Concurrency 

without Pain 

in Pure Java





Palanivelrajan Balasubramanian <prajan@manh.com>

R & D, Architecture

12-02-2013

About Me

  • Joined Manhattan Associates in 2006
  • Member of R&D Architecture
  • Java Developer 
    • Spring
    • Hibernate
    • Application Servers
    • Scripting
  • Current Project - System Director 3.0


    Agenda

    • Bank Account System 
    • Problems with Synchronization 
    • Introduction to Software Transactional Memory
    • Clojure
    • Akka
    • Discussion

    DISCLAIMER

    • I am not a expert in this topic
    • Developed interest after attending "Concurrency without pain in pure java" session by Venkat Subramaniam
    • If i am not sure about your question, i will go back do my research and will get back to you.

    Bank Account System

    Lets use the java 'synchronization' and develop a bank account system

    Problems with Java Synchronization

    • No errors/warning from JDK
    • Programmers can get synchronization wrong
    • Too conservative - Back to single threaded execution
    • Thread Contention - waiting for lock
    • Deadlock - Waiting infinitely for locks
    • Conflating Identity with State 

    Conflating Identity with State

    • When we create an object (Employee) in java
      • we receive a pointer to the instance that serves as both
        • its identity - Object represents Palanivel , Employee
        • State - Object also hold info about my start data July 2006
    • Anyone having access to the object can change the start date
    • Previous start date is lost forever
    • A thread with a pointer to Employee Palanivel has to assume that its value can change at any moment - So we add 'synchronization'

    Software TRANSACTIONal Memory

    • Identity and state are managed separately
    • Identity - a stable logical entity associated with a series of different values over time 
    • value - something that doesn't change. All values are immutable
    • The date May 03, 1985 never change
    • You might have an identity called “today”
      • At one point, “today” was associated with “May 03, 1985”
      • The next day, it was associated with “May 04, 1985” and now that identity is associated with “Dec 02, 2013”

    STM

    • The identity is ALWAYS associated with a single immutable value at a given time; someone (a thread) can request that it be associated with a different immutable value (perhaps creating the new value based on the old value)
      • the association is then changed, not the values
    • This immutability is good in concurrent situations, since there is never any danger of a value changing out from under you

    STM


    STM BENEFITS

    • separating identity from state
    • lock-free programming
    • Keep tracks of mutable identity and screams if its modified outside of a transaction
    • Multiple threads can read this value with no contention

    clojure

    • Clojure is a dynamic programming language
    • Implements STM
    • Can be used in Java apps by including the library to classpath
    • http://clojure.org/

    Using Clojure in our  BANK ACCOUNT SYSTEM 

    Akka

    • Actor based system supporting STM
    • Written in Scala, but also in Java as well
    • Highly scalable
    • Akka's unit of code organization is called an Actor
    • Like Java EE servlets and session beans
    • Runs on a dedicated threads, managed by Akka framework
      • Never think about threads, locks
    • Distributed by Design
      • Actors can run on local or can be run on a remote machine, producing the same result

    Akka - Actors


    Akka Demo

    Questions

    Thanks you

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