This Refcard presents Apache Hadoop, a software framework that enables distributed storage and processing of large datasets using simple high-level programming models. The card covers the most important concepts of Hadoop, describes its architecture, and explains how to start using it as well as write and execute various applications on Hadoop.
This Refcard presents Apache Hadoop, a software framework that enables distributed storage and processing of large datasets using simple high-level programming models. We cover the most important concepts of Hadoop, describe its architecture, guide how to start using it as well as write and execute various applications on Hadoop.
In the nutshell, Hadoop is an open-source project of the Apache Software Foundation that can be installed on a set of standard machines, so that these machines can communicate and work together to store and process large datasets. Hadoop has become very successful in recent years thanks to its ability to effectively crunch big data. It allows companies to store all of their data in one system and perform analysis on this data that would be otherwise impossible or very expensive to do with traditional solutions.
Many companion tools built around Hadoop offer a wide variety of processing techniques. Integration with ancillary systems and utilities is excellent, making real-world work with Hadoop easier and more productive. These tools together form the Hadoop Ecosystem.
Visit ttp://hadoop.apache.org to get more information about the project and access detailed documentation.
Note: By a standard machine, we mean typical servers that are available from many vendors and have components that are expected to fail and be replaced on a regular base. Because Hadoop scales nicely and provides many fault-tolerance mechanisms, you do not need to break the bank to purchase expensive top-end servers to minimize the risk of hardware failure and increase storage capacity and processing power.
To solve the challenge of processing and storing large datasets, Hadoop was built according to the following core characteristics:
Hadoop is divided into two core components
Many execution frameworks run on top of YARN, each tuned for a specific use-case. The most important are discussed under ‘YARN Applications’ below.
Let’s take a closer look on their architecture and describe how they cooperate.
Note: YARN is the new framework that replaces the former implementation of the processing layer in Hadoop. You can find how YARN addresses shortcomings of previous version on the Yahoo blog: https://developer.yahoo.com/blogs/hadoop/next-generation-apache-hadoop-mapreduce-3061.html.
HDFS is a Hadoop distributed file system. It can be installed on commodity servers and run on as many servers as you need - HDFS easily scales to thousands of nodes and petabytes of data.
The larger HDFS setup is, the bigger probability that some disks, servers or network switches will fail. HDFS survives these types of failures by replicating data on multiple servers. HDFS automatically detects that a given component has failed and takes necessary recovery actions that happen transparently to the user.
HDFS is designed for storing large files of the magnitude of hundreds of megabytes or gigabytes and provides high-throughput streaming data access to them. Last but not least, HDFS supports the write-once-read-many model. For this use case HDFS works like a charm. If you need, however, to store a large number of small files with a random read-write access, then other systems like RDBMS and Apache HBase can do a better job.
Note: HDFS does not allow you to modify a file’s content. There is only support for appending data at the end of the file. However, Hadoop was designed with HDFS to be one of many pluggable storage options – for example, with MapR-Fs, a proprietary filesystem, files are fully read-write. Other HDFS alternatives include Amazon S3 and IBM GPFS.
HDFS consists of following daemons that are installed and run on selected cluster nodes:
Figure 1 illustrates installation of HDFS on a 4-node cluster. One of the node hosts the NameNode daemon while the other three run DataNode daemons.
Note: NameNode and DataNode are Java processes that run on top of Linux distribution such as RedHat, Centos, Ubuntu and more. They use local disks for storing HDFS data.
HDFS splits each file into a sequence of smaller, but still large, blocks (default block size equals to 128MB – bigger blocks mean fewer disk seek operations, which results in large throughput). Each block is stored redundantly on multiple DataNodes for fault-tolerance. The block itself does not know which file it belongs to – this information is only maintained by the NameNode that has a global picture of all directories, files and blocks in HDFS.
Figure 2 illustrates the concept of splitting files into blocks. File X is split into blocks B1 and B2 and File Y comprises of only one block B3. All blocks are replicated 2 times within the cluster. As mentioned, information about which blocks compose a file is kept by the NameNode while raw data is stored by DataNodes.
HDFS provides a simple POSIX-like interface to work with data. You perform file system operations using hdfs dfs command.
To start playing with Hadoop you don’t have to go through process of setting up a whole cluster. Hadoop can run in so called pseudo-distributed mode on a single machine. You can download the sandbox Virtual Machine with all HDFS components already installed and start using Hadoop in no time! Just follow one of these links:
The following steps illustrate typical operations that a HDFS user can perform:
|List the content of home directory
$ hdfs dfs -ls /user/adam
|Upload a file from local file system to HDFS
$ hdfs dfs -put songs.txt /user/adam
|Read the content of the file from HDFS
$ hdfs dfs -cat /user/adam/songs.txt
|Change the permission of a file
$ hdfs dfs -chmod 700 /user/adam/songs.txt
|Set the replication factor of a file to 4
$ hdfs dfs -setrep -w 4 /user/adam/songs.txt
|Check the size of the file
$ hdfs dfs -du -h /user/adam/songs.txt
|Move the file to the newly created subdirectory
$ hdfs dfs -mv songs.txt songs/
|Remove directory from HDFS
$ hdfs dfs -rm -r songs
You can type hdfs dfs without any parameters to get a full list of available commands
YARN is a framework that manages resources on the cluster and enables running various distributed applications that process data stored (usually) on HDFS.
YARN, similarly to HDFS, follows the master-slave design with single ResourceManager daemon and multiple NodeManagers daemons. These types of daemons have different responsibilities.
YARN assigns cluster resources to various applications in the form of resource containers which represent a combination of resource elements such as memory and CPU.
Each application that executes on YARN cluster has its own ApplicationMaster process. This process starts when the application is scheduled on the cluster and coordinates the execution of all tasks within this application.
Each task runs within a container managed by the selected NodeManager. The ApplicationMaster negotiates resources (in form of containers) with the ResourceManager. On a successful negotiation, the ResourceManager delivers a container specification to the ApplicationMaster. This specification is then handed over to a NodeManager which launches the container and executes a task within it.
Figure 3 illustrates cooperation of YARN daemons on 4-node cluster running two applications that spawned 7 tasks in total.
HDFS and YARN daemons running on the same cluster give us a powerful platform for storing and processing large datasets.
Interestingly, DataNode and NodeManager processes are collocated on the same nodes to enable one of the biggest advantages of Hadoop called data locality. Data locality allows us to perform computations on the machines that actually store the data, thus minimizing the necessity of sending large chunks of data over the network. This technique known as “sending computation to the data” causes significant performance improvements while processing large data.
Figure 4: collocating HDFS and YARN daemons on a Hadoop cluster.
YARN is merely a resource manager that knows how to allocate distributed compute resources to various applications running on a Hadoop cluster. In other words, YARN itself does not provide any processing logic that can analyze data in HDFS. Hence various processing frameworks must be integrated with YARN (by providing a specific implementation of the ApplicationMaster) to run on a Hadoop cluster and process data in HDFS.
The table below provides a list and short descriptions of the most popular distributed computation frameworks that can run on a Hadoop cluster powered by YARN.
|MapReduce||The most popular processing framework for Hadoop that expresses computation as a series of map and reduce tasks. MapReduce is explained in the next section.|
|Apache Spark||A fast and general engine for large-scale data processing that optimizes the computation by aggressively caching data in memory.|
|Apache Tez||Generalizes the MapReduce paradigm to a more powerful and faster framework that executes computation as complex directed acyclic graphs of general data processing tasks.|
|Apache Giraph||An iterative graph processing framework for big data.|
|Apache Storm||A realtime stream processing engine.|
|Cloudera Impala||Fast SQL on Hadoop.|
Map(k1, v1) -> list(k2, v2)
Reduce(k2, list(v2)) -> list(k3, v3)
The general purpose of Map function is to transform or filter the input data. On the other hand Reduce function typically aggregates or summarizes the data produced by Map functions.
Figure 6 shows an example of using MapReduce to count occurrences of distinct words in a sentence. Map function splits the sentence and produces intermediate <key, value> pairs where a key is the word and a value equals to 1. Then reduce function sums all the 1s associated with a given word returning the total number of occurrences of that word.
MapReduce on YARN is a framework that enables running MapReduce jobs on the Hadoop cluster powered by YARN. It provides a high-level API for implementing custom Map and Reduce functions in various languages as well as the code-infrastructure needed to submit, run and monitor MapReduce jobs.
Note: MapReduce was historically the only programming model that you could use with Hadoop. It is no longer the case after the introduction of YARN. MapReduce is still the most popular application running on YARN clusters, though.
The execution of each MapReduce job is managed and coordinated by an instance of a special process called MapReduce Application Master (MR AM). MR AM spawns Map tasks that runs map() functions and Reduce tasks that run reduce() functions. Each Map task processes a separate subset of the input dataset (one block in HDFS by default). Each reduce task processes a separate subset of the intermediate data produced by the Map tasks. What’s more, Map and Reduce tasks run in isolation from one another, which allows for parallel and fault-tolerant computations.
To optimize the computation, MR AM tries to schedule data-local Map tasks. Such tasks execute in the containers running on the NodeManagers that are collocated with DataNodes that already store the data we want to process. Because by default each block in HDFS is redundantly stored on three DataNodes there are three NodeManagers that can be asked to run a given Map task locally.
Let’s see MapReduce in action and run a MapReduce job on a Hadoop cluster.
To get started quickly we use a jar file with MapReduce examples that is supplied with Hadoop packages. On Linux systems it can be found under
We run the Word Count job explained in the previous section.
To be or not to be
# hdfs dfs -mkdir input
# hadoop jar /usr/lib/hadoop-mapreduce/hadoop-mapreduce-examples.jar wordcount input hamlet-output
After a successful submission, track the progress of this job on the ResourceManager web UI.
If you use a sandbox, the ResourceManager UI is available at http://localhost:8088
Figure 7: ResourceManager UI with running Job
# hadoop fs -cat hamlet-output/*
Apart from the Word Count job, the jar file contains several other MapReduce examples. You can list them by typing the following command:
# hadoop jar /usr/lib/hadoop-mapreduce/hadoop-mapreduce-examples.jar
The table below provides a list and short descriptions of a couple of interesting MapReduce examples:
|grep||Counts the matches of a given regular expression in the input dataset.|
|pi||Estimates Pi using a quasi-Monte Carlo method.|
|terasort||Sorts the input dataset. Often used in conjunction with teragen and teravalidate. Find more details at|
|wordmean||Counts the average length of the words in the input dataset.|
Developing applications in native MapReduce can be a time-consuming and daunting work reserved only for programmers.
Fortunately, there are a number of frameworks that make the process of implementing distributed computation on Hadoop cluster easy and quicker, even for non-developers. The most popular ones are Hive and Pig.
Hive provides a SQL-like language, called HiveQL, for easier analysis of data in Hadoop cluster. When using Hive our datasets in HDFS are represented as tables that have rows and columns. Therefore, Hive is easy to learn and appealing to use for those who already know SQL and have experience in working with relational databases.
Having this said, Hive can be considered as a data warehouse infrastructure built on top of Hadoop.
A Hive query is translated into a series of MapReduce jobs (or a Tez directed acyclic graph) that are subsequently executed on a Hadoop cluster.
Let’s process a dataset about songs listened to by users in a given time. The input data consists of a tab-separated file songs.txt:
“Creep” Radiohead piotr 2014-07-20 “Desert Rose” Sting adam 2014-07-14 “Desert Rose” Sting piotr 2014-06-10 “Karma Police” Radiohead adam 2014-07-23 “Everybody” Madonna piotr 2014-07-01 “Stupid Car” Radiohead adam 2014-07-18 “All This Time” Sting adam 2014-07-13
We use Hive to find the two most popular artists in July 2014:
Note: We assume that commands below are executed as user “training”.
# hdfs dfs -mkdir songs # hdfs dfs -put songs.txt songs/
# hive hive>
hive> CREATE TABLE songs( title STRING, artist STRING, user STRING, date DATE ) ROW FORMAT DELIMITED FIELDS TERMINATED BY ‘\t’ LOCATION ‘/user/training/songs’;
hive> SHOW tables;
Apart from information about column names and types, you can see other interesting properties:
# Detailed Table Information Database: default Owner: root CreateTime: Tue Jul 29 14:08:49 PDT 2014 LastAccessTime: UNKNOWN Protect Mode: None Retention: 0 Location: hdfs://localhost:8020/user/root/songs Table Type: EXTERNAL_TABLE
SELECT artist, COUNT(*) AS total FROM songs WHERE year(date) = 2014 AND month(date) = 7 GROUP BY artist ORDER BY total DESC LIMIT 2;
This query is translated to two MapReduce jobs. Verify it by reading the standard output log messages generated by a Hive client or by tracking jobs executed on Hadoop cluster using ResourceManager web UI.
Note: at the time of this writing, MapReduce was the default execution engine for Hive. It may change in the future. See next section for instructions how to set other execution engine for Hive.
Hive is not constrained to translate queries into MapReduce jobs only. You can also instruct Hive to express its queries using other distributed frameworks such as Apache Tez.
Tez is an efficient framework that executes computation in form of a DAG (directed acyclic graph) of tasks. With Tez, a complex Hive query can be expressed as a single Tez DAG rather than multiple MapReduce jobs. This way we do not introduce the overhead of launching multiple jobs and avoid the cost of storing data between jobs on HDFS what saves I/O.
To benefit from Tez’s fast response times, simply overwrite hive.execution.engine property and set it to tez.
Follow these steps to execute the Hive query from the previous section as a Tez application:
# hive hive>
hive> SET hive.execution.engine=tez;
Note: now you can see different logs displayed on the console than when executing the query on MapReduce:
Total Jobs = 1 Launching Job 1 out of 1 Status: Running application id: application_123123_0001 Map 1: -/- Reducer 2: 0/1 Reducer 3: 0/1 Map 1: 0/1 Reducer 2: 0/1 Reducer 3: 0/1 … Map 1: 1/1/ Reducer 2: 1/1 Reducer 3: 1/1 Status: Finished successfully OK Radiohead 3 Sting 2
The query is now executed as only one Tez job instead of two MapReduce jobs as before. Tez isn’t tied to a strict MapReduce model - it can execute any sequence of tasks in a single job, for example Reduce tasks after Reduce tasks, what brings significant performance benefits.
Find out more about Tez on the blog: http://hortonworks.com/blog/apache-tez-a-new-chapter-in-hadoop-data-processing.
Apache Pig is another popular framework for large-scale computations on Hadoop. Similarly to Hive, Pig allows you to implement computations in an easier, faster and less-verbose way than using MapReduce. Pig introduces a simple, yet powerful, scripting-like language called PigLatin. PigLatin supports many common and ready-to-use data operations like filtering, aggregating, sorting and joining. Developers can also implement own functions (UDFs) that extend Pig’s core functionality.
Like Hive queries, Pig scripts are translated to MapReduce jobs scheduled to run on Hadoop cluster.
We use Pig to find the most popular artists as we did with Hive in previous example.
a = LOAD ‘songs/songs.txt’ as (title, artist, user, date); b = FILTER a BY date MATCHES ‘2014-07-.*’; c = GROUP b BY artist; d = FOREACH c GENERATE group, COUNT(b) AS total; e = ORDER d by total DESC; f = LIMIT e 2; STORE f INTO ‘top-artists-pig’;
# pig top-artists.pig
When developing Pig scripts you can iterate in local mode and catch mistakes before submitting jobs to the cluster. To enable local mode add -x local option to pig command.
Apache Hadoop is one of the most popular tools for big data processing thanks to its great features such as a high-level API, scalability, the ability to run on commodity hardware, fault tolerance and an open source nature. Hadoop has been successfully deployed in production by many companies for several years.
The Hadoop Ecosystem offers a variety of open-source tools for collecting, storing and processing data as well as cluster deployment, monitoring and data security. Thanks to this amazing ecosystem of tools, each company can now easily and relatively cheaply store and process a large amount of data in a distributed and highly scalable way.
This table contains names and short descriptions of the most useful and popular projects from the Hadoop Ecosystem that have not been mentioned yet:
|Oozie||Workflow scheduler system to manage Hadoop jobs.|
|Zookeeper||Framework that enables highly reliable distributed coordination.|
|Sqoop||Tool for efficient transfer of bulk data between Hadoop and structured datastores such as relational databases.|
|Flume||Service for aggregating, collecting and moving large amounts of log data.|
|Hbase||Non-relational, distributed database running on top of HDFS. It enables random realtime read/write access to your Big Data.|