Transcript characterization of distributed systems

Introduction to Distributed System and
Computing
From Coulouris, Dollimore and Kindberg
Distributed Systems:
Concepts and Design
Edition 5, © Pearson Education 2012
Minqi Zhou @ ECNU
Information
Office :数学馆东111
Tel:021-62235022
Email: [email protected]
2
Overview
This course focuses on the issues fundamental to
the understanding of distributed systems.
Text and reference books
Distributed Systems - Concept and Design, G. Coulouris,
J, Dollimore, and T. Kindberg, Addison-Wesley, 5th Edition,
2012.
Distributed Systems – Principles and Paradigms, A.
Tanenbaum and M. van Steen, Prentice Hall, 2nd Edition
2008.
 Course Webpage: http://dase.ecnu.edu.cn/mqzhou/Teaching.html

Check Regularly
Tentative Grading
Class
Projects
Final Reports
Open
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25 %
40%
25 %
10%
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Building a Distributed System
“The number of people who know how to build really
solid distributed systems…is about ten”
Scott Shenker, Professor at UC Berkeley
The point: it’s hard to build a solid distributed
system.
So, why is it hard?...but first of all…
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Why distributed computing?
 Resource sharing/Economics: distributed systems allow
the pooling of resources, including CPU cycles, data
storage, input/output devices, and services.
 Scalability: Increasing demands and users can be easily
addressed by adding more resources and the system can
still run effectively.
 Reliability: a distributed system allow replication of
resources and/or services, thus reducing service outage
due to failures.
 The affordability of computers and availability of network
access: The Internet has become a universal platform for
distributed computing.
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 5
© Pearson Education 2012
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Topics
 Defining Distributed Systems
 Resource sharing and the Web
 Design Challenges of Distributed Systems
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Defining Distributed Systems
 Various definition of distributed systems have
been given in the literature, for example:
 A collection of logically related data that is
distributed over different processing nodes of
computer network.
 Definition above does not provide all
characteristics of distributed systems.
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Defining Distributed Systems
 It is difficult to find a definition that provides all
characteristics of distributed systems.
 Another way is a definition that gives a loose
characterization for distributed systems such as:
 A distributed system is a collection of independent
computers that appear to the users of the system as a
single computer.
 With any definition, sharing of resources is a main
motivation for constructing distributed systems.
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Defining Distributed Systems
 In this course, we define distributed systems
more precisely as :
 A distributed system is one in which hardware
or software components located at networked
computers communicate and coordinate their
actions only by message passing.
 Definition above covers the entire range of
distributed systems in which networked
computers can usefully be deployed.
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Defining Distributed Systems
 Networks of computers are everywhere!
 Examples of networks of computers are:
 Mobile phone networks
 Corporate networks
 Factory networks
 Campus networks
 Home networks
 In-car networks
 On board networks in aero planes and trains
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Defining Distributed Systems
 Our definition of distributed systems has the
following significant consequences:
 Concurrency
 Tasks carry out independently
 No global clock
 Tasks coordinate their actions by exchanging
messages
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Defining Distributed Systems
 Independent Failures
 Faults in the network result in the isolation of the
computers that are connected to it.
 Each component of the system can fail
independently, leaving the others still running.
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
General Examples of Distributed Systems
 Internet
 Intranets
 Mobile networks
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
General Examples of Distributed Systems
 The Internet
 The Internet is a vast interconnected collection
of computer networks of many different types.
 Multimedia services are available in the
Internet enabling users to access audio and
video data including music, radio, TV
channels, phone, and video conferencing.
(Figure 1)
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
General Examples of Distributed Systems
intranet
%
ISP
%
%
%
backbone
satellite link
desktop computer:
server:
network link:
Figure 1. A typical portion of the Internet
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
General Examples of Distributed Systems
 Intranet
 An intranet is a portion of the Internet that is
separately administered and has a boundary
that can be configured to enforce local security
policies.
(Figure 2)
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
General Examples of Distributed Systems
email s erv er
Desktop
computers
print and ot her servers
Web server
Local area
netw ork
email s erv er
File s erv er
print
ot her servers
the rest of
the Int ernet
router/f irew all
Figure 2. A typical Interanet
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
General Examples of Distributed Systems
 Mobile networks
 Technological advances in device
miniatorization and wireless networking have
led increasingly to the integration of small and
portable computing devices into distributed
systems.
 These devices include:
 Laptop computers
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
General Examples of Distributed Systems
 Handheld devices
•
•
•
•
•
Personal digital assistants(PDAs)
Mobile phones
Pagers
Video cameras
Digital cameras
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
General Examples of Distributed Systems
 Wearable devices
 Smart watches with functionality similar to a
PDA
 Devices embedded in appliances
 Washing machines
 Hi-fi systems
 Cars
 Refrigerators
(Figure 3)
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
General Examples of Distributed Systems
Internet
Host intranet
WAP
gatew ay
Wireles s LAN
Mobile
phone
Laptop
Printer
Camera
Home intranet
Host site
Figure 3. Portable and handheld devices in a distributed system.
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Other Examples of Distributed Systems
 Cluster
 A type of parallel or distributed processing
system, which consists of a collection of
interconnected stand-alone computers
cooperatively working together as a single,
integrated computing resource. The computers
may be standard per uniprocessor or
multiprocessor.
 A cluster can be used for providing highly
scalable services such as search engines
provide for users all over the Internet.
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Other Examples of Distributed Systems
 Grid
 A type of parallel and distributed system that
enables the sharing, selection, and
aggregation of geographically distributed
autonomous resources dynamically at runtime
depending on their availability, capability,
performance, cost, and users' quality-ofservice requirements.
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Examples of Distributed Systems
 Web Search: major growth industry in the last decade. 10 billion per
month for global number of searches. Complex task for searching a big
database with 63 billion pages. (e.g. Google distributed infrastructure, file
system, storage, lock service, parallel computing)
 Massively multiplayer online games: Large number of people interact
through the Internet with a virtual world. Challenges include fast response
time, real-time propagation of events.
 Financial trading: provides real-time access to a wide range of information
sources such as current share prices and trends, economic and political
development news. Challenges include how to deliver events reliably and
in a timely manner to very large numbers of clients.
 Peer-to-Peer system
Trend in Distributed Systems
 Pervasive networking and modern Internet: Wifi, WiMax, Bluetooth, the
third-generation of mobile phone networks. The result is that networking
has become a pervasive resource and devices can be connected at any
time and in any place.
 Mobile and ubiquitous computing: small and portable computing devices
are integrated into the distributed system such as laptop, handheld
devices( PDA, cell phone, camera etc).
 Distributed multimedia systems: it support a range of media types such as
audio, video in a distributed system. So desktop can access live
television, file libraries, music libraries, telephone IP phone (Skype) in
distributed system. QoS issue
 Distributed Computing as a utility: A number of companies provide the
computing, storage and application resources as a commodity or utility to
users. So users do not need to maintain local IT. Pay as use. Analogy
between distributed resources and other utilities such a water or
electricity.
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Design Challenges of Distributed Systems

Designers of distributed systems need to take
the following challenges into account:
 Heterogeneity
 Heterogeneous components must be able to
interoperate.
 Openness
 Interfaces should allow components to be added
or replaced.
 Security
 The system should only be used in the way
intended.
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Design Challenges of Distributed Systems
 Scalability
 System should work efficiently with an
increasing number of users.
 System performance should increase with
inclusion of additional resources.
 Failure handling
 Failure of a component (partial failure) should
not result in failure of the whole system.
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Design Challenges of Distributed Systems
 Transparency
 Distribution should be hidden from the user as
much as possible.
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Heterogeneity
 Heterogeneous components that must be able
to interoperate, apply to all of the following:
 Networks
 Hardware architectures
 Operating systems
 Programming languages
30
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Heterogeneity
 Examples that mask differences in network,
operating systems, hardware and software to
provide heterogeneity are
 Middleware
 Internet protocols
 Mobile code
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Heterogeneity
 Middleware
 Middleware applies to a software layer.
 Middleware provides a programming abstraction.
 Middleware masks the heterogeneity of the
underlying networks, hardware, operating
systems and programming languages.
 The Common Object Request Broker (CORBA)
is a middleware example.
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Heterogeneity
 Mobile code
 Mobile code is the code that can be sent from
one computer to another and run at the
destination.
 Java applets are the example of mobile codes.
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Heterogeneity
 Virtual machine
 Virtual machine provides a way of making code
executable on any hardware.
34
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Openness
 Distributed systems must be extensible.
 Openness of a computer system is the
characteristic that determines whether the
system can be extended and re-implemented
in various ways.
35
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Openness
 The first step in openness is publishing the
documentation of software components and
interfaces of the components to make them
available to software developers.
36
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Security
 Security of a computer system is the
characteristic that the resources are
accessible to authorized users and used in
the way they are intended.
 Security for information resources has three
components:
 Confidentiality
 Protection against disclosure to unauthorized
individual.
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Security
 Integrity
 Protection against alteration or corruption.
 Availability
 Protection against interference with the means
to access the resources.
38
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Security
 Security Mechanisms are:
 Encryption
 Authentication
 Authorization
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Security challenges
 Denial of service attacks
 Denial of service attacks is an attempt to make
a computer resource unavailable to its
intended users.
 Security of mobile code
 Mobile code needs to be handled with care.
 E.g. receiving an executable program as an electronic mail
attachment to display an interesting picture but in reality it
may access local resources, or perhaps be part of a denial
of service attack.
40
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Scalability
 Scalable distributed systems operate
effectively and efficiently at many different
scales, ranging from a small Intranet to the
Internet.
 Scalable distributed systems remain effective
when there is a significant increase in the
number of resources and the number of users.
41
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Scalability
 Challenges of designing scalable distributed
systems are:
 Controlling the cost of physical resources
 Cost should linearly increase with the system
size.
 Controlling the performance loss
 For example, in hierarchically structured data,
search performance loss due to data growth
should not be beyond O(log n), where n is the
size of data.
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Scalability
 Preventing software resources running out
 An example is the numbers used as Internet
addresses (IP)(32 bit->128-bit)
 Y2K like problem.
 Avoiding performance bottlenecks
 Using decentralized algorithms to avoid having
performance bottlenecks.
 Caching and replication in Web are examples
of providing scalability.
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CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Failure handling
 Failures in distributed systems are partial, that
is some components fail while others continue
to function.
 Techniques for dealing with failures:
 Detecting failures
 E.g. Checksums
 Masking failures
 E.g. Retransmission of corrupt messages
 E.g. File redundancy
44
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Failure handling
 Tolerating failures
 E.g. Exception handling
 E.g. Timeouts
 Recovery from Failure
 E.g. Rollback mechanisms
 Redundancy
 E.g. Redundant components
45
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Concurrency
 With concurrency, services and applications
can be shared by clients in a distributed
system.
 For an object to be safe in a concurrent
environment, its operations must be
synchronized in such a way that its data
remains consistent.
46
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Concurrency
 Concurrency can be achieved by standard
techniques such as semaphores, which are
used in most operating systems.
47
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Transparency
 Transparency is defined as the hiding of the
separation of components in a distributed
systems from the user and the application
programmer.
 With transparency the system is perceived as
a whole rather than a collection of
independent components.
48
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Transparency
 Forms of transparencies:
 Access transparency
 Enables local and remote resources to be
accessed using identical operations.
 Location transparency
 Enables resources to be accessed without
knowledge of their physical or network location
(for example, which building or IP address).
49
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Transparency
 Concurrency transparency
 Enables several processes to operate
concurrently using shared resources without
interference between them.
 Replication transparency
 Enables multiple instances of resources to be
used to increase reliability and performance
without knowledge of the replicas by users or
application programmers.
50
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Transparency
 Failure transparency
 Enables the concealment of faults, allowing
users and application programs to complete their
tasks despite the failure of hardware or software
components.
 Mobility transparency
 Allows the movement of resources and clients
within a system without affecting the operation of
users or programs.
51
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Transparency
 Performance transparency
 Allows the system to be reconfigured to improve
performance as loads vary.
 Scaling transparency
 Allows the system and applications to expand in
scale without change to the system structure or
the application algorithms.
52
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Transparency
 The two most important transparencies are
access and location transparency referred to
together as network transparency.
 Presence or absence of network transparency
most strongly affects the utilization of
distributed resources.
53
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Resource sharing and the Web
 The World Wide Web
 The World Wide Web is an evolving system for
publishing and accessing resources and
services across the Internet.
(Figure 4)
54
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Resource sharing and the Web
www.google.com
Web servers
www.cdk3.net
http://www.google.com/search?q=Buyya
Browsers
http://www.cdk3.net/
www.w3c.org
File system of
www.w3c.org
http://www.w3c.org/Protocols/Activity.html
Protocols
Activity.html
Figure 4. Web servers and web browsers.
55
CHARACTERIZATION OF DISTRIBUTED SYSTEMS
Question 1.10
 The INFO service manages a potentially very large
set of resources, each of which can be accessed by
users throughout the Internet by means of a key (a
string name). Discuss an approach to the design of
the names of the resources that achieves the
minimum loss of performance as the number of
resources in the service increases. Suggest how the
INFO service can be implemented so as to avoid
performance bottlenecks when the number of users
becomes very large
56
Figure 1.5
Computers in the Internet
Computers
Date
1979, Dec.
1989, July
1999, July
2003, Jan.
Web servers
188
0
130,000
56,218,000
171,638,297
0
5,560,866
35,424,956
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Centralized vs. Distributed Computing
terminal
mainframe computer
workstation
network link
network host
centralized computing
distributed computing
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Evolution of paradigms
 Client-server: Socket API, remote method invocation
 Distributed objects
 Object broker: CORBA
 Network service: Jini
 Object space: JavaSpaces
 Mobile agents
 Message oriented middleware (MOM): Java Message
Service
 Collaborative applications
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Operating Systems Basics
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Process and program
A process consists of an executing program, its
current values, state information, and the
resources used by the operating system to
manage its execution.
A program is an artifact constructed by a
software developer; a process is a dynamic
entity which exists only when a program is run.
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Process State Transition Diagram
terminated
start
queued
exit
dispatch
ready
event completion
running
waiting
for event
blocked
Simplifed finite state diagram for a process's lifetime
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Java processes
 There are three types of Java program:
applications, applets, and servlets, all are written as
a class.
 A Java application program has a main method, and is
run as an independent(standalone) process.
 An applet does not have a main method, and is run
using a browser or the appletviewer.
 A servlet does not have a main method, and is run in the
context of a web server.
 A Java program is compiled into bytecode, a
universal object code. When run, the bytecode is
interpreted by the Java Virtual Machine (JVM).
Three Types of Java programs
 Applications
a program whose byte code can be run on any system
which has a Java Virtual Machine. An application may
be standalone (monolithic) or distributed (if it interacts
with another process).
 Applets
A program whose byte code is downloaded from a
remote machine and is run in the browser’s Java Virtual
Machine.
 Servlets
A program whose byte code resides on a remote
machine and is run at the request of an HTTP client (a
browser).
Three Types of Java programs
A standalone Java application is run on a local machine
computer
Java object
Java Virtual Machine
An applet is an object downloaded (transferred) from a remote machine,
then run on a local machine.
an applet
Java object
Java Virtual Machine
A servlet is an object that runs on a remote machine and
interacts with a local program using a request-response protocol
a servlet
request
response
a process
A sample Java application
/*******************************************************
* A sample of a simple Java application.
1/8/02
* M. Liu
******************************************************/
import java.io.*;
class MyProgram{
public static void main(String[ ] args)
throws IOException{
BufferedReader keyboard = new
BufferedReader(new InputStreamReader(System.in));
String theName;
System.out.println("What is your name?");
theName = keyboard.readLine( );
System.out.print("Hello " + theName );
System.out.println(" - welcome to CSC369.\n");
} // end main
} //end class
A Sample Java Applet
/***************************************************
* A sample of a simple applet.
* M. Liu
1/8/02
***************************************************/
import java.applet.Applet;
import java.awt.*;
public class MyApplet extends Applet{
public void paint(Graphics g){
setBackground(Color.blue);
Font Claude = new Font("Arial", Font.BOLD, 40);
g.setFont(Claude);
g.setColor(Color.yellow);
g.drawString("Hello World!", 100, 100);
} // end paint
} //end class
<!-- A web page which, when browsed, will run >
<!-- the MyApplet applet>
<!-- M. Liu 1/8/02>
<title>SampleApplet</title>
<hr>
<applet code="MyApplet.class" width=500 height=500>
</applet>
<hr>
<a href="Hello.java">The source.</a>
A Sample Java Servlet
/*******************************************************
* A sample of a simple Java servlet.
* M. Liu
1/8/02
******************************************************/
import java.io.*;
import java.text.*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class MyServlet extends HttpServlet {
public void doGet (HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
PrintWriter out;
String title = "MyServlet Output";
// set content type and other response header
// fields first
response.setContentType("text/html");
// then write the data of the response
out = response.getWriter();
out.println("<HTML><HEAD><TITLE>");
out.println(title);
out.println("</TITLE></HEAD><BODY>");
out.println("<H1>" + title + "</H1>");
out.println("<P>Hello World!");
out.println("</BODY></HTML>");
out.close();
} //end doGet
} //end class
Concurrent Processing
On modern day operating systems, multiple
processes appear to be executing concurrently
on a machine by timesharing resources.
Processes
P1
P2
P3
P4
time
Timesharing of a resource
Concurrent processing within a process
It is often useful for a process to have parallel threads of
execution, each of which timeshare the system
resources in much the same way as concurrent
processes.
A parent process may spawn child processes.
A process may spawn child threads
a process
parent process
main thread
child thread 1
child thread 2
child processes
Concurrent processing within a process
Java threads
 The Java Virtual Machine allows an application to have multiple
threads of execution running concurrently.
 Java provides a Thread class:
public class Thread
extends Object
implements Runnable

When a Java Virtual Machine starts up, there is usually a single
thread (which typically calls the method named main of some
designated class). The Java Virtual Machine continues to
execute threads until either of the following occurs:
 The exit method of class Runtime has been called and the security
manager has permitted the exit operation to take place.
 All threads have terminated, either by returning from the call to the
run method or by throwing an exception that propagates beyond
the run method.
Using a subclass of the Thread class
Using a class that implements the Runnable interface
Create a class that is a subclass of the Thread class
Declare a class to be a subclass of
Thread. This subclass should override
the run method of class Thread. An
instance of the subclass can then be
allocated and started:
import SomeThread;
public class RunThreads
{
public static void main (String[] args)
{
SomeThread p1 = new SomeThread(1);
p1.start();
SomeThread p2 = new SomeThread(2);
p2.start();
SomeThread p3 = new SomeThread(3);
p3.start();
}
}// end class RunThreads
public class SomeThread extends Thread {
int myID;
SomeThread(int id) {
this.myID = id;
}
public void run( ) {
int i;
for (i = 1; i < 11; i++)
System.out.println ("Thread"+myID + ": " + i);
}
} //end class SomeThread
Create a class that implements the Runnable interface
The other way to create a thread is to declare a class that
implements the Runnable interface. That class then
implements the run method. An instance of the class can
then be allocated, passed as an argument when creating
Thread, and started.
public class RunThreads2
{
public static void main (String[] args)
{
Thread p1 = new Thread(new SomeThread2(1));
p1.start();
Thread p2 = new Thread(new SomeThread2(2));
p2.start();
Thread p3 = new Thread(new SomeThread2(3));
p3.start();
}
}
class SomeThread2 implements Runnable {
int myID;
SomeThread2(int id) {
this.myID = id;
}
public void run() {
int i;
for (i = 1; i < 11; i++)
System.out.println ("Thread"+myID + ": " + i);
}
} //end class SomeThread
Thread-safe Programming
 When two threads independently access and
update the same data object, such as a counter,
as part of their code, the updating needs to be
synchronized. (See next slide.)
 Because the threads are executed concurrently,
it is possible for one of the updates to be
overwritten by the other due to the sequencing
of the two sets of machine instructions
executed on behalf of the two threads.
 To protect against the possibility, a
synchronized method can be used to provide
mutual exclusion.
Race Condition
time
fetch value in counter and load into a register
fetch value in counter and load into a register
increment value in register
fetch value in counter and load into a register
store value in register to counter
increment value in register
fetch value in counter and load into a register
increment value in register
increment value in register
store value in register to counter
store value in register to counter
store value in register to counter
This execution results in the
value 2 in the counter
This execution results in the
value 1 in the counter
instruction executed in concurrent process or thread 1
instruction executed in concurrent process or thread 2
Synchronized method in a thread
class SomeThread3 implements Runnable {
static int count=0;
SomeThread3() {
super();
}
public void run() {
update( );
}
static public synchronized void update( ){
int myCount = count;
myCount++;
count = myCount;
System.out.println("count="+count+
"; thread count=" + Thread.activeCount( ));
}
}
Network Basics
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
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Network resources
 Network resources are resources available to
the participants of a distributed computing
community.
 Network resources include hardware such as
computers and equipment, and software such
as processes, email mailboxes, files, web
documents.
 An important class of network resources is
network services such as the World Wide Web
and file transfer (FTP), which are provided by
specific processes running on computers.
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Network standards and protocols
 On public networks such as the Internet, it is
necessary for a common set of rules to be
specified for the exchange of data.
 Such rules, called protocols, specify such
matters as the formatting and semantics of
data, flow control, error correction.
 Software can share data over the network using
network software which supports a common set
of protocols.
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Protocols



In the context of communications, a protocol is a set of
rules that must be observed by the participants.
In communications involving computers, protocols must
be formally defined and precisely implemented. For
each protocol, there must be rules that specify the
followings:
 How is the data exchanged encoded?
 How are events (sending , receiving) synchronized
so that the participants can send and receive in a
coordinated order?
The specification of a protocol does not dictate how the
rules are to be implemented.
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
The network architecture
 Network hardware transfers electronic
signals,which represent a bit stream, between two
devices.
 Modern day network applications require an
application programming interface (API) which
masks the underlying complexities of data
transmission.
 A layered network architecture allows the
functionalities needed to mask the complexities to
be provided incrementally, layer by layer.
 Actual implementation of the functionalities may not
be clearly divided by layer.
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Protocol layers in the ISO Open Systems Interconnection (OSI) model
Messag e received
Messag e sent
Layers
Application
Presentation
Session
Transport
Network
Data link
Physical
Sender
Communication
medium
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Recipient
OSI protocol summary
Layer
Application
Presentation
Session
Transport
Network
Data link
Physical
Description
Protocols that are designed to meet the communication requirements of
specific applications, often defining the interface to a service.
Protocols at this level transmit data in a network representation that is
independent of the representations used in individual computers, which may
differ. Encryption is also performed in this layer, if required.
At this level reliability and adaptation are performed, such as detection of
failures and automatic recovery.
This is the lowest level at which messages (rather than packets) are handled.
Messages are addressed to communication ports attached to processes,
Protocols in this layer may be connection-oriented or connectionless.
Transfers data packets between computers in a specific network. In a WAN
or an internetwork this involves the generation of a route passing through
routers. In a single LAN no routing is required.
Responsible for transmission of packets between nodes that are directly
connected by a physical link. In a WAN transmission is between pairs of
routers or between routers and hosts. In a LAN it is between any pair of hosts.
The circuits and hardware that drive the network. It transmits sequences of
binary data by analogue signalling, using amplitude or frequency modulation
of electrical signals (on cable circuits), light signals (on fibre optic circuits)
or other electromagnetic signals (on radio and microwave circuits).
Examples
HTTP, FTP , SMTP,
CORBA IIOP
Secure Sockets
(SSL),CORBA Data
Rep.
TCP, UDP
IP, ATM virtual
circuits
Ethernet MAC,
ATM cell transfer,
PPP
Ethernet base- band
signalling, ISDN
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Figure 3.12
TCP/IP layers
The Transmission Control Protocol/Internet Protocol suite is a set of
network protocols which supports a four-layer network
architecture.
The Internet layer implements the Internet Protocol, which provides the
functionalities for allowing data to be transmitted between any two hosts
on the Internet.
The Transport layer delivers the transmitted data to a specific process running
on an Internet host.
The Application layer supports the programming interface used for building a
program.
Message
Layers
Application
Messages (UDP) or Streams (TCP)
Transport
UDP or TCP packets
Internet
IP datagrams
Network interface
Network-specific frames
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Underlying network
Figure 3.14
The programmer's conceptual view of a TCP/IP Internet
Application
Application
TCP
UDP
IP
 Socket programming in UDP and
TCP.
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Identification of Network Resources
One of the key challenges in distributed
computing is the unique identification of
resources available on the network, such as email mailboxes, and web documents.
Addressing an Internet Host
Addressing a process running on a host
Email Addresses
Addressing web contents: URL
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
The Internet Topology
 The internet consists of an hierarchy of
networks, interconnected via a network
backbone.
 Each network has a unique network address.
 Computers, or hosts, are connected to a
network. Each host has a unique ID within its
network.
 Each process running on a host is associated
with zero or more ports. A port is a logical
entity for data transmission.
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Network Basics
1. Addressing an Internet Host
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Figure 3.15
Internet address structure, showing field sizes in bits
7
Class A:
Class B:
0
24
Network ID
1 0
Host ID
14
16
Network ID
Host ID
21
Class C :
1 1 0
8
Network ID
Host ID
28
Class D (multicast):
1 1 1 0
Multicast address
27
Class E (reserved):
1 1 1 1 0
unused
Internet routing scheme developed in the 1970s. Class A addresses are the
largest, but there are few of them. Class Cs are the smallest, but they are
numerous. Classes D and E are also defined, but not used in normal
operation.
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Figure 3.16
Decimal representation of Internet addresses
octet 1
octet 2
Network ID
192 to 223
Range of addresses
Host ID
Class A: 1 to 126
0 to 255
(0, 127
Network ID
reserved)
Class B:
128 to 191
0 to 255
Class C:
octet 3
Network ID
0 to 255
0 to 255
1.0.0.0 to
126.255.255.255
0 to 255
0 to 255
128.0.0.0 to
191.255.255.255
0 to 255
Host ID
1 to 254
0 to 255
Host ID
192.0.0.0 to
223.255.255.255
Multicast address
Class D (multicast):
224 to 239
0 to 255
0 to 255
1 to 254
224.0.0.0 to
239.255.255.255
Class E (reserved):
240 to 255
0 to 255
0 to 255
1 to 254
240.0.0.0 to
255.255.255.255
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
The Internet addressing scheme
Subdividing the host portion of an Internet address:
byte 0
class B address
10
byte 1
network address
byte 2
byte 3
host portion
A class A/C address space can
also be similarly subdivided..
Which portion of the host address
is used for the subnet identification
is determined by a subnet mask.
subnet address
local host address
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Example
Example:
Suppose the dotted-decimal notation for a particular Internet address
is129.65.24.50. The 32-bit binary expansion of the notation is as
follows:
129.65.24.50
10000001
01000001
00011000
00110010is a Class B
Since the leading bit sequence is 10, the address
address. Within the class, the network portion is identified by the
remaining bits in the first two bytes, that is, 1000000101000001,
and the host portion is the values in the last two bytes, or
0001100000110010. so that we would say that this particular
address is at network 129.65 and then at host address 24.50 on
that network.
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
The Internet Address Scheme
 For human readability, Internet addresses are
written in a dotted decimal notation:
nnn.nnn.nnn.nnn, where each nnn group is a decimal
value in the range of 0 through 255
# Internet host table (found in /etc/hosts file)
127.0.0.1
localhost
129.65.242.5 falcon.csc.calpoly.edu falcon loghost
129.65.241.9 falcon-srv.csc.calpoly.edu
falcon-srv
129.65.242.4 hornet.csc.calpoly.edu hornet
129.65.241.8 hornet-srv.csc.calpoly.edu
hornet-srv
129.65.54.9 onion.csc.calpoly.edu onion
129.65.241.3 hercules.csc.calpoly.edu
hercules
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
The Domain Name System (DNS)
For user friendliness, each Internet address is
mapped to a symbolic name, using the DNS, in the
format of:
<computer-name>.<subdomain hierarchy>.<organization>.<sector name>{.<country code>}
root
e.g., www.csc.calpoly.edu.us
top-level domain
com
edu
gov
net
in the U.S.
org
mil
country code
Top-level domain name has to be applied for.
Subdomain hierachy and names are assigned
by the organization.
organization
...
...
subdomain
host name
4/2/2016
94
The Domain Name System
 For network applications, a domain name
must be mapped to its corresponding Internet
address.
 Processes known as domain name system
servers provide the mapping service, based
on a distributed database of the mapping
scheme.
 The mapping service is offered by thousands
of DNS servers on the Internet, each
responsible for a portion of the name space,
called a zone. The servers that have access
to the DNS information (zone file) for a zone is
said to have authority for that zone.
Top-level Domain Names
 .com: For commercial entities, which anyone, anywhere in the
world, can register.
 .net : Originally designated for organizations directly involved
in Internet operations. It is increasingly being used by
businesses when the desired name under "com" is already
registered by another organization. Today anyone can register
a name in the Net domain.
 .org: For miscellaneous organizations, including non-profits.
 .edu: For four-year accredited institutions of higher learning.
 .gov: For US Federal Government entities
 .mil: For US military
 Country Codes :For individual countries based on the
International Standards Organization. For example, ca for
Canada, and jp for Japan.
Example
Another example:
Given the address 224.0.0.1, one can expand it as
follows:
224.0.0.1
1110000
00000000
00000000
00000001
The binary prefix of 1110 signifies that this is class D, or
multicast, address. Data packets sent to this
address should therefore be delivered to the
multicast group 0000000000000000000000000001.
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Network Basics
2. Addressing a process running on a host
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Addressing a process running on a host
host A
...
host B
...
process
port
Each host has 65536 ports.
The Internet
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Well Known Ports
Each Internet host has 216 (65,535) logical ports.
Each port is identified by a number between 1
and 65535, and can be allocated to a particular
process.
Port numbers beween 1 and 1023 are reserved
for processes which provide well-known services
such as finger, FTP, HTTP, and email.
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Well Known Ports
Assignment of some well-known ports
Protocol
Port
Service
echo
7
IPC testing
daytime
13
provides the current date and time
ftp
21
file transfer protocol
telnet
23
remote, command-line terminal session
smtp
25
simple mail transfer protocol
time
37
provides a standard time
finger
79
provides information about a user
http
80
web server
RMI Registry
1099
registry for Remote Method Invocation
special web server
8080
web server which supports
servlets, JSP, or ASP
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Choose a port
For our programming exercises: when a port is
needed, choose a random number above the
well known ports: 1,024- 65,535.
If you are providing a network service for the
community, then arrange to have a port
assigned to and reserved for your service.
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
Network Basics
3. Addressing a Web Document
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005
The Uniform Resource Identifier (URI)
 Resources to be shared on a network need to be
uniquely identifiable.
 On the Internet, a URI is a character string which allows
a resource to be located.
 There are two types of URIs:
 URL (Uniform Resource Locator) points to a specific resource
at a specific location
 URN (Uniform Resource Name) points to a specific resource at
a nonspecific location.
 “A URN is like a person's name, while a URL is like their street
address. The URN defines something's identity, while the URL
provides a method for finding something. Essentially, "what" vs.
"where".” (from Wiki)
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© Pearson Education 2005
URL
A URL has the format of:
protocol://host address[:port]/directory path/file name#section
A sample URL:
http://www.csc.calpoly.edu:8080/~mliu/CSC369/hw.html # hw1
section name
file name
host name
protocol of server
directory path
port number of server process
Other protocols that can appear in a URL are:
file
ftp
gopher
news
telnet
WAIS
Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4
© Pearson Education 2005