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Learning Concurrent Programming in Scala
Learning Concurrent Programming in Scala

Learning Concurrent Programming in Scala: Practical Multithreading in Scala , Second Edition

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Learning Concurrent Programming in Scala

Chapter 2. Concurrency on the JVM and the Java Memory Model

 

"All non-trivial abstractions, to some degree, are leaky."

 
 --Jeff Atwood

Since its inception, Scala has run primarily on top of JVM, and this fact has driven the design of many of its concurrency libraries. The memory model in Scala, its multithreading capabilities, and its inter-thread synchronization are all inherited from the JVM. Most, if not all, higher-level Scala concurrency constructs are implemented in terms of the low-level primitives presented in this chapter. These primitives are the basic way to deal with concurrency-in a way, the APIs and synchronization primitives in this chapter constitute the assembly of concurrent programming on the JVM.

In most cases, you should avoid low-level concurrency in place of higher-level constructs introduced later, but we felt it was important for you to understand what a thread is, that a guarded block is better than busy-waiting, or why a memory model is useful. We are convinced that...

Processes and threads


In modern, pre-emptive, multitasking operating systems, the programmer has little or no control over the choice of processor on which the program will be executed. In fact, the same program might run on many different processors during its execution and sometimes even simultaneously on several processors. It is usually the task of the Operating System (OS) to assign executable parts of the program to specific processors--this mechanism is called multitasking, and it happens transparently for the computer user.

Historically, multitasking was introduced to operating systems to improve the user experience by allowing multiple users or programs to use resources of the same computer simultaneously. In cooperative multitasking, programs were able to decide when to stop using the processor and yield control to other programs. However, this required a lot of discipline on the programmer's part and programs could easily give the impression of being unresponsive. For example,...

Monitors and synchronization


In this section, we will study inter-thread communication using the synchronized statement in more detail. As we saw in the previous sections, the synchronized statement serves both to ensure the visibility of writes performed by different threads, and to limit concurrent access to a shared region of memory. Generally speaking, a synchronization mechanism that enforces access limits on a shared resource is called a lock. Locks are also used to ensure that no two threads execute the same code simultaneously; that is, they implement mutual exclusion.

As mentioned previously, each object on the JVM has a special built-in monitor lock, also called the intrinsic lock. When a thread calls the synchronized statement on an x object, it gains ownership of the monitor lock of the x object, given that no other thread owns the monitor. Otherwise, the thread is blocked until the monitor is released. Upon gaining ownership of the monitor, the thread can witness the memory writes...

Volatile variables


The JVM offers a more lightweight form of synchronization than the synchronized block, called volatile variables. Volatile variables can be atomically read and modified, and are mostly used as status flags; for example, to signal that a computation is completed or canceled. They have two advantages. First, writes to and reads from volatile variables cannot be reordered in a single thread. Second, writing to a volatile variable is immediately visible to all the other threads.

Note

Reads and writes to variables marked as volatile are never reordered. If a write W to a volatile v variable is observed on another thread through a read R of the same variable, then all the writes that preceded the write W are guaranteed to be observed after the read R.

In the following example, we search for at least one ! character in several pages of the text. Separate threads start scanning separate pages p of the text written by a person that is particularly fond of a popular fictional hero...

The Java Memory Model


While we were never explicit about it throughout this chapter, we have actually defined most of the JMM. What is a memory model in the first place?

A language memory model is a specification that describes the circumstances under which a write to a variable becomes visible to other threads. You might think that a write to a variable v changes the corresponding memory location immediately after the processor executes it, and that other processors see the new value of v instantaneously. This memory consistency model is called sequential consistency.

As we already saw in the ThreadSharedStateAccessReordering example, sequential consistency has little to do with how processors and compilers really work. Writes rarely end up in the main memory immediately; instead, processors have hierarchies of caches that ensure a better performance and guarantee that the data is only eventually written to the main memory. Compilers are allowed to use registers to postpone or avoid memory...

Summary


In this chapter, we showed how to create and start threads, and wait for their termination. We have shown how to achieve inter-thread communication by modifying the shared memory and by using the synchronized statement, and what it means for a thread to be in a blocked state. We have studied approaches to prevent deadlocks by imposing the ordering on the locks and avoided busy-waits in place of guarded blocks. We have seen how to implement a graceful shutdown for thread termination and when to communicate using volatiles. We witnessed how the correctness of a program can be compromised by undesired interactions known as race conditions as well as data races due to the lack of synchronization. And, most importantly, we have learned that the only way to correctly reason about the semantics of a multithreaded program is in terms of happens-before relationships defined by the JMM.

The language primitives and APIs presented in this section are low-level; they are the basic building blocks...

Exercises


In the following set of exercises, you are required to implement higher-level concurrency abstractions in terms of basic JVM concurrency primitives. Some of these exercises introduce concurrent counterparts of sequential programming abstractions, and, in doing so, highlight important differences between sequential and concurrent programming. The exercises are not ordered in any particular order, but some of them rely on specific content from earlier exercises or this chapter:

  1. Implement a parallel method, which takes two computation blocks, a and b, and starts each of them in a new thread. The method must return a tuple with the result values of both the computations. It should have the following signature:

                def parallel[A, B](a: =>A, b: =>B): (A, B) 
    
  2. Implement a periodically method, which takes a time interval duration specified in milliseconds, and a computation block b. The method starts a thread that executes the computation block b every duration milliseconds...

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Key benefits

  • Make the most of Scala by understanding its philosophy and harnessing the power of multicores
  • Get acquainted with cutting-edge technologies in the field of concurrency, through practical, real-world applications
  • Get this step-by-step guide packed with pragmatic examples

Description

Scala is a modern, multiparadigm programming language designed to express common programming patterns in a concise, elegant, and type-safe way. Scala smoothly integrates the features of object-oriented and functional languages. In this second edition, you will find updated coverage of the Scala 2.12 platform. The Scala 2.12 series targets Java 8 and requires it for execution. The book starts by introducing you to the foundations of concurrent programming on the JVM, outlining the basics of the Java Memory Model, and then shows some of the classic building blocks of concurrency, such as the atomic variables, thread pools, and concurrent data structures, along with the caveats of traditional concurrency. The book then walks you through different high-level concurrency abstractions, each tailored toward a specific class of programming tasks, while touching on the latest advancements of async programming capabilities of Scala. It also covers some useful patterns and idioms to use with the techniques described. Finally, the book presents an overview of when to use which concurrency library and demonstrates how they all work together, and then presents new exciting approaches to building concurrent and distributed systems. Who this book is written for If you are a Scala programmer with no prior knowledge of concurrent programming, or seeking to broaden your existing knowledge about concurrency, this book is for you. Basic knowledge of the Scala programming language will be helpful.

Who is this book for?

If you are a Scala programmer with no prior knowledge about concurrent programming, or seeking to broaden your existing knowledge about concurrency, this book is for you. Basic knowledge of the Scala programming language will be helpful. Also if you have a solid knowledge in another programming language, such as Java, you should find this book easily accessible.

What you will learn

  • What you will learn from this book
  • ? Get to grips with the fundamentals of concurrent programming on modern multiprocessor systems
  • ? Build high-performance concurrent systems from simple, low-level concurrency primitives
  • ? Express asynchrony in concurrent computations with futures and promises
  • ? Seamlessly accelerate sequential programs by using data-parallel collections
  • ? Design safe, scalable, and easy-to-comprehend in-memory transactional data models
  • ? Transparently create distributed applications that scale across multiple machines
  • ? Integrate different concurrency frameworks together in large applications
  • ? Develop and implement scalable and easy-to-understand concurrent applications in Scala 2.12
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Table of Contents

10 Chapters
Introduction Chevron down icon Chevron up icon
Concurrency on the JVM and the Java Memory Model Chevron down icon Chevron up icon
Traditional Building Blocks of Concurrency Chevron down icon Chevron up icon
Asynchronous Programming with Futures and Promises Chevron down icon Chevron up icon
Data-Parallel Collections Chevron down icon Chevron up icon
Concurrent Programming with Reactive Extensions Chevron down icon Chevron up icon
Software Transactional Memory Chevron down icon Chevron up icon
Actors Chevron down icon Chevron up icon
Concurrency in Practice Chevron down icon Chevron up icon
Reactors Chevron down icon Chevron up icon

Customer reviews

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Full star icon Full star icon Full star icon Full star icon Half star icon 4.8
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4 star 25%
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Tomer Ben David May 06, 2015
Full star icon Full star icon Full star icon Full star icon Full star icon 5
I'm reading it like its prose. Enjoying every minute. I wish this was my first concurrency book as the author manages to put things clear and well ordered.It's so well written, so clear that I have to applause the writer for such an amazing work. For me, even if I would not be interested in scala at all it would be a great concurrency book especially on the jvm. It is not encrypted, it's description are decrypted and well organized.
Amazon Verified review Amazon
Tihomir Gvero Mar 01, 2015
Full star icon Full star icon Full star icon Full star icon Full star icon 5
This is an interesting and useful book that explores topics that help programmers understand and write correct and efficient concurrent programs in Scala. It is written for both the beginners and the programmers with more expertise in Scala programming, but who still want to develop and/or improve their concurrent programming skills. I really like this book, because it has many clear and easy to follow examples used to introduce many advanced aspects of concurrent programming. I enjoyed reading them. They cover several different topics introducing the most important concurrent programming abstractions. This includes the concurrency on the JVM (e.g., how to use Threads), traditional building blocks (thread pools, atomic variables and concurrent collections), asynchronous abstractions (e.g. Futures and Promises), the Scala parallel collections, the Scala reactive extension framework, transnational programming in Scala and Actors (e.g., the famous Akka framework). I bought a hard copy and so far I did not have any complaints about the quality. To sum up, it is the very informative and helpful book and the must-have in your Scala book collection. I sincerely recommend it!
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I. Maier Apr 02, 2015
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You can tell right away that this book was written by a true authority on concurrent programming in Scala. It’s written for programmers already familiar with Scala or for those with a strong background in related languages and/or the JVM. If you are one of them and are interested in getting an overview of different abstractions for concurrent programming in Scala, or more broadly on the JVM, go pick up this book!The book is very well written and the examples are interesting. It is roughly organized going from low-level to high-level concepts, concluding with a practical example that connects the different abstractions introduced in earlier chapters. Chapter 5 is a bit different than the others as it mostly deals with parallel programming (with collections) rather than concurrent programming, although it shortly relates the two. Since the book can’t cover every single aspect of each subject on 300 pages, most chapters give helpful references to obtain a deeper understanding and background information on the subject. I think the book strikes a very good balance between breadth and depth. It covers all important abstractions and discusses them in just the right amount of detail.My only gripe is with the book’s design and typesetting. Many figures don’t look too professional and have a rather low resolution, which is noticeable in my paper copy as well as the ebook version. For the price of the book, I’d expect a little more care. Nonetheless, the figures are helpful for the understanding of the subject.In the end, it’s the content that matters most to me, that’s why I won’t hesitate to give it 5 stars.
Amazon Verified review Amazon
Thomas Ulrich Mar 21, 2015
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Very good, in-depth study of JVM concurrency intrinsics followed by Scala concurrency frameworks. In addition to the well-written study, each topic includes short, pragmatic, runnable example idioms demonstrating the current topic. The book concludes with a more comprehensive example comprising key concurrency libraries previously covered in book. Very well done! Highly recommended.
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Swarnali May 25, 2021
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there appears to be a typo while describing ABA problem in chapter 3. the final CAS should be from T2 thread and T1. It caused me some trouble while trying to understand what's going on. so wanted to call it out
Amazon Verified review Amazon
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