Chapter 1: Characterization of Distributed
Systems
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•
•
•
•
Introduction
Examples of distributed systems
Resource sharing and the web
Challenges
Summary
Ubiquitous networks
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Internet
Mobile phone networks
Corporation networks
Factory networks
Campus networks
Home networks
……
Distributed System Definition
• A distributed system is one in which
hardware or software components
located at networked computers
communicate and coordinate their
actions only by passing messages.
Characteristics of Distributed System
• Concurrency
• concurrent programs execution – share
resource
• No global clock
• programs coordinate actions by exchanging
messages
• Independent failures
• when some systems fail, others may not know
Share resources
• It characterizes the range of the things that can
usefully be shared in a networked computer
• It extends from hardware components to
software-defined entities.
• It includes the stream of video frames and
the audio connection.
Chapter 1: Characterization of Distributed
Systems
•
•
•
•
•
Introduction
Examples of distributed systems
Resource sharing and the web
Challenges
Summary
Familiar and widely used computer networks
• The Internet
• Intranet
• Mobile computing
Internet
• It is a very large distributed system that allows
users throughout the world to make use of its
services.
• Internet protocols is a major technical
achievement.
The Internet
intranet
ISP
%
%
%
%
backbone
satellite link
desktop computer:
server:
network link:
Intranet
• What is Intranet?
– A portion of the Internet that is separately
administered and has a boundary that can be
configured to enforce local security policies
– Composed of several LANs linked by
backbone connections
– Be connected to the Internet via a router
A typical Intranet
email s erv er
Desktop
computers
print and other servers
Web server
Local area
netw ork
email s erv er
File s erv er
print
other servers
the rest of
the Internet
router/firew all
Three main issues in the design of
components for the use in intranet
• File services
• Firewall
• The cost of software installation and
support
Mobile and ubiquitous computing
• Mobile devices
– Laptop computers
– Handheld devices
• PDA, mobile phone, pager, video camera, digital
camera
– Wearable devices
• e.g. smart watches, digital glasses
– Network appliances
• e.g. washing machines, hi-fi systems, cars and
refrigerators
Mobile and ubiquitous computing … continued
• Mobile computing (nomadic computing)
– Access resources while on the move or in an
unusual environment
– Location-aware computing: utilize resources
that are conveniently nearby
• Ubiquitous computing (pervasive computing)
– The harnessing of many small, cheap
computational devices
Portable and handheld devices in a
distributed system
Internet
Host intranet
WAP
gatew ay
Wireles s LAN
Mobile
phone
Printer
Laptop
Camera
Host site
Home intranet
Issues in the design of components for the
use in Mobile and ubiquitous computing
• Discovery of resources
• Eliminating the need for users to
reconfigure their mobile devices
• To cope with limited connectivity as they
travel
• Provide privacy and other security
guarantees
Chapter 1: Characterization of Distributed
Systems
•
•
•
•
•
Introduction
Examples of distributed systems
Resource sharing and the web
Challenges
Summary
Resource sharing
• Is the primary motivation of distributed
computing
• Resources types
– Hardware, e.g. printer, scanner, camera
– Data, e.g. file, database, web page
– More specific functionality, e.g. search
engine, file
Some definitions
• Service
– manage a collection of related resources and present their functionalities to
users and applications
• Server
– a process on networked computer that accepts requests from processes on
other computers to perform a service and responds appropriately
• Client
– the requesting process
• Remote invocation
– A complete interaction between client and server, from the point when the
client sends its request to when it receives the server’s response
Case study: the World Wide Web
• Motivation of WWW
– Documents sharing between physicists of CERN
• Web is an open system: it can be extended
and implemented in new ways without
disturbing its existing functionality.
– Its operation is based on communication standards
and document standards
– Respect to the types of ‘resource’ that can be
published and shared on it.
Three main components of the Web
• HyperText Markup Language
– A language for specifying the contents and layout of
pages
• Uniform Resource Locators
– Identify documents and other resources
• A client-server architecture with HTTP
– By with browsers and other clients fetch
documents and other resources from web
servers
HTML
<IMG SRC = http://www.cdk3.net/WebExample/Images/earth.jpg>
<P>
Welcome to Earth! Visitors may also be interested in taking a look at the
<A HREF = “http://www.cdk3.net/WebExample/moon.html>Moon</A>.
<P>
(etcetera)
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
HTML text is stored in a file of a web server.
A browser retrieves the contents of this file from a web
server.
-The browser interprets the HTML text
-The server can infer the content type from the filename
extension.
URL
Scheme: scheme-specific-location
e.g:
mailto:[email protected]
ftp://ftp.downloadIt.com/software/aProg.exe
http://net.pku.cn/
….


HTTP URLs are the most widely used
An HTTP URL has two main jobs to do:
- To identify which web server maintains the resource
- To identify which of the resources at that server
Web servers and web browsers
http://e.pku.cn/cgi-bin/allsearch?word=distributed+system
e.pku.cn
Browsers
Web servers
Internet
www.cdk3.net
http://www.cdk3.net/
www.w3c.org
File system of
www.w3c.org
http://www.w3c.org/Protocols/Activity.html
Protocols
Activity.html
HTTP URLs
• http://servername[:port]//pathNameOnServer][?arguments]
• e.g.
http://www.cdk3.net/
http://www.w3c.org/Protocols/Activity.html
http://e.pku.cn/cgi-bin/allsearch?word=distributed+system
----------------------------------------------------------------------------------------------------
Server DNS name
www.cdk3.net
www.w3c.org
e.pku.cn
Pathname on server
(default)
Protocols/Activity.html
cgi-bin/allsearch
Arguments
(none)
(none)
word=distributed+system
-------------------------------------------------------------------------------------------------------
• Publish a resource remains unwieldy
HTTP
• Defines the ways in which browsers and
any other types of client interact with web
servers (RFC2616)
• Main features
– Request-replay interaction
– Content types. The strings that denote the type of
content are called MIME (RFC2045,2046)
– One resource per request. HTTP version 1.0
– Simple access control
More features-services and dynamic pages
• Dynamic content
– Common Gateway Interface: a program that
web servers run to generate content for their
clients
• Downloaded code
– JavaScript
– Applet
Discussion of Web

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Dangling: a resource is deleted or moved, but links
to it may still remain
Find information easily: e.g. Resource Description
Framework which standardize the format of
metadata about web resources
Exchange information easily: e.g. XML – a self
describing language
Scalability: heavy load on popular web servers
More applets or many images in pages increase in
the download time
Chapter 1: Characterization of Distributed
Systems
•
•
•
•
•
Introduction
Examples of distributed systems
Resource sharing and the web
Challenges
Summary
Heterogeneity
• Networks
– Ethernet, token ring, etc
• Computer hardware
– big endian / little endian
• Operating systems
– different API of Unix and Windows
• Programming languages
– different representations for data structures
• Implementations from different developers
– no application standards
Heterogeneity… continued
• Middleware
– applies to a software layer that provides a
programming abstraction as well as masking the
heterogeneity of the underlying networks,
hardware, OSs and programming languages
• Mobile code
– is used to refer to code that can be sent from one
computer to another and run at the destination
Openness
• Openness of a computer system
-- is the characteristic that determines whether the system
can be extended and re-implemented in various way.
e.g. Unix
• Openness of distributed systems
-- is determined by the degree to witch new resource
sharing services can be added and be made available for
use by A variety of client programs.
e.g. Web
• How to deal with openness?
-- key interfaces are published, e.g. RFC
Security
• Confidentiality
– protection against disclosure to
unauthorized individuals,
e.g. ACL in Unix File System
• Integrity
– protection against alteration or corruption,
e.g. checksum
• Availability
– protection against interference with the
means to access the resources,
e.g. Denial of service
Scalability
• A system is described as scalable
–
if will remain effective when there is a significant increase
in the number of resources and the number of users
• A scalable example system: the Internet
• design challenges
–
–
–
–
The cost of physical resources, e.g., servers support users at most O(n)
The performance loss, e.g., DNS no worse than O(logn)
Prevent software resources running out, e.g., IP address
Avoid performance bottlenecks, e.g., partitioning name table of DNS,
cache and replication
Date
1993, July
1995, July
1997, July
1999, July
Computers
1,776,000
6,642,000
19,540,000
56,218,000
Web servers
130
23,500
1,203,096
6,598,697
Percentage
0.008
0.4
6
12
Failure handling
• Detecting
– e.g. checksum for corrupted data
– Sometimes impossible so suspect, e.g. a remote
crashed server in the Internet
• Masking
– e.g. Retransmit message, standby server
• Tolerating
– e.g. a web browser cannot contact a web server
• Recovery
– e.g. Roll back
• Redundancy
– e.g. IP route, replicated name table of DNS
Concurrency
• Correctness
– ensure the operations on shared resource
correct in a concurrent environment
e.g. records bids for an auction
• Performance
– Ensure the high performance of concurrent
operations
Transparency
• Access transparency
– using identical operations to access local and remote
resources, e.g. a graphical user interface with folders
• Location transparency
– resources to be accessed without knowledge of their
location, e.g. URL
• Concurrency transparency
– several processed operate concurrently using shared
resources without interference with between them
• Replication transparency
– multiple instances of resources to be used to increase
reliability and performance without knowledge of the
replicas by users or application programmers,
e.g. realcourse(http://vod.yf.pku.edu.cn/)
Transparency … continued
• Failure transparency
– users and applications to complete their tasks despite the
failure of hardware and software components, e.g., email
• Mobility transparency
– movement of resources and clients within a system
without affecting the operation of users and programs, e.g.,
mobile phone
• 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
Chapter 1: Characterization of Distributed
Systems
•
•
•
•
•
Introduction
Examples of distributed systems
Resource sharing and the web
Challenges
Summary
Summary
• Distributed systems are pervasive
• Resource sharing is the primary motivation for
constructing distributed systems
• Characterization of Distributed System
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–
–
Concurrency
No global clock
Independent failures
• Challenges to construct distributed system
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Heterogeneity
Openness
Security
Scalability
Failure handling
Concurrency
Transparency
Backup
OceanStore overview
The JXTA Search network architecture