Introduccion a Hadoop / Introduction to Hadoop

Gerardo Barberena
gerardo.barberena@sensecloudnetwork.com

Hadoop
Apache Hadoop is a framework for running applications on
large cluster built of commodity hardware.
A common way of avoiding data loss is through replication:
redundant copies of the data are kept by the system so that in the
event of failure, there is another copy available. The Hadoop
Distributed Filesystem (HDFS), takes care of this problem.
The second problem is solved by a simple programming model-
Mapreduce. Hadoop is the popular open source implementation
of MapReduce, a powerful tool designed for deep analysis and
transformation of very large data sets.

Introduccion
Big Data:
•Big data is a term used to describe the voluminous amount of unstructured
and semi-structured data a company creates.
•Data that would take too much time and cost too much money to load into
a relational database for analysis.
• Big data doesn't refer to any specific quantity, the term is often used when
speaking about petabytes and exabytes of data.

• The New York Stock Exchange generates about one terabyte of new trade data per day.
• Facebook hosts approximately 10 billion photos, taking up one petabyte of storage.
• Ancestry.com, the genealogy site, stores around 2.5 petabytes of data.
• The Internet Archive stores around 2 petabytes of data, and is growing at a rate of 20
terabytes per month.
• The Large Hadron Collider near Geneva, Switzerland, produces about 15 petabytes of
data per year.

Cual es el Problema?
 The transfer speed is around 100 MB/s
 A standard disk is 1 Terabyte
 Concurrency Users
 Time to read entire disk= 10000 seconds or 3 Hours!
 Increase in processing time may not be as helpful because
• Network bandwidth is now more of a limiting factor
• Physical limits of processor chips have been reached

Computacion Distribuida vs
Paralelizacion
 Parallelization- Multiple processors or CPU’s
in a single machine
 Distributed Computing- Multiple computers
connected via a network

Computacion Distribuida
The key issues involved in this Solution:
 Hardware failure
 Combine the data after analysis
 Network Associated Problems

Problemas en la Computacion
Distribuida
• Hardware Failure:
As soon as we start using many pieces of
hardware, the chance that one will fail is fairly
high.
• Combine the data after analysis:
Most analysis tasks need to be able to combine
the data in some way; data read from one
disk may need to be combined with the data
from any of the other 99 disks.

Componentes Claves de Hadoop
A reliable shared storage and analysis system.
There are other subprojects of Hadoop that provide complementary
services, or build on the core to add higher-level abstractions The various
subprojects of hadoop include:
1. Core
2. Avro
3. Pig
4. HBase
5. Zookeeper
6. Hive

Funcionalidades de Hadoop en la
computacion distribuida
 The theoretical 1000-CPU machine would cost a very large amount of
money, far more than 1,000 single-CPU.
 Hadoop will tie these smaller and more reasonably priced machines together
into a single cost-effective compute cluster.
 Hadoop provides a simplified programming model which allows the user to
quickly write and test distributed systems, and its’ efficient, automatic
distribution of data and work across machines and in turn utilizing the
underlying parallelism of the CPU cores.

MapReduce
 Hadoop limits the amount of communication which can be performed by the
processes, as each individual record is processed by a task in isolation from one another
 By restricting the communication between nodes, Hadoop makes the distributed system
much more reliable. Individual node failures can be worked around by restarting tasks
on other machines.
 The other workers continue to operate as though nothing went wrong, leaving the
challenging aspects of partially restarting the program to the underlying Hadoop layer.
Map : (in_value,in_key)(out_key, intermediate_value)
Reduce: (out_key, intermediate_value) (out_value list)

Que es Map Reduce?
 MapReduce is a programming model
 Programs written in this functional style are automatically parallelized and
executed on a large cluster of commodity machines
 MapReduce is an associated implementation for processing and generating
large data sets.

Modelo de Programacion de MapReduce
 Map, written by the user, takes an input pair and produces a set of intermediate
key/value pairs. The MapReduce library groups together all intermediate values
associated with the same intermediate key I and passes them to the Reduce
function.

 The Reduce function, also written by the user, accepts an intermediate key I and a set of values
for that key. It merges together these values to form a possibly smaller set of values

Orientacion de los Nodos
Data Locality Optimization:
The computer nodes and the storage nodes are the same. The Map-Reduce
framework and the Distributed File System run on the same set of nodes. This
configuration allows the framework to effectively schedule tasks on the nodes where
data is already present, resulting in very high aggregate bandwidth across the
cluster.
If this is not possible: The computation is done by another processor on the same
rack.
“Moving Computation is Cheaper than Moving Data”

Como trabaja MapReduce
 A Map-Reduce job usually splits the input data-set into independent chunks which are
processed by the map tasks in a completely parallel manner.
 The framework sorts the outputs of the maps, which are then input to the reduce tasks.
 Typically both the input and the output of the job are stored in a file-system. The
framework takes care of scheduling tasks, monitoring them and re-executes the failed
tasks.
 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 information. Hadoop runs
the job by dividing it into tasks, of which there are two types: map tasks and reduce
tasks

Tolerancia a Fallas
 There are two types of nodes that control the job execution process: tasktrackers and
jobtrackers
 The jobtracker coordinates all the jobs run on the system by scheduling tasks to run on
tasktrackers.
 Tasktrackers run tasks and send progress reports to the jobtracker, which keeps a record
of the overall progress of each job.
 If a tasks fails, the jobtracker can reschedule it on a different tasktracker.

Introduccion a Hadoop / Introduction to Hadoop

Division de Entrada
 Input splits: Hadoop divides the input to a MapReduce job into fixed-size
pieces called input splits, or just splits. Hadoop creates one map task for each
split, which runs the user-defined map function for each record in the split.
 The quality of the load balancing increases as the splits become more fine-
grained.
 BUT if splits are too small, then the overhead of managing the splits and of map
task creation begins to dominate the total job execution time. For most jobs, a
good split size tends to be the size of a HDFS block, 64 MB by default.
WHY?
 Map tasks write their output to local disk, not to HDFS. Map output is
intermediate output: it’s processed by reduce tasks to produce the final output,
and once the job is complete the map output can be thrown away. So storing it
in HDFS, with replication, would be a waste of time. It is also possible that the
node running the map task fails before the map output has been consumed by
the reduce task.

Entrada a las Tareas de Reduccion
 Reduce tasks don’t have the advantage of
data locality—the input to a single reduce
task is normally the output from all mappers.

MapReduce data flow with a single reduce task

•Many MapReduce jobs are limited by the bandwidth available on the cluster.
•In order to minimize the data transferred between the map and reduce tasks, combiner
functions are introduced.
•Hadoop allows the user to specify a combiner function to be run on the map output—the
combiner function’s output forms the input to the reduce function.
•Combiner finctions can help cut down the amount of data shuffled between the maps and
the reduces.
Combinacion de Funciones

•Hadoop provides an API to MapReduce that allows you to
write your map and reduce functions in languages other than
Java.
•Hadoop Streaming uses Unix standard streams as the
interface between Hadoop and your program, so you can use
any language that can read standard input and write to
standard output to write your MapReduce program.
Hadoop Streaming:

•Hadoop Pipes is the name of the C++ interface to Hadoop MapReduce.
•Unlike Streaming, which uses standard input and output to communicate with
the map and reduce code, Pipes uses sockets as the channel over which the
tasktracker communicates with the process running the C++ map or reduce
function. JNI is not used.
Hadoop Pipes:

 Filesystems that manage the storage across a network of machines are called
distributed filesystems.
 Hadoop comes with a distributed filesystem called HDFS, which stands for
Hadoop Distributed Filesystem.
 HDFS, the Hadoop Distributed File System, is a distributed file system
designed to hold very large amounts of data (terabytes or even petabytes),
and provide high-throughput access to this information.
HADOOP DISTRIBUTED
FILESYSTEM (HDFS)

Problemas con HDFS
Making distributed filesystems is more complex than regular disk filesystems. This
is because the data is spanned over multiple nodes, so all the complications of
network programming kick in.
•Hardware Failure
An HDFS instance may consist of hundreds or thousands of server machines, each storing
part of the file system’s data. The fact that there are a huge number of components and that
each component has a non-trivial probability of failure means that some component of HDFS
is always non-functional. Therefore, detection of faults and quick, automatic recovery from
them is a core architectural goal of HDFS.
•Large Data Sets
Applications that run on HDFS have large data sets. A typical file in HDFS is gigabytes to
terabytes in size. Thus, HDFS is tuned to support large files. It should provide high
aggregate data bandwidth and scale to hundreds of nodes in a single cluster. It should
support tens of millions of files in a single instance.

Metas del HDFS
Streaming Data Access
Applications that run on HDFS need streaming access to their data sets. They are
not general purpose applications that typically run on general purpose file systems.
HDFS is designed more for batch processing rather than interactive use by users.
The emphasis is on high throughput of data access rather than low latency of data
access. POSIX imposes many hard requirements that are not needed for
applications that are targeted for HDFS. POSIX semantics in a few key areas has
been traded to increase data throughput rates.
Simple Coherency Model
HDFS applications need a write-once-read-many access model for files. A file once
created, written, and closed need not be changed. This assumption simplifies data
coherency issues and enables high throughput data access. A Map/Reduce
application or a web crawler application fits perfectly with this model. There is a plan
to support appending-writes to files in the future.

 Low-latency data access
Applications that require low-latency access to data, in the tens
of milliseconds
range, will not work well with HDFS. Remember HDFS is
optimized for delivering a high throughput of data, and this may
be at the expense of latency. HBase (Chapter 12) is currently a
better choice for low-latency access.
 Multiple writers, arbitrary file modifications
Files in HDFS may be written to by a single writer. Writes are
always made at the end of the file. There is no support for
multiple writers, or for modifications at arbitrary offsets in the
file. (These might be supported in the future, but they are likely
to be relatively inefficient.)

Block Abstraction
 Blocks:
• A block is the minimum amount of data that can be read or
written.
• 64 MB by default.
• Files in HDFS are broken into block-sized chunks, which are
stored as independent units.
• HDFS blocks are large compared to disk blocks, and the
reason is to minimize the cost of seeks. By making a block
large enough, the time to transfer the data from the disk can be
made to be significantly larger than the time to seek to the start
of the block. Thus the time to transfer a large file made of
multiple blocks operates at the disk transfer rate.

Benefits of Block Abstraction
 A file can be larger than any single disk in the network. There’s
nothing that requires the blocks from a file to be stored on the
same disk, so they can take advantage of any of the disks in
the cluster.
 Making the unit of abstraction a block rather than a file
simplifies the storage subsystem.
 Blocks provide fault tolerance and availability. To insure against
corrupted blocks and disk and machine failure, each block is
replicated to a small number of physically separate machines
(typically three). If a block becomes unavailable, a copy can be
read from another location in a way that is transparent to the
client.

Hadoop Archives
 HDFS stores small files inefficiently, since each file is stored in
a block, and block metadata is held in memory by the
namenode. Thus, a large number of small files can eat up a lot
of memory on the namenode.
 Hadoop Archives, or HAR files, are a file archiving facility that
packs files into HDFS blocks more efficiently, thereby reducing
namenode memory usage while still allowing transparent
access to files.
 Hadoop Archives can be used as input to MapReduce.

Limitations of Archiving
 There is currently no support for archive
compression, although the files that go into
the archive can be compressed
 Archives are immutable once they have been
created. To add or remove files, you must
recreate the archive

Namenodes and Datanodes
 A HDFS cluster has two types of node operating in a master-
worker pattern: a namenode (the master) and a number of
datanodes (workers).
 The namenode manages the filesystem namespace. It
maintains the filesystem tree and the metadata for all the files
and directories in the tree.
 Datanodes are the work horses of the filesystem. They store
and retrieve blocks when they are told to (by clients or the
namenode), and they report back to the namenode periodically
with lists of blocks that they are storing.

 Without the namenode, the filesystem cannot
be used. In fact, if the machine running the
namenode were obliterated, all the files on
the filesystem would be lost since there
would be no way of knowing how to
reconstruct the files from the blocks on the
datanodes.

 Important to make the namenode resilient to failure, and
Hadoop provides two mechanisms for this:
1. is to back up the files that make up the persistent state of the
filesystem metadata. Hadoop can be configured so that the
namenode writes its persistent state to multiple filesystems.
2. Another solution is to run a secondary namenode. The
secondary namenode usually runs on a separate physical
machine, since it requires plenty of CPU and as much memory
as the namenode to perform the merge. It keeps a copy of the
merged namespace image, which can be used in the event of
the namenode failing

File System Namespace
 HDFS supports a traditional hierarchical file organization. A user or an
application can create and remove files, move a file from one directory
to another, rename a file, create directories and store files inside these
directories.
 HDFS does not yet implement user quotas or access permissions.
HDFS does not support hard links or soft links. However, the HDFS
architecture does not preclude implementing these features.
 The Namenode maintains the file system namespace. Any change to
the file system namespace or its properties is recorded by the
Namenode. An application can specify the number of replicas of a file
that should be maintained by HDFS. The number of copies of a file is
called the replication factor of that file. This information is stored by the
Namenode.

Data Replication
 The blocks of a file are replicated for fault tolerance.
 The NameNode makes all decisions regarding replication of
blocks. It periodically receives a Heartbeat and a Blockreport
from each of the DataNodes in the cluster. Receipt of a
Heartbeat implies that the DataNode is functioning properly.
 A Blockreport contains a list of all blocks on a DataNode.
 When the replication factor is three, HDFS’s placement policy
is to put one replica on one node in the local rack, another on a
different node in the local rack, and the last on a different node
in a different rack.

Introduccion a Hadoop / Introduction to Hadoop

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