Chapter 1: Characterization of Distributed Systems

• Introduction• Examples of distributed systems• Resource sharing and the web• Challenges• Summary

Characteristics of Distributed System

• Ubiquitous networks• Internet, mobile phone network, corporate network,

campus network, home network, in-car network, personal network …

• Tremendous applications are based on these networks, e.g., Web, ICQ

• Distributed System Definition• a distributed system is one in which components located at networked

computers communicate and coordinate their actions only by passing messages.

• Characteristics• 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

Chapter 1: Characterization of Distributed Systems

• Introduction• Examples of distributed systems• Resource sharing and the web• Challenges• Summary

• A vast interconnected collection of computer networks of many different types– TCP/IP

• A very large distributed system– WWW, email, FTP, VOD, etc

The Internet

intranet

ISP

desktop computer:

backbone

satellite link

server:

network link:

• 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• Main issues arising in the design of components for use in intranets

– File services: enable users to share data– Firewalls: impede legitimate access to services– The cost of software installation and support: reduce cost by the use of system

architectures such as network computers and thin clients

Intranet

A typical intranet

the rest of

email server

Web server

Desktopcomputers

File server

router/firewall

print and other servers

other servers

print

Local areanetwork

email server

the Internet

• Mobile devices– Laptop computers– Handheld devices, including PDAs, cell phones, pagers, video cameras and digital

cameras– Wearable devices, such as smart watches– Devices embedded in appliances such as washing machines, hi-fi systems, cars

• Mobile computing (nomadic computing)– People can still access resources while he is on the move, or visiting places other th

an their usual environment– Location-aware computing: utilize resources that are conveniently nearby

• Ubiquitous computing (pervasive computing)– The harnessing of many small, cheap computational devices that are present in use

r’s physical environments, including the home, office and elsewhere• A example about portable and handheld devices in a distributed system

Mobile and ubiquitous computing

Portable and handheld devices in a distributed system

Laptop

Mobile

PrinterCamera

Internet

Host intranet Home intranetWAP

Wireless LAN

phone

gateway

Host site

• Computational Grid– Meta Computing

• Idle computers are ubiquitous• Computers collaborate together as a whole system in the range of WAN• Transparently resource (process, storage, network, etc) sharing for the end use

rs– Examples: Globus, CERN Data Grid, ChinaGrid, Entropia.com

• Distributed Object Computing– Object Oriented Middleware for applications– Examples: CORBA, DCOM, EJB, Globe

• Peer to Peer applications– Distributed system architecture in contrast to Client/Server– Examples: Napster, Gnutella, FreeNet, OceanStore, JXTA

• Commercial giants’ perspective– Distributed Computing Environment– Examples: .NET, Autonomous Computing

What are people doing now? – Ongoing projects

OceanStore overview

The JXTA Search network architecture

Chapter 1: Characterization of Distributed Systems

• Introduction• Examples of distributed systems• Resource sharing and the web• Challenges• Summary

• Motivation of distributed computing: resource sharing• Resources types

– Hardware, e.g. printer, scanner, camera– Data, e.g. file, database, web page– Service, e.g. search engine, file

• Some definitions– Service: manages a collection of related resources and presents 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: interaction between client and server, from the point when the client sends its request to when it receives the server’s response

Resource sharing

Case study: World Wide Web

• Motivation of WWW– documents sharing between physicists of CERN

• WWW is an open system– be extended and be differently implemented based on standard protocols, different ser

ver & different browser– types of sharing resource can be extended, MIMES

• Basic technological components– HTML: HyperText Markup Language– URL: e.g. Ftp://ftp.cs.pku.edu.cn, http://net.cs.pku.edu.cn/~hh, http://

www.google.com/search?q=network– HTTP: Request-reply interactions, Content types, One resource per request, Simple ac

cess control• Advance Features

– Dynamic content: CGI, ASP, Servlet, etc– Dynamic web page: JavaScript, Applet, etc

• Discussion– 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

Chapter 1: Characterization of Distributed Systems

• Introduction• Examples of distributed systems• Resource sharing and the web• Challenges• Summary

Challenges (1)

1. Heterogeneity• networks: ethernet, token ring, etc• computer hardware: big endian/ little endian• operating systems: different message interfaces of Unix and Windows• programming languages: different representations for data structures• implementations by different developers: no command standardsMiddleware is a software layer that provides a programming abstraction as well as mask

ing the heterogeneity of the underlying platform. E.g., CORBA, DCOM, Java RMI, etc.

2. Openness• characterized by the fact that it can be extended and re-implemented in various wa

ys, e.g. Unix, InternetHow to deal with openness? key interfaces are published, e.g. RFC

3. Security• confidentiality: protection against disclosure to unauthorized individ

uals,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

Challenges (2)

Security challenges that are not yet fully met: Denial of service attacks, Security of mobile code, etc.

4. Scalability• A system is described as scalable if will remain effective when there is a sig

nificant increase in the number of resources and the number of users• The Internet is an example distributed system that is scalable

• design challenges– controlling the cost of physical resources. E.g., at most O(n)– controlling the performance loss. E.g., no worse than O(logn)– preventing software resources running out. E.g., IP address– avoiding performance bottlenecks. E.g., name table partitioned and cached in D

NS

Date Computers Web servers Percentage

1993, July 1,776,000 130 0.008

1995, July 6,642,000 23,500 0.41997, July 19,540,000 1,203,096 61999, July 56,218,000 6,598,697 12

Challenges (3)

5. Failure handling• Detecting, e.g. checksum for corrupted data. Sometimes impossible so sus

pect, e.g. a remote crashed server in the Internet• Masking, e.g. Retransmit message, standby server• Tolerating, e.g. Inform problems• Recovery, e.g. Roll back• Redundancy, e.g. IP route, replicated name table of DNSAvailability: measure of the proportion of time that a system is available for us

e

6. Concurrency• ensure the operations on shared resource correct in a concurrent environme

nt

7. Transparency• Access transparency: using identical operations to access local and remote

resources, e.g. Hyperlink in web page• Location transparency: resources to be accessed without knowledge of the

ir location, e.g. URL• Concurrency transparency: several processed operate concurrently using s

hared resources without interference with between them

Challenges (4)

• 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. Web cache

• 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 main motivation for constructing

distributed systems• Characterization of Distributed System

– Concurrency– No global clock– Independent failures

• Challenges to construct distributed system– Heterogeneity– Openness– Security– Scalability– Failure handling– Concurrency– Transparency