1 cosc3657:distributed systems haibin zhu, phd. assistant professor department of computer science...
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COSC3657:Distributed Systems
Haibin Zhu, PhD.
Assistant Professor
Department of Computer Science
Nipissing University
(C) 2002
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Instructor
Haibin Zhu, Ph. D. Assistant Professor Department of CS and Math Nipissing University Room: A124A Ext.: 4434 Email: [email protected] URL: http://www.nipissingu.ca/faculty/haibinz Office Hour: Mon.-Fri. 2:30pm-4:30pm, and by appointment
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Course
Introduction to computer networks and computer systems interconnected by networks, including both the hardware and software of selected LANs and WANs, the issues of reliable and secure communication, layered models, distributed file systems and concurrency will be discussed.
In this course, we will study some of the most important concepts in the design of distributed operating systems.
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Introduction to Distributed Systems
Distributed SystemsNetwork OSs vs distributed OSsResearch and design issuesReading: CDK, chapters 1
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What is a distributed system?
CDK’s Definition: A system in which hardware and software components
located on networked computers communicate and coordinate their actions only by passing messages.
Galli’s Definition A distributed system is a collection of heterogeneous
computers and processors connected via a network. This collection works closely together to accomplish a common goal.
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To understand the definition
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.
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Problems
Why do we require distributed systems?How does this compare with a traditional operating
system such as Unix?What implications does this definition have on
implementation?
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Distributed Operating Systems
Requirements: Provide user with convenient virtual computer. Hide distribution of resources. Mechanisms for protecting resources. Secure communication.
Definition Distributed OS looks to user like ordinary centralized
OS, but runs on multiple, independent CPUs. Use of multiple processors is invisible. User views system as virtual uniprocessor.
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Distributed vs Centralized Systems
Advantages of Distributed Systems: Reliability. Sharing of resources. Aggregate computing power. Openness/Scalability
Disadvantages of distributed systems: Security. Physical distribution of resources vs demand. Computing power per node is limited.
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Distributed vs Networked OS
Transparency: How aware are users of the fact that multiple computers
are being used?
Network OS: Users are aware where resources are located Network OS is built on top of centralized OS. Handles interfacing and coordination between local OSs.
Distributed OS: Designed to control and optimize operations and
resources in distributed system.
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Network OSs
Definition: A network OS is a collection of OSs of computers
connected through a network incorporating modules to provide access to remote resources.
Characteristics: Each computer has a private OS. A user works on his own machine and remotely logs in to
other computers. Users are aware of location of files. Limited fault tolerance.
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Internet
The Internet is a very large distributed system that allows users throughout the world to make use of its services.
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intranet
ISP
desktop computer:
backbone
satellite link
server:
network link:
Figure 1.1A typical portion of the Internet
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Intranet
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.
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Figure 1.2A typical intranet
the rest of
email server
Web server
Desktopcomputers
File server
router/firewall
print and other servers
other servers
Local areanetwork
email server
the Internet
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Mobile computing
Distributed systems techniques are equally applicable to mobile computing (involving laptops, PDAs and wearable computing devices) as well as ubiquitous computing (involving the small computers embedded in appliances).
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Fig. 1.3 Portable and handheld devices in a distributed system
Laptop
Mobile
PrinterCamera
Internet
Host intranet Home intranetWAP
Wireless LAN
phone
gateway
Host site
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Fig. 1.4 Web servers and web browsers
Internet
BrowsersWeb servers
www.google.com
www.cdk3.net
www.w3c.org
Protocols
Activity.html
http://www.w3c.org/Protocols/Activity.html
http://www.google.comlsearch?q=kindberg
http://www.cdk3.net/
File system ofwww.w3c.org
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Figure 1.5Computers in the Internet
Date Computers Web servers
1979, Dec. 188 0
1989, July 130,000 0
1999, July 56,218,000 5,560,866
2004 605, 600, 000 ???
2003 171, 638, 297 35, 424, 956
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Figure 1.6Computers vs Web servers in the Internet
Date Computers Web servers Percentage
1993, July 1,776,000 130 0.008
1995, July 6,642,000 23,500 0.4
1997, July 19,540,000 1,203,096 6
1999, July 56,218,000 6,598,697 12
42, 298, 371
125, 888, 197 31, 299, 5922001
2003
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World Wide Web
Ultimate distributed system. Developed in 1989 in CERN Switzerland for document
exchange among physicists. Documents have a hypertext structure. New resources are located by following these hypertext links. Uses a client-server model. A browser (client) on a user machine makes requests and
handles the displayNew document formats can be handled by plug-ins or helpers without changing the browser software. .The web server only delivers documents and is not concerned with user
interfaces or document format.
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World Wide Web Components
HTML (HyperText Markup Language) - specifies the format for the documents delivered by the server URL (Uniform Resource Locator) - specifies a resource that is accessible via the web.
HTTP (HyperText Transfer Protocol) - specifies the interaction between browsers and web servers
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HTML format for web documents
Text format can be created using Text editor or HTML tool.
Uses tags to specify content: <P> A paragraph </P> <IMG SRC=“Book.gif”> <A HREF=“http://www.nipissingu.ca”>NU</A>
The <A HREF…..> X</A> specifies a link identified by X in the text.
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URLS
Format scheme: scheme-specific-location
Format of scheme-specific-location when scheme is HTTP: http://servername[:port][/pathOnServer][/arguments]
Examples http://www.nipissingu.ca http://www.nipissingu.ca/faculty/haibinz http://www.google.ca/search?q=distributed+systems
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HTTP
Built using TCP socket connections. Request-reply protocol:
Client initiates with a request (GET) Server responds with requested document or an error
Content types identify document types for browser. HTTP 1.0 - need a separate request for each resource HTTP 1.1 - requests pipelined and served by a single
connection. Executables:
CGI (Common Gateway Interface) executes on server Java Applets execute on browser
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Research and Design Issues
Communication modelParadigms for process interactionTransparencyHeterogeneityAutonomy and/or interdependenceReliable distributed computingReplication
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Challenges
Heterogeneity Networks Hardware OS Programming languages Different implementations
Openness System can be extended and re-implemented interfaces published uniform mechanism to access resources heterogeneous h/w and software, provided that they conform to the
specification. Security
naming capacity, e.g., factor of 105 for Web servers
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Challenges cont’d
Failure handling hardware redundancy software recovery
Concurrency Many users, hence many concurrent pieces of work Servers need to be responsible to many clients, so they
need to be concurrent Many computers, hence many resources available for
parallelism Concurrency ?parallelism
Transparency
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Transparency
Transparency: Make the network invisible to user/applications.
Various degrees of transparency: Access Transparency Location Transparency Name Transparency Data Transparency Execution Transparency Performance Transparency Scaling transparency
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Figure 1.7Transparencies
Access transparency: enables local and remote resources to be accessed using identical operations.Location transparency: enables resources to be accessed without knowledge of their location.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.
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Autonomy and Interdependence
Disadvantage generated by interdependence: cannot work stand-alone globally controlled difficult to identify source of authority and responsibility what about mutual suspicion?
Reasons for autonomy: policy freedom robustness cooperation between mutually suspicious users
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Summary
Distributed Systems Definitions One system Many computers Connected by network Cooperate on the same task
Internet and World Wide Web Challenges
Heterogeneity Openness Security Failure handling Concurrency Transparency