HEIG-VD DAI - Java network concurrency

Objectives

• Learn what is concurrency
• Learn the different ways to handle multiple clients at the same time:
  • Multi-processing
  • Multi-threading
  • Asynchronous programming
• Learn how to implement and manage concurrency in Java network applications

Disclaimer

• This is not a course on concurrency
• Many many things are not covered
• Focus on concurrency for network applications
• Other ways and mechanisms to handle concurrency will be covered in other courses

Explore the code examples

Individually, or in pair/group, take 10 minutes to explore and discuss the code examples.

Answer the questions available in the course material:

• How do the code examples work?
• What are the main takeaways of the code examples?
• What are the main differences between the code examples?

If needed, use the theoretical content to help you.

Concurrency: an introduction

• Ability to handle multiple tasks at the same time
• Differs from parallelism, the ability to execute multiple tasks simultaneously
• Before we dive into concurrency, let's take a step back and remember what a processor is

What is a processor?

• Piece of hardware that executes millions of instructions every second
• Gives the illusion of executing multiple tasks at the same time
• However, it can execute only one instruction at a time
• When managing multiple tasks, the processor switches between tasks

What is a core?

• A physical unit inside a processor that executes instructions (always one at a time)
• A processor can have multiple cores, called multi-core processors
• Cores can execute multiple instructions at the same time (parallelism) - not covered in this course

What is a process?

• A process is an instance of a program that is being executed
• A process can have multiple threads (such as picocli)
• Processes are isolated from each other and have their own memory space
• Heavier than threads: expensive to create and manage

What is a thread?

• A thread is a sequence of instructions that can be executed by a core
• A core can manage multiple threads at the same time
• The main thread is the thread that starts when the application starts
• More lightweight than processes and share the same memory space

What problems can concurrency cause?

• What happens when a client connects to the server?
• How to isolate the data sent by one client from the others?
• What happens when multiple clients want to access the same resource (variables) at the same time?

What happens when multiple threads access the same resource?

• The processor has to switch between threads
• The order in which the threads are executed is not guaranteed
• The threads can access the same resource at the same time

Handling one client at a time

1. Create a socket to listen to incoming connections
2. Create a new socket for the client
3. Handle the connection

Analogy: a restaurant with one table

Simple but quite useless...

Handling multiple clients at the same time

Handling multiple clients at the same time is called concurrency.

Concurrency can be achieved with:

• Multi-processing
• Multi-threading
• Asynchronous programming

Multi-processing

• Create an entirely new process for each client
• Heavyweight and slow
• Not recommended
• Analogy: a new restaurant for each customer, including the kitchen, waiters, etc.

Multi-threading

• Create a new thread for each client
• More lightweight and faster than multi-processing
• Recommended with thread pool to limit resource usage
• Not enough or too many threads can slow down the system
• Analogy: a new or limited number of tables for each customer

Asynchronous programming

• Handle multiple clients with a single thread
• Very performant!
• Analogy: a food truck - you get a ticket, wait, get your food and leave
• Out of scope for this course but interesting to know! Node.js is a good example for this approach

Handling multiple threads in Java

• Java provides the ExecutorService interface to manage threads
• It uses different Executors implementations
• Each implementation has its own pros and cons
• You have used threads with picocli (each sub-command is a thread)

@Override
public Integer call() {
  try (ExecutorService executorService = Executors.newFixedThreadPool(2); ) {
    executorService.submit(this::emittersWorker);
    executorService.submit(this::operatorsWorker);
  } catch (Exception e) {
    System.out.println("[Receiver] Exception: " + e);

    return 1;
  }

  return 0;
}

public Integer emittersWorker() {
  // Manage emitters
}


public Integer operatorsWorker() {
  // Manage operators
}

Concurrent safe variable types and data structures in Java

• A thread-safe variable type and/or data structure is a variable type/data structure that can be accessed by multiple threads at the same time
• Java provides several thread-safe variable types and data structures

• Variables types:
  • AtomicBoolean instead of boolean/Boolean
  • AtomicInteger instead of int/Integer
  • AtomicLong instead of long/Long
• Data structures:
  • ConcurrentHashMap instead of HashMap
  • ConcurrentLinkedQueue instead of LinkedList
  • CopyOnWriteArrayList instead of ArrayList
  • CopyOnWriteArraySet instead of HashSet

• Multiple concurrent variable types and/or data structures can be co-dependent
• You must use them together with locks (the equivalent to mutexes and semaphores in C/C++) to avoid inconsistencies

Locks will not be covered (and not expected to be used) in this course but you can find additional information in the course material.

What will you do?

• Run full client/server examples and understand how concurrent clients are handled
• Explore and answer questions about some concurrent strategies
• Update previous game/application to handle multiple clients (optional)

Now it's your turn!

• Read the course material.
• Do the practical content.
• Ask questions if you have any.

Find the course on GitHub.

Do not hesitate to help each other! There's no need to rush!

Finished? Was it easy? Was it hard?

Can you let us know what was easy and what was difficult for you during this chapter?

This will help us to improve the course and adapt the content to your needs. If we notice some difficulties, we will come back to you to help you.

GitHub Discussions

You can use reactions to express your opinion on a comment!

What will you do next?

• Learn about electronic mail protocols
• Experiment with the SMTP protocol
  • Start a local SMTP server for testing
  • Send an email using ncat
  • Send an email using a SMTP client written in Java

Sources

• Main illustration by Brent Olson on Unsplash
• Illustration by Aline de Nadai on Unsplash
• Illustration by Bernard Hermant on Unsplash
• Illustration by alexey turenkov on Unsplash
• Illustration by Andrey Matveev on Unsplash
• Illustration by BoliviaInteligente on Unsplash
• Illustration by BoliviaInteligente on Unsplash
• Illustration by BoliviaInteligente on Unsplash

• Illustration by That's Her Business on Unsplash
• Illustration by Jens Herrndorff on Unsplash
• Illustration by Elliott Stallion on Unsplash
• Illustration by K8 on Unsplash
• Illustration by bckfwd on Unsplash
• Illustration by James Elchico on Unsplash
• Illustration by micheile henderson on Unsplash
• Illustration by Afrah on Unsplash
• Illustration by Pop & Zebra on Unsplash
• Illustration by Kateryna Miller on Unsplash

• Illustration by MChe Lee on Unsplash