Learning Concurrent Programming in Scala - Second Edition.pdf

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Learning Concurrent Programming

in Scala

Second Edition

Learn the art of building intricate, modern, scalable, and

concurrent applications using Scala

Aleksandar Prokopec

BIRMINGHAM - MUMBAI

Learning Concurrent Programming in Scala

Second Edition

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First published: November 2014

Second edition: February 2017

Production reference: 1170217

Published by Packt Publishing Ltd.

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35 Livery Street

Birmingham

B3 2PB, UK.

ISBN 978-1-78646-689-1

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Credits

Author

Aleksandar Prokopec

Reviewers

Vikash Sharma

Dominik Gruntz

Zhen Li

Lukas Rytz

Michel Schinz

Samira Tasharofi

Commissioning Editor

Aaron Lazar

Acquisition Editor

Sonali Vernekar

Content Development Editor

Rohit Kumar Singh

Technical Editor

Pavan Ramchandani

Copy Editor

Safis Editing

Project Coordinator

Vaidehi Sawant

Proofreader

Safis Editing

Indexer

Aishwarya Gangawane

Graphics

Jason Monteiro

Production Coordinator

Shantanu Zagade

Foreword

Concurrent and parallel programming have progressed from niche disciplines, of interest

only to kernel programming and high-performance computing, to something that every

competent programmer must know. As parallel and distributed computing systems are

now the norm, most applications are concurrent, be it for increasing the performance or for

handling asynchronous events.

So far, most developers are unprepared to deal with this revolution. Maybe they have

learned the traditional concurrency model, which is based on threads and locks, in school,

but this model has become inadequate for dealing with massive concurrency in a reliable

manner and with acceptable productivity. Indeed, threads and locks are hard to use and

harder to get right. To make progress, one needs to use concurrency abstractions that are at

a higher level and composable.

15 years ago, I worked on a predecessor of Scala: Funnel was an experimental programming

language that had concurrent semantics at its core. All the programming concepts were

explained in this language as syntactic sugar on top of functional nets, an object-oriented

variant of join calculus . Even though join calculus is a beautiful theory, we realized after

some experimentation that the concurrency problem is more multifaceted than what can be

comfortably expressed in a single formalism. There is no silver bullet for all concurrency

issues; the right solution depends on what one needs to achieve. Do you want to define

asynchronous computations that react to events or streams of values? Or have autonomous,

isolated entities communicating via messages? Or define transactions over a mutable store?

Or, maybe the primary purpose of parallel execution is to increase the performance? For

each of these tasks, there is an abstraction that does the job: futures, reactive streams, actors,

transactional memory, or parallel collections.

This brings us to Scala and this book. As there are so many useful concurrency abstractions,

it seems unattractive to hardcode them all in a programming language. The purpose behind

the work on Scala was to make it easy to define high-level abstractions in user code and

libraries. This way, one can define the modules handling the different aspects of concurrent

programming. All of these modules would be built on a low-level core that is provided by

the host system. In retrospect, this approach has worked well. Today, Scala has some of the

most powerful and elegant libraries for concurrent programming. This book will take you

on a tour of the most important ones, explaining the use case for each and the application

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