Preface

Data Addiction

Data is addictive. Our ability to collect and store data has grown massively in the last several decades. Yet our appetite for ever more data shows no sign of being satiated. Scientists want to be able to store more data in order to build better mathematical models of the world. Marketers want better data to understand their customers’ desires and buying habits. Financial analysts want to better understand the workings of their markets. And everybody wants to keep all their digital photographs, movies, emails, etc.

The computer and Internet revolutions have greatly increased our ability to collect and store data. Before these revolutions, the US Library of Congress was one of the largest collections of data in the world. It is estimated that its printed collections contain approximately 10 terabytes (TB) of information. Today large Internet companies collect that much data on a daily basis. And it is not just Internet applications that are producing data at prodigious rates. For example, the Large Synoptic Survey Telescope (LSST) planned for construction in Chile is expected to produce 20 TB of data every day.

Part of the reason for this massive growth in data is our ability to collect much more data. Every time someone clicks on a website’s links, the web server can record information about what page the user was on and which link he clicked. Every time a car drives over a sensor in the highway, its speed can be recorded. But much of the reason is also our ability to store that data. Ten years ago, telescopes took pictures of the sky every night. But they could not store it at the same detail level that will be possible when the LSST is operational. The extra data was being thrown away because there was nowhere to put it. The ability to collect and store vast quantities of data only feeds our data addiction.

One of the most commonly used tools for storing and processing data in computer systems over the last few decades has been the relational database management system (RDBMS). But as data sets have grown large, only the more sophisticated (and hence more expensive) RDBMSs have been able to reach the scale many users now desire. At the same time, many engineers and scientists involved in processing the data have realized that they do not need everything offered by an RDBMS. These systems are powerful and have many features, but many data owners who need to process terabytes or petabytes of data need only a subset of those features.

The high cost and unneeded features of RDBMSs have led to the development of many alternative data-processing systems. One such alternative system is Apache Hadoop. Hadoop is an open source project started by Doug Cutting. Over the past several years, Yahoo! and a number of other web companies have driven the development of Hadoop, which was based on papers published by Google describing how their engineers were dealing with the challenge of storing and processing the massive amounts of data they were collecting. For a history of Hadoop, see Hadoop: The Definitive Guide, by Tom White (O’Reilly). Hadoop is installed on a cluster of machines and provides a means to tie together storage and processing in that cluster.

The development of new data-processing systems such as Hadoop has spurred the porting of existing tools and languages and the construction of new tools, such as Apache Pig. Tools like Pig provide a higher level of abstraction for data users, giving them access to the power and flexibility of Hadoop without requiring them to write extensive data-processing applications in low-level Java code.

Who Should Read This Book

This book is intended for Pig programmers, new and old. Those who have never used Pig will find introductory material on how to run Pig and to get them started writing Pig Latin scripts. For seasoned Pig users, this book covers almost every feature of Pig: different modes it can be run in, complete coverage of the Pig Latin language, and how to extend Pig with your own User Defined Functions (UDFs). Even those who have been using Pig for a long time are likely to discover features they have not used before.

Being a relatively young project, Pig has changed and grown significantly over the last four years. In that time we have released versions 0.1 through 0.9. This book assumes Pig 0.7 as the base version. Wherever features are only in versions 0.8 or 0.9, this is called out. The biggest change from 0.6 to 0.7 is that load and store function interfaces were rewritten, so Chapter 11 will not be usable by those on 0.6 or earlier versions. However, the rest of the book will still be applicable.

Some knowledge of Hadoop will be useful for readers and Pig users. Appendix B provides an introduction to Hadoop and how it works. “Pig on Hadoop” walks through a very simple example of a Hadoop job. These sections will be helpful for those not already familiar with Hadoop.

Small snippets of Java, Python, and SQL are used in parts of this book. Knowledge of these languages is not required to use Pig, but knowledge of Python and Java will be necessary for some of the more advanced features. Those with a SQL background may find “Comparing query and dataflow languages” to be a helpful starting point in understanding the similarities and differences between Pig Latin and SQL.

Conventions Used in This Book

The following typographical conventions are used in this book:

Italic

Indicates new terms, URLs, email addresses, filenames, and file extensions.

Constant width

Used for program listings, as well as within paragraphs to refer to program elements such as variable or function names, databases, data types, environment variables, statements, and keywords.

Constant width bold

Shows commands or other text that should be typed literally by the user.

Constant width italic

Shows text that should be replaced with user-supplied values or by values determined by context.

This icon signifies a tip, suggestion, or general note.

This icon indicates a warning or caution.

Code Examples in This Book

Many of the example scripts, User Defined Functions (UDFs), and data used in this book are available for download from my GitHub repository. README files are included to help you get the UDFs built and to understand the contents of the datafiles. Each example script in the text that is available on GitHub has a comment at the beginning that gives the filename. Pig Latin and Python script examples are organized by chapter in the examples directory. UDFs, both Java and Python, are in a separate directory, udfs. All data sets are in the data directory.

For brevity, each script is written assuming that the input and output are in the local directory. Therefore, when in local mode, you should run Pig in the directory that the input data is in. When running on a cluster, you should place the data in your home directory on the cluster.

Example scripts were tested against Pig 0.8.0 or 0.8.1, except those scripts that use functionality newly introduced in version 0.9. These were run against builds from the 0.9 branch because 0.9 was not released until much of the book had been written.

The three data sets used in the examples are real data sets, though quite small. The file baseball contains baseball player statistics. A second set contains New York Stock Exchange data in two files: NYSE_daily and NYSE_dividends. This data was trimmed to include only stock symbols, starting with C from the year 2009, to make the data small enough to download easily. However, the schema of the data has not changed. If you want to download the entire data set and place it on a cluster (only a few nodes would be necessary), it would be a more realistic demonstration of Pig and Hadoop. Instructions on how to download the data are in the README files. The third data set is a very brief web crawl started from Pig’s web page.

Using Code Examples

This book is here to help you get your job done. In general, you may use the code in this book in your programs and documentation. You do not need to contact us for permission unless you’re reproducing a significant portion of the code. For example, writing a program that uses several chunks of code from this book does not require permission. Selling or distributing a CD-ROM of examples from O’Reilly books does require permission. Answering a question by citing this book and quoting example code does not require permission. Incorporating a significant amount of example code from this book into your product’s documentation does require permission.

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Acknowledgments

A book is like a professional football team. Much of the glory goes to the quarterback or a running back. But if the team has a bad offensive line, the quarterback never gets the chance to throw the ball. Receivers must be able to catch, and the defense must be able to prevent the other team from scoring. In short, the whole team must play well in order to win. And behind those on the field there is an array of coaches, trainers, and managers who prepare and guide the team. So it is with this book. My name goes on the cover. But without the amazing group of developers, researchers, testers, documentation writers, and users that contribute to the Pig project, there would be nothing worth writing about.

In particular, I would like to acknowledge Pig contributors and users for their contributions and feedback on this book. Chris Olston, Ben Reed, Richard Ding, Olga Natkovitch, Thejas Nair, Daniel Dai, and Dmitriy Ryaboy all provided helpful feedback on draft after draft. Julien Le Dem provided the example code for embedding Pig in Python. Jeremy Hanna wrote the section for Pig and Cassandra. Corrine Chandel deserves special mention for reviewing the entire book. Her feedback has added greatly to the book’s clarity and correctness.

Thanks go to Tom White for encouraging me in my aspiration to write this book, and for the sober warnings concerning the amount of time and effort it would require. Chris Douglas of the Hadoop project provided me with very helpful feedback on the sections covering Hadoop and MapReduce.

I would also like to thank Mike Loukides and the entire team at O’Reilly. They have made writing my first book an enjoyable and exhilarating experience. Finally, thanks to Yahoo! for nurturing Pig and dedicating more than 25 engineering years (and still counting) of effort to it, and for graciously giving me the time to write this book.