1. 1 Chapter 1: Characterization of Distributed Systems From Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edition 3, © Addison-Wesley 2001 Slides modified by Jens B. Jørgensen and Jonas Thomsen, University of Aarhus

2. 2 Distributed systems zDefinition: A distributed system is one in which components located at networked computers communicate and coordinate their actions only by passing messages. zPrimary purpose: Sharing of resources, e.g., discs, printers, files, and databases. zExamples: yDAIMI’s IT infrastructure. yThe Internet. yÅrhus Amt’s electronic patient record. y…

3. 3 Distributed systems – main characteristics zConcurrency: Parallel activities, shared resources, coordination. zNo global clock: No common notion of time, timing problems. zIndependent failures: Network, software, hardware, undiscovered errors. yLamport’s definition of a distributed system: “One in which the failure of a computer you didn't even know existed can render your own computer unusable”.

4. 4 The Internet (extract) Standardized protocols; Services: www, email, ftp,..

5. 5 The Internet – size Date Computers Web servers 1979, Dec. 1880 1989, July130,0000 1999, July56,218,0005,560,866

6. 6 An intranet and other servers other servers print Local area network email server the Internet Separately administered, local security policies

7. 7 Mobile, ubiquitous, and pervasive computing Example: Pervasive healthcare

8. 8 Client-server communication zService: Distinct part of a computer system that manages a collection of related resources and presents their functionality to users and applications. zServer: A running program (a process) on a networked computer that accepts requests from programs running on other computers to perform a service and respond appropriately. zClient: User of a server (also a process). z‘Client’ and ‘Server’ are roles. They only exist during a request. A Server may be client to another server.

9. 9 World Wide Web Internet Browsers Web servers www.google.com www.cdk3.net www.w3c.org Protocols Activity.html http://www.w3c.org/Protocols/Activity.html http://www.google.com/search?q=kindberg http://www.cdk3.net/ File system of www.w3c.org Server DNS (Domain Name Service) name Pathname on server Arguments

10. 10 Distributed systems design issues zHeterogeneity yDifferent hardware, software, network, dev. tools. ySolution: Middleware zOpenness yPublished interfaces zSecurity yConfidentiality, integrity, availability zScalability yO(n) resources for n users; Hierarchical O(n log n). zFailure handling yDetecting, masking, tolerating, recovering, redundancy  availability zConcurrency ySoftware must be written for concurrent access zTransparencies yNetwork (access, location), concurrency, replication, failure, mobility, performance, scaling

11. 11 Distributed systems – summary zDefinition and characterization of distributed systems. zExamples, e.g., the Internet. zClient/server communication. zWorld Wide Web. zDesign issues: Heterogeneity, openness, security, scalability, failure handling, concurrency, transparency.