OS Support for Building Distributed Applications: Multithreaded Programming using Java Threads
Cloud Computing and Distributed Systems (CLOUDS) Laboratory School of Computing and Information Systems
The University of Melbourne, Australia http://www.buyya.com
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Introduction to Middleware Thread Applications
Defining Threads
Java Threads and States
Architecture of Multithreaded servers Threads Synchronization
Introduction
Middleware is a layer of software (system) between Applications and Operating System (OS) powering the nodes of a distributed system.
The OS facilitates:
Encapsulation and protection of
resources inside servers;
Invocation of mechanisms required to access those resources including concurrent access/processing.
Applications
Middleware
Distributed Nodes with OS
Middleware and Network Operating System (NOS)
Many DOS (Distributed OS) have been investigated, but there are none in general/wide use. But NOS are in wide use for various reasons both technical and non- technical.
Users have much invested in their application software; they will not adopt a new OS that will not run their applications.
Users tend to prefer to have a degree of autonomy of their machines, even in a closely knit organisation.
A combination of middleware and NOSs provides an acceptable balance between the requirement of autonomy and network transparency.
NOS allows users to run their favorite word processor.
Middleware enables users to take advantage of services that become available in their distributed systems.
Introducing a middleware
Building Distributed Systems
DOS or NOS are not enough to build a DS!
NOS are a good starting point but ….
… we need an additional layer “gluing” all together
Middleware NOS
Building Distributed Systems
Middleware
High-level features for DS Communication
Management
Application specific
Uniform layer where to build DS services Runtime environment of applications
Operating System
Low / medium level (core) features Process / threads management
Local hardware (CPU, disk, memory)
Security (users, groups, domain, ACLs) Basic networking
Operating system layers and Middleware
Applications, services
Middlew are
OS1 Processes,threads,
communication, …
OS2 Processes,threads,
communication, …
Computer & netw ork hardware
Computer & netw ork hardware
OS: kernel, libraries & servers
Pl a t f o r m
Unix and Windows are two examples of Network Operating Systems – have a networking capability built into them and so can be used to access remote resources using basic services such as rlogin, telnet.
Core OS components and functionality
Process manager
Thread manager
Communic ation manager
Memory manager
Supervisor
Threaded Applications
Modern Applications and Systems Operating System Level
Multitasking: multiple applications running at once
Application Level
Multithreading: multiple operations performed at
the same time within an application.
Bottom Line:
Illusion of concurrency
Threaded Applications
Modern Systems
Multiple applications run concurrently!
This means that… there are multiple processes on your computer
web & email
games office automation
multimedia
Multitasking
A single threaded program
public void main(..) {
Threaded Applications
Modern Systems
Applications perform many tasks at once!
This means that… there are multiple threads within a single process.
Background printing
GUI rendering
Application core logic
A Multithreaded Program
Main Thread
Threads may switch or exchange data/results
Single and Multithreaded Processes
threads are light-weight processes within a process
Single-threaded Process
Multi-threaded Process Threads of
Single instruction stream
Multiple instruction stream Address Space
Multithreaded Server: For Serving Multiple Clients Concurrently
Modern Applications
Example: Multithreaded Web Server
Web/FTP server
Process Request Client 1
Process Request Client 2
Process Request Client N
Threaded Applications
Modern Applications
Example: Internet Browser + Youtube
Video Streaming
Favorities, Share, Comments Posting
Modern Applications need Threads (ex1): Editing and Printing documents in background.
Printing Thread
Editing Thread
Multithreaded/Parallel File Copy
reader() {
——— –
lock(buff[i]);
read(src,buff[i]);
unlock(buff[i]);
——— –
writer() {
———- lock(buff[i]); write(src,buff[i]); unlock(buff[i]); ———-
Cooperative Parallel Synchronized Threads
Defining Threads
Applications – Threads are used to perform:
Parallelism and concurrent execution of independent tasks / operations.
Implementation of reactive user interfaces.
Non blocking I/O operations.
Asynchronous behavior.
Timer and alarms implementation.
Defining Threads
Example: Web/FTP Server
while
Main Thread
Worker Thread
Worker Thread
Web/FTP server
Worker Thread
Execution Timeline
Defining Threads
A Thread is a piece of code that runs in concurrent with other threads.
Each thread is a statically ordered sequence of instructions.
Threads are used to express concurrency on both single and multiprocessors machines.
Programming a task having multiple threads of control is called: Multithreading or Multithreaded Programming.
Java Threads
Java has built in support for Multithreading
Synchronization
Thread Scheduling
Inter-Thread Communication: currentThread start
yield run
sleep stop
setPriority getPriority suspend
Java Garbage Collector is a low-priority thread.
Threading Mechanisms…
Create a class that extends the Thread class Create a class that implements the Runnable
(objects are threads)
(objects with run() body)
1st method: Extending Thread class
run() method:
class MyThread extends Thread
public void run()
// thread body of execution
Create a thread:
MyThread thr1 = new MyThread();
Start Execution of threads: thr1.start();
Create and Execute:
new MyThread().start();
Create a class by extending Thread class and override
An example
class MyThread extends Thread { public void run() {
System.out.println(” this thread is running … “); }
class ThreadEx1 {
public static void main(String [] args ) {
MyThread t = new MyThread(); t.start();
2nd method: Threads by implementing Runnable interface
Create a class that implements the interface Runnable and override run() method:
class MyThread implements Runnable {
public void run()
// thread body of execution
Creating Object:
MyThread myObject = new MyThread();
CreatingThreadObject:
Thread thr1 = new Thread( myObject );
StartExecution: thr1.start();
An example
class MyThread implements Runnable { public void run() {
System.out.println(” this thread is running … “); }
class ThreadEx2 {
public static void main(String [] args ) {
Thread t = new Thread(new MyThread()); t.start();
Life Cycle of Thread
I/O completed
Time expired/ interrupted
Block on I/O
completion
A Program with Three Java Threads
Write a program that creates 3 threads
Three threads example
class A extends Thread {
public void run() {
for(int i=1;i<=5;i++)
System.out.println("\t From ThreadA: i= "+i); }
System.out.println("Exit from A");
class B extends Thread {
public void run() {
for(int j=1;j<=5;j++)
System.out.println("\t From ThreadB: j= "+j); }
System.out.println("Exit from B");
Three threads example
class C extends Thread {
public void run() {
for(int k=1;k<=5;k++)
System.out.println("\t From ThreadC: k= "+k); }
System.out.println("Exit from C"); }
class ThreadTest
public static void main(String args[]) {
new A().start();
new B().start();
new C().start(); }
threads [1:76] java ThreadTest From ThreadA: i= 1
From ThreadA: i= 2 From ThreadA: i= 3 From ThreadA: i= 4 From ThreadA: i= 5
Exit from A
From ThreadC: k= 1
From ThreadC: k= 2 From ThreadC: k= 3 From ThreadC: k= 4 From ThreadC: k= 5
Exit from C
From ThreadB: j= 1
From ThreadB: j= 2 From ThreadB: j= 3 From ThreadB: j= 4 From ThreadB: j= 5
Exit from B
threads [1:77] java ThreadTest From ThreadA: i= 1
From ThreadA: i= 2 From ThreadA: i= 3 From ThreadA: i= 4 From ThreadA: i= 5 From ThreadC: k= 1 From ThreadC: k= 2 From ThreadC: k= 3 From ThreadC: k= 4 From ThreadC: k= 5
Exit from C
From ThreadB: j= 1
From ThreadB: j= 2 From ThreadB: j= 3 From ThreadB: j= 4 From ThreadB: j= 5
Exit from B Exit from A
Thread Priority
In Java, each thread is assigned priority, which affects the order in which it is scheduled for running. The threads so far had same default priority (NORM_PRIORITY) and they are served using FCFS policy.
Java allows users to change priority:
ThreadName.setPriority(intNumber) MIN_PRIORITY = 1
NORM_PRIORITY=5
MAX_PRIORITY=10
Thread Priority Example
class A extends Thread
public void run()
System.out.println("Thread A started"); for(int i=1;i<=4;i++)
System.out.println("\t From ThreadA: i= "+i);
System.out.println("Exit from A");
class B extends Thread
public void run()
System.out.println("Thread B started"); for(int j=1;j<=4;j++)
System.out.println("\t From ThreadB: j= "+j);
System.out.println("Exit from B");
Thread Priority Example
class C extends Thread
public void run()
System.out.println("Thread C started"); for(int k=1;k<=4;k++)
System.out.println("\t From ThreadC: k= "+k);
System.out.println("Exit from C");
class ThreadPriority
public static void main(String args[])
A threadA=new A();
B threadB=new B();
C threadC=new C();
threadC.setPriority(Thread.MAX_PRIORITY); threadB.setPriority(threadA.getPriority()+1); threadA.setPriority(Thread.MIN_PRIORITY); System.out.println("Started Thread A");
threadA.start(); System.out.println("Started Thread B");
threadB.start(); System.out.println("Started Thread C");
threadC.start();
System.out.println("End of main thread"); }
Assignment 1 at a Glance: Multithreaded Dictionary Server – Using Sockets and Threads
A Client Program Meaning(“guru”)?
Meaning (“guru”) “master or teacher”
Multithreaded Dictionary Server
A Client Program Meaning(“love”)?
A Client Program in “C”
Meaning(“java”)?
A Client Program in “C++”
Meaning(“channel”)?
Accessing Shared Resources
Applications access to shared resources need to be coordinated.
Printer (two person jobs cannot be printed at the same time)
Simultaneous operations on your bank account.
Can the following operations be done at the same time on the same account?
Deposit()
Withdraw() Enquire()
Online Bank: Serving Many Customers and Operations
Internet Bank Server
Bank Operator 1 Bank Local Area Network
Bank Operator M
Bank Database
Shared Resources
If one thread tries to read the data and other thread tries to update the same data, it leads to inconsistent state.
This can be prevented by synchronising access to the data.
Use “synchronized” method:
public synchronized void update()
the driver: 3 Threads sharing the same object
class InternetBankingSystem {
public static void main(String [] args ) {
Account accountObject = new Account ();
Thread t1 = new Thread(new MyThread(accountObject)); Thread t2 = new Thread(new YourThread(accountObject)); Thread t3 = new Thread(new HerThread(accountObject));
t1.start(); t2.start(); t3.start();
// DO some other operation } // end main()
Shared account object between 3 threads
class MyThread implements Runnable { Account account;
public MyThread (Account s) { account = s;}
public void run() { account.deposit(); } } // end class MyThread
class YourThread implements Runnable { Account account;
public YourThread (Account s) { account = s;}
public void run() { account.withdraw(); } } // end class YourThread
class HerThread implements Runnable { Account account;
public HerThread (Account s) { account = s; }
public void run() {account.enquire(); } } // end class HerThread
account (shared object)
Monitor (shared object access): serializes operation on shared objects
class Account { // the 'monitor' int balance;
// if 'synchronized' is removed, the outcome is unpredictable public synchronized void deposit( ) {
// METHOD BODY : balance += deposit_amount; }
public synchronized void withdraw( ) {
// METHOD BODY: balance -= deposit_amount; }
public synchronized void enquire( ) {
// METHOD BODY: display balance. }
Architecture for Multithread Servers
Multithreading enables servers to maximize their throughput, measured as the number of requests processed per second.
Threads may need to treat requests with varying priorities:
A corporate server could prioritize request processing according to class of customers.
Architectures:
Worker pool
Thread-per-request
Thread-per-connection Thread-per-object
Client and server with threads (worker-pool architecture)
Thread 2 makes
requests to server
Receipt & queuing
Input-output
generates results
In worker-pool architectures, the server creates a fixed pool of worker threads to process requests.
The module “receipt and queuing” receives requests from sockets/ports and places them on a shared request queue for retrieval by the workers.
N threads Server
Alternative server threading architectures
remote ob je ct s
per-connection threads
remote ob je ct s
per-object threads
remote ob je ct s
a. Thread-per-request
b. Thread-per-connection
c. Thread-per-object
Server associates a Thread with each connection and destroys when client closes the connection. Client may make many requests over the connection.
Associates Thread with each object. An IO thread receives request and queues them for workers, but this time there is a per-object queue.
IO Thread creates a new worker thread for each request and worker thread destroys itself after serving the request.
Invocations between address spaces
(a) Systemcall
Control transfer via trapinstruction
Control transfer via privileged instructions
Us er (b) RPC/RMI(withinonecomputer)
(c) RPC/RMI(betweencomputers)
Thread 1 Us er 1
Protection domain boundary
Thread 2 User 2
Thread 2 Us er 2
Operating system provides various types of facilities to support middleware for distributed system:
encapsulation, protection, and concurrent access and management of node resources.
Multithreadingenablesserverstomaximizetheirthroughput, measured as the number of requests processed per second.
Threads support treating of requests with varying priorities.
Various types of architectures can be used in concurrent
processing:
Workerpool
Thread-per-request
Thread-per-connection Thread-per-object
Threadsneedtobesynchronizedwhenaccessingand manipulating shared resources.
S designs provide flexibility in terms of separating mechanisms from policies.
References
CDK Book (Text Book)
Chapter 7 – “Operating System Support”
Chapter 14: Multithread Programming
R. Buyya, S. Selvi, X. Chu, “Object Oriented Programming with Java: Essentials and Applications”, McGraw Hill, , India, 2009.
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