1. Introduction to DISTRIBUTED COMPUTING Tran, Van Hoai Department of Systems & Networking Faculty of Computer Science & Engineering HCMC University of Technology 12009-2010

2. Outline Why distributed computing needed ? – performed by distributed systems Examples Definitions Goals to build distributed systems 22009-2010

3. Why distributed systems needed ? (1) Functional distribution: computers have different functional capabilities – Client/server – Host/terminal – Data gathering/data processing sharing of resources with specific functionalities Inherent distribution: stemming from application domain, e.g., – cash register and inventory systems for supermarket chains – computer supported collaborative work 32009-2010

4. Why distributed systems needed ? (2) Load distribution/balancing: assign tasks to computers such that overall performance is optimized Replication of processing power: independent computers working on the same task – collection of microcomputers may have processing power that no supercomputer will ever achieve 42009-2010

5. Why distributed systems needed ? (3) Physical separation: relying on the fact that computers are physically separated (e.g., to satisfy reliability requirements) Economics: collections of microprocessors offer a better price/performance ratio than large mainframes – mainframes: 10 times faster, 1000 times as expensive 52009-2010

6. Examples (1) Network of workstations – all files accessible from all machines in the same way and using the same path name – system looks for the best place to execute a command distributed system Workflow information system: automatic order processing – people from several departments at different locations – users unaware how an order to be processed distributed system 62009-2010

7. Examples (2) World Wide Web: offering uniform model of distributed documents – in theory, no need to know where the document is fetched – in practice, the location should be awared 72009-2010

8. Examples (3) Internet interconnected collection of computer networks of many different types computer interacts by passing messages using a common means of communication 82009-2010

9. Examples (4) Intranet resources shared to different computers 92009-2010

10. Definitions (1)  “A system in which hardware or software located at networked computers communicate and coordinate their actions only by message passing”. [Coulouris]  “A system that consists of a collection of two or more independent computers which coordinate their processing through exchange of synchronous or asynchronous message passing”. 102009-2010

11. Definitions (2)  “A distributed system is a collection of independent computers that appear to the users of the system as a single computer”. [Tanenbaum]  “A distributed system is a collection of autonomous computers linked by a network with software designed to produce an integrated computing facility”. 112009-2010

12. Definitions (3) There are several autonomous computational entities, each of which has its own local memory. [Andrews et al] 2009-201012

13. Computer networks vs. Distributed systems Computer network: autonomous computers are explicitly visible (have to be explicitly addressed) Distributed system: existence of multiple computers is transparent However, – many problems in common – in some sense networks (or parts of them, e.g. name services) are also distributed systems – normally, every distributed system relies on services provided by a computer network 132009-2010

14. Which examples are distributed systems ? Network of workstations distributed system Workflow information system: automatic order processing distributed system World Wide Web not fully qualified as a distributed system (Tanenbaum) distributed system (Coulouris) 142009-2010

15. Machine A Local OS Machine B Local OS Machine C Local OS Distributed applications Middleware service To guarantee – supporting heterogeneous computers – providing single view to users 152009-2010

16. Goals to build a distributed systems (1) Connecting users and resources – sharing resource – easier to collaborate and exchange information disadvantage: security (intrusion), privacy violation (communication tracking) 162009-2010

17. Goals to build a distributed systems (2) Transparency Description Access Hide differences in data representation and how a resource is accessed LocationHide where a resource is located MigrationHide that a resource may move to another location Relocation Hide that a resource may be moved to another location while in use ReplicationHide that a resource may have many copies ConcurrencyHide that a resource may be shared by several competitive users FailureHide the failure and recovery of a resource PersistenceHide whether a (software) resource is in memory or on disk tradeoff between a high degree of transparency and the performance of system 172009-2010

18. Goals to build a distributed systems (3) Openness – Offering services according to standard rules that describe syntax and semantics of those services syntax specification: in interface definition language semantic specification: in natural language – Interoperability and portability – Flexibility: using different components from different developers 182009-2010

19. Goals to build a distributed systems (4) Scalability – Measured in three dimensions size: more users, resources can be added easily geographics: users, resources may lie far apart administration: still easy to manage even spanning many independent administrative organizations – Some problems must be solved size: centralization – centralized service: single server for all users – centralized data: single online telephone book – centralized algorithm: routing based on complete information 192009-2010

20. Goals to build a distributed systems (5) size: centralization – centralized service: single server for all users – centralized data: single online telephone book – centralized algorithm: routing based on complete information geographics: synchronous & unreliable communication, – some system only designed for LAN (blocking communication depends strongly on quick response) administration: conflicting policies w.r.t. resource usage, management, security 202009-2010

21. Scaling techniques Asynchronous communication Distribution Replication, caching 212009-2010

22. Typical properties tolerate failures in individual computers The structure of the system (network topology, network latency, number of computers) is not known in advance Each computer has only a limited, incomplete view of the system 2009-201022

23. Architectures Client-server: – permanent data on server 3-tier architecture: – stateless client, – N-tier: web applications Tightly-coupled (clustered): – NOW, cluster of machines Peer-to-peer – Grid computing (VO level) Space-based – virtualization as one single address-space 2009-201023

24. 2009-201024 source:wikipedia.org

25. Some numbers (1) Computers in the Internet 2009-201025 Date Computers Web servers 1979, Dec. 188 0 1989, July130,0000 1999, July56,218,0005,560,866 2003, Jan.171,638,29735,424,956

26. Some numbers (2) Computers vs. Web servers in the Internet 2009-201026 DateComputersWeb serversPercentage 1993, July 1,776,000 130 0.008 1995, July 6,642,000 23,500 0.4 1997, July19,540,0001,203,0966 1999, July56,218,0006,598,69712 2001, July125,888,197 31,299,592 25

27. Text books & materials Andrew S. Tanenbaum, Maaten Van Steen, Distributed Systems: Principles and Paradigms, Prentice Hall, Second Edition, 2007 George Coulouris, Jean Dollimore, Tim Kindberg, Distributed Systems: Concepts and Design, Addison Wesley, Fourth Edition, 2005 Google 2009-201027