also rack failures
2. Computations must be divided into independent tasks
■ that can be restarted in case of a failure
switch
racks with compute nodes
Computing cluster architecture:
source: J. Leskovec, A. Rajaraman, and J. D. Ullman, Mining of Massive Datasets. 2014. 5
Agenda
● Distributed Data Processing
● Google MapReduce
○ Motivation and History
○ Google File System (GFS)
○ MapReduce: Schema, Example, MapReduce Framework
● Apache Hadoop
○ Hadoop Modules and Related Projects
○ Hadoop Distributed File System (HDFS)
○ Hadoop MapReduce
● MapReduce in Other Systems 6
PageRank
PageRank works by counting the number and quality of links
to a page to determine a rough estimate of how important the
website is.
The underlying assumption is that
more important websites are likely
to receive more links from other
websites.
https://en.wikipedia.org/wiki/PageRank 7
● Additional factors:
1. Individual data files can be enormous (terabyte or more)
2. The files were rarely updated
■ the computations were read-heavy, but not very write-heavy
■ If writes occurred, they were appended at the end of the file
MapReduce: Origins
● In 2003, Google had the following problem:
1. How to rank tens of billions of webpages by their
“importance” (PageRank) in a “reasonable” amount of time?
2. How to compute these rankings efficiently when the data is
scattered across thousands of computers?
8
Google Solution
● Google found the following solutions:
○ Google File System (GFS)
■ A distributed file system
○ MapReduce
■ A programming model for distributed data processing
9
Google File System (GFS)
● One machine is a master, the other chunkservers
○ The master keeps track of all file metadata
■ mappings from files to chunks and locations of the chunks
○ To find a file chunk, client queries the master,
and then contacts the relevant chunkservers
○ The master’s metadata files are also replicated
● Files are divided into chunks (typically 64 MB)
○ The chunks are replicated at three different machines
■ ...in an “intelligent” fashion, e.g. never all on the same computer rack
○ The chunk size and replication factor are tunable
10
GFS: Schema
source: http://dl.acm.org/citation.cfm?id=945450 11
MapReduce (1)
● MapReduce is a programming model sitting
on the top of a Distributed File System
○ Originally: no data model – data stored directly in files
● A distributed computational task has three phases:
1. The map phase: data transformation
2. The grouping phase
■ done automatically by the MapReduce Framework
3. The reduce phase: data aggregation
● User must define only map & reduce functions
12
Map
● Map function simplifies the problem in this way:
○ Input: a single data item (e.g., line of text) from a data file
○ Output: zero or more (key, value) pairs
● The keys are not typical “keys”:
○ They do not have to be unique
○ A map task can produce several key-value pairs with the
same key (even from a single input)
● Map phase applies the map function to all items.
13
input data
map function
output data
(color indicates key)
14
Grouping Phase
● Grouping (Shuffling): The key-value outputs from
the map phase are grouped by key
○ Values sharing the same key are sent to the same reducer.
○ These values are consolidated into a single list (key, list).
■ This is convenient for the reduce function
○ This phase is realized by the MapReduce framework.
intermediate output
(color indicates key)
shuffle (grouping) phase
15
Reduce Phase
● Reduce: combine the values for each key
■ to achieve the final result(s) of the computational task
○ Input: (key, value-list)
■ the value-list contains all values generated for a given key in the Map
phase
○ Output: (key, value-list)
■ zero or more output records
16
input data
map function
intermediate output
(color indicates key)
input data
reduce function
output data
shuffle (grouping) phase
17
Example: Word Count
Task: Calculate word frequency in a set of documents
map(String key, Text value):
// key: document name (ignored)
// value: content of document (words)
foreach word w in value:
emitIntermediate(w, 1);
reduce(String key, Iterator values):
// key: a word
// values: a list of counts
int result = 0;
foreach v in values:
result += v;
emit(key, result); 18
Example: Word Count (2)
source: http://www.cs.uml.edu/~jlu1/doc/source/report/MapReduce.html 19
MapReduce: Combiner
● If the reduce function is commutative & associative
○ The values can be combined in any order
and combined per partes (grouped)
■ with the same result (e.g., Word Counts)
● ...then we can do "partial reductions"
○ Apply the same reduce function right after the map phase,
before shuffling and redistribution to reducer nodes
● This (optional) step is known as the combiner
○ Note: it is still necessary to run the reduce phase.
20
Example: Word Count, Combiner
Task: Calculate word frequency in a set of documents
combine(String key, Iterator values):
// key: a word
// values: a list of local counts
int result = 0;
foreach v in values:
result += v;
emit(key, result);
21
Example: Word Count with Combiner
source: http://www.admin-magazine.com/HPC/Articles/MapReduce-and-Hadoop 22
MapReduce Framework
● MapReduce framework takes care about
○ Distribution and parallelizing of the computation
○ Monitoring of the whole distributed task
○ The grouping (shuffling) phase
■ putting together intermediate results
○ Recovering from any failures
● User must define only map & reduce functions
○ but can define also other additional functions (see below)
23
MapReduce Framework (2)
source: Dean, J. & Ghemawat, S. (2004). MapReduce: Simplified Data Processing on Large Clusters 24
MapReduce Framework: Details
1. Input reader (function)
○ defines how to read data from underlying storage
2. Map (phase)
○ master node prepares M data splits and M idle Map tasks
○ pass individual splits to the Map tasks that run on workers
○ these map tasks are then running
○ when a task is finished, its intermediate results are stored
3. Combiner (function, optional)
○ combine local intermediate output from the Map phase
25
MapReduce Framework: Details (2)
4. Partition (function)
○ to partition intermediate results for individual Reducers
5. Comparator (function)
○ sort and group the input for each Reducer
6. Reduce (phase)
○ master node creates R idle Reduce tasks on workers
○ Partition function defines a data batch for each reducer
○ each Reduce task uses Comparator to create key-values pairs
○ function Reduce is applied on each key-values pair
7. Output writer (function)
○ defines how the output key-value pairs are written out 26
MapReduce: Example II
Task: Calculate a graph of web links
● what pages reference (<a href=””>) each page (backlinks)
map(String url, Text html):
// url: web page URL
// html: HTML text of the page (linearized HTML tags)
foreach tag t in html:
if t is <a> then:
emitIntermediate(t.href, url);
reduce(String key, Iterator values):
// key: target URLs
// values: a list of source URLs
emit(key, values);
27
Example II: Result
Input: (page_URL, HTML_code)
("http://cnn.com", "<html>...<a href="http://cnn.com">link</a>...</html>")
("http://ihned.cz", "<html>...<a href="http://cnn.com">link</a>...</html>")
("http://idnes.cz",
"<html>...<a href="http://cnn.com">x</a>...
<a href="http://ihned.cz">y</a>...<a href="http://idnes.cz">z</a>
</html>")
Intermediate output after Map phase:
("http://cnn.com", "http://cnn.com")
("http://cnn.com", "http://ihned.cz")
("http://cnn.com", "http://idnes.cz")
("http://ihned.cz", "http://idnes.cz")
("http://idnes.cz", "http://idnes.cz")
Intermediate result after shuffle phase (the same as output after Reduce phase):
("http://cnn.com", ["http://cnn.com", "http://ihned.cz", "http://idnes.cz"] )
("http://ihned.cz", [ "http://idnes.cz" ])
("http://idnes.cz", [ "http://idnes.cz" ])
28
MapReduce: Example III
Task: What are the lengths of words in the input text
● output = how many words are in the text for each length
map(String key, Text value):
// key: document name (ignored)
// value: content of document (words)
foreach word w in value:
emitIntermediate(length(w), 1);
reduce(Integer key, Iterator values):
// key: a length
// values: a list of counts
int result = 0;
foreach v in values:
result += v;
emit(key, result); 29
MapReduce: Features
● MapReduce uses a “shared nothing” architecture
○ Nodes operate independently, sharing no memory/disk
○ Common feature of many NoSQL systems
● Data partitioned and replicated over many nodes
○ Pro: Large number of read/write operations per second
○ Con: Coordination problem – which nodes have my data,
and when?
30
Applicability of MapReduce
● MR is applicable if the problem is parallelizable
● Two problems:
1. The programming model is limited
(only two phases with a given schema)
2. There is no data model - it works only on “data chunks”
● Google’s answer to the 2nd problem was BigTable
○ The first column-family system (2005)
○ Subsequent systems: HBase (over Hadoop), Cassandra,...
31
Agenda
● Distributed Data Processing
● Google MapReduce
○ Motivation and History
○ Google File System (GFS)
○ MapReduce: Schema, Example, MapReduce Framework
● Apache Hadoop
○ Hadoop Modules and Related Projects
○ Hadoop Distributed File System (HDFS)
○ Hadoop MapReduce
● MapReduce in Other Systems 32
Apache Hadoop
● Open-source software framework
○ Implemented in Java
● Able to run applications on large clusters of
commodity hardware
○ Multi-terabyte data-sets
○ Thousands of nodes
● Derived from the idea of Google's
MapReduce and Google File System
web: http://hadoop.apache.org/ 33
Hadoop: Modules
● Hadoop Common
○ Common support functions for other Hadoop modules
● Hadoop Distributed File System (HDFS)
○ Distributed file system
○ High-throughput access to application data
● Hadoop YARN
○ Job scheduling and cluster
resource management
● Hadoop MapReduce
○ YARN-based system for
parallel data processing source: https://goo.gl/NPuuJr 34
HDFS (Hadoop Distributed File System)
● Free and open source
● Cross-platform (pure Java)
○ Bindings for non-Java programming languages
● Highly scalable
● Fault-tolerant
○ Idea: “failure is the norm rather than exception”
■ A HDFS instance may consist of thousands of machines and each can fail
○ Detection of faults
○ Quick, automatic recovery
● Not the best in efficiency
35
HDFS: Data Characteristics
● Assumes:
○ Streaming data access
■ reading the files from the beginning till the end
○ Batch processing rather than interactive user access
● Large data sets and files
● Write-once / read-many
○ A file once created does not need to be changed often
○ This assumption simplifies coherency
● Optimal applications for this model: MapReduce,
web-crawlers, data warehouses, … 36
HDFS: Basic Components
● Master/slave architecture
● HDFS exposes file system namespace
○ File is internally split into blocks
● NameNode - master server
○ Manages the file system namespace
■ Opening/closing/renaming files and directories
■ Regulates file accesses
○ Determines mapping of blocks to DataNodes
● DataNode - manages file blocks
○ Block read/write/creation/deletion/replication
○ Usually one per physical node
37
HDFS: Schema
38
HDFS: NameNode
● NameNode has a structure called FsImage
○ Entire file system namespace + mapping of blocks to files
+ file system properties
○ Stored in a file in NameNode’s local file system
○ Designed to be compact
■ Loaded in NameNode’s memory (4 GB of RAM is sufficient)
● NameNode uses a transaction log called EditLog
○ to record every change to the file system’s meta data
■ E.g., creating a new file, change in replication factor of a file, ..
○ EditLog is stored in the NameNode’s local file system
39
HDFS: DataNode
● Stores data in files on its local file system
○ Each HDFS block in a separate file
○ Has no knowledge about HDFS file system
● When the DataNode starts up:
○ It generates a list of all HDFS blocks = BlockReport
○ It sends the report to NameNode
40
HDFS: Blocks & Replication
● HDFS can store very large files across a cluster
○ Each file is a sequence of blocks
○ All blocks in the file are of the same size
■ Except the last one
■ Block size is configurable per file (default 128MB)
○ Blocks are replicated for fault tolerance
■ Number of replicas is configurable per file
● NameNode receives HeartBeat and BlockReport
from each DataNode
○ BlockReport: list of all blocks on a DataNode
41
HDFS: Block Replication
42
HDFS: Reliability
● Primary objective: to store data reliably in case of:
○ NameNode failure
○ DataNode failure
○ Network partition
■ a subset of DataNodes can lose connectivity with NameNode
● In case of absence of a HeartBeat message
○ NameNode marks “dead” the DataNodes without
HeartBeat, and does not send any I/O requests to them.
○ The death of a DataNode typically results in re-replication
43
Hadoop: MapReduce
● Hadoop MapReduce requires:
○ Distributed file system (typically HDFS)
○ Engine that can distribute, coordinate, monitor and gather
the results (typically YARN)
● Two main components:
○ JobTracker (master) = scheduler
■ tracks the whole MapReduce job
■ communicates with HDFS NameNode to run the task close to the data
○ TaskTracker (slave on each node) – is assigned a Map or
a Reduce task (or other operations)
■ Each task runs in its own JVM
44
Hadoop HDFS + MapReduce
source: http://bigdata.black/architecture/hadoop/what-is-hadoop/ 45
Hadoop MapReduce: Schema
46
output
file 1
output
file 2
Map phase Reduce phase
input
files
block 1
block 2
block 3
block 4
block 5
Hadoop MR: WordCount Example (1)
public class Map
extends Mapper<LongWritable, Text, Text, IntWritable> {
private final static IntWritable one = new IntWritable(1);
private final Text word = new Text();
@Override protected void map(LongWritable key, Text value,
Context context) throws ... {
String string = value.toString()
StringTokenizer tokenizer = new StringTokenizer(string);
while (tokenizer.hasMoreTokens()) {
word.set(tokenizer.nextToken());
context.write(word, one);
}
}
} 47
Hadoop MR: WordCount Example (2)
public class Reduce
extends Reducer<Text, IntWritable, Text, IntWritable> {
@Override
public void reduce (Text key, Iterable<IntWritable> values,
Context context) throws ... {
int sum = 0;
for (IntWritable val : values) {
sum += val.get();
}
context.write(key, new IntWritable(sum));
}
}
48
source: http://www.dineshonjava.com/2014/11/hadoop-architecture.html#.WLU6aBLyso8 49
Hadoop: Related Projects
● Avro: a data serialization system
● HBase: scalable distributed column-family database
● Cassandra: scalable distributed column-family database
● ZooKeeper: high-performance coordination service for
distributed applications
● Hive: data warehouse: ad hoc querying & data summarization
● Mahout: scalable machine learning and data mining library
50
Agenda
● Distributed Data Processing
● Google MapReduce
○ Motivation and History
○ Google File System (GFS)
○ MapReduce: Schema, Example, MapReduce Framework
● Apache Hadoop
○ Hadoop Modules and Related Projects
○ Hadoop Distributed File System (HDFS)
○ Hadoop MapReduce
● MapReduce in Other Systems 51
MapReduce: Implementation
Amazon Elastic
MapReduce
52
Apache Spark
● Engine for distributed data processing
○ Runs over Hadoop Yarn, Apache Mesos, standalone, …
○ Can access data from HDFS, Cassandra, HBase, AWS S3
● Can do MapReduce
○ Is much faster than pure Hadoop
■ They say 10x on the disk, 100x in memory
○ The main reason: intermediate data in memory
● Different languages to write MapReduce tasks
○ Java, Scala, Python, R
homepage: http://spark.apache.org/ 53
Apache Spark: Example
● Example of a MapReduce task in Spark Shell
○ The shell works with Scala language
○ Example: Word count
val textFile = sc.textFile("hdfs://...")
val counts = textFile.flatMap(line => line.split(" "))
.map(word => (word, 1))
.reduceByKey(_+_)
counts.saveAsTextFile("hdfs://...")
● Comparison of Hadoop and Spark: link
54
MapReduce in MongoDB
collection "accesses":
{
"user_id": <ObjectId>,
"login_time": <time_the_user_entered_the_system>,
"logout_time": <time_the_user_left_the_system>,
"access_type": <type_of_the_access>
}
● How much time did each user spend logged in
○ Counting just accesses of type “regular”
db.accesses.mapReduce(
function() { emit (this.user_id, this.logout_time - this.login_time); },
function(key, values) { return Array.sum( values ); },
{
query: { access_type: "regular" },
out: "access_times"
}
)
55
References
● RNDr. Irena Holubova, Ph.D. MMF UK course NDBI040:
Big Data Management and NoSQL Databases
● Dean, J. & Ghemawat, S. MapReduce: Simplified Data
Processing on Large Clusters. In OSDI 2004 (pp 137-149)
● Firas Abuzaid, Perth Charernwattanagul (2014). Lecture 8
“NoSQL” of Stanford course CS145. link
● J. Leskovec, A. Rajaraman, and J. D. Ullman, Mining of
Massive Datasets. 2014.
● I. Holubová, J. Kosek, K. Minařík, D. Novák. Big Data a
NoSQL databáze. Praha: Grada Publishing, 2015. 288 p.
56