Transcript Figure 15.1 A distributed multimedia system

Chapter 1
Characterization of Distributed Systems
Coulouris, Dollimore and Kindberg
Distributed Systems:
Concepts and Design
Edition 4, © Pearson Education 2005
Distributed Computing
A distributed system is defined as one in which
components at networked computers communicate
and coordinate their actions only by passing
messages.
This definition allows for concurrent execution of
programs, but prevents the possibility of a global
clock and means that components can fail
independently of one another.
Internet and Internet
The Internet is a very large distributed system that
allows users throughout the world to make use of its
services.
An intranet is a part of the Internet that is separately
administered and uses a firewall to enforce its own
local security policies. Users in an intranet share
data by means of file services.
A typical portion of the Internet
intranet
ISP
%
%
%
%
backbone
satellite link
desktop computer:
server:
network link:
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
Portable and handheld devices in a distributed system
Internet
Host intranet
WAP
gatew ay
Wireles s LAN
Mobile
phone
Laptop
Printer
Camera
Home intranet
Host site
Distributed systems techniques are equally applicable to mobile computing
(involving laptops, PDAs and wearable computing devices)
Client-Server Model
Shared resources are managed by server
processes, which provide client processes with
access to those resources via a well-defined set of
operations.
In a distributed system written in an object-oriented
language, resources may be encapsulated as
objects whose methods are invoked by client
objects.
Web servers and web browsers
www.google.com
http://www.google.comlsearch?q=kindberg
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
Computers in the Internet
Date
1979, Dec.
1989, July
1999, July
2003, Jan.
Computers
Web servers
188
0
130,000
56,218,000
171,638,297
0
5,560,866
35,424,956
Computers vs. Web servers in the Internet
Date
1993, July
1995, July
1997, July
1999, July
2001, July
Computers
Web servers
Percentage
1,776,000
130
0.008
6,642,000
19,540,000
56,218,000
125,888,197
23,500
1,203,096
6,598,697
31,299,592
42,298,371
0.4
6
12
25
Challenges
Heterogeneity
Openness
Discovery
Scalability
The Web (hierarchic naming, partitioned data, caching
and replication).
failure handling
Concurrency
transparency
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).
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.
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.
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.