1. Kyung Hee University 1/41 Introduction Chapter 1

2. Kyung Hee University 2/41 Agenda -Definition -Hardware concept -Software concept -The Client-Server model

3. Kyung Hee University 3/41 Definition of a Distributed System (1) A distributed system is: A collection of independent computers that appears to its users as a single coherent system.

4. Kyung Hee University 4/41 Definition of a Distributed System (2) A distributed system organized as middleware. Note that the middleware layer extends over multiple machines. 1.1

5. Kyung Hee University 5/41 Goal of Distributed system Connecting users to resources Openness Transparency Scalability

6. Kyung Hee University 6/41 Transparency in a Distributed System Different forms of transparency in a distributed system. TransparencyDescription 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 Replication Hide that a resource is replicated Concurrency Hide that a resource may be shared by several competitive users FailureHide the failure and recovery of a resource Persistence Hide whether a (software) resource is in memory or on disk

7. Kyung Hee University 7/41 Scalability Problems Examples of scalability limitations. ConceptExample Centralized servicesA single server for all users Centralized dataA single on-line telephone book Centralized algorithmsDoing routing based on complete information

8. Kyung Hee University 8/41 Scaling Techniques (1) 1.4 The difference between letting: a)a server or b)a client check forms as they are being filled

9. Kyung Hee University 9/41 Scaling Techniques (2) 1.5 An example of dividing the DNS name space into zones.

10. Kyung Hee University 10/41 Hardware Concepts 1.6 Different basic organizations and memories in distributed computer systems

11. Kyung Hee University 11/41 Multiprocessors (1) A bus-based multiprocessor. 1.7

12. Kyung Hee University 12/41 Multiprocessors (2) a)A crossbar switch b)An omega switching network 1.8

13. Kyung Hee University 13/41 Multicomputers Each node is an autonomous machine  Private memory Lower traffic than multiprocessors  CPU – CPU versus CPU – Memory traffic Homogeneous or Heterogeneous

14. Kyung Hee University 14/41 Similar nodes  Same processors and memory space Homogeneous access to network  Single network Bus-based or point-to-point communication Homogeneous Multicomputer Systems (1)

15. Kyung Hee University 15/41 Homogeneous Multicomputer Systems (2) a)Grid b)Hypercube 1-9

16. Kyung Hee University 16/41 Heterogeneous Multicomputers Different nodes  Nodes can be complex system None-homogeneous access to network  Different network Distributed systems are commonly built on this H/W category  Need S/W to make it transparent

17. Kyung Hee University 17/41 Software Concepts An overview between DOS (Distributed Operating Systems) NOS (Network Operating Systems) Middleware SystemDescriptionMain Goal DOS Tightly-coupled operating system for multi- processors and homogeneous multicomputers Hide and manage hardware resources NOS Loosely-coupled operating system for heterogeneous multicomputers (LAN and WAN) Offer local services to remote clients Middleware Additional layer atop of NOS implementing general-purpose services Provide distribution transparency

18. Kyung Hee University 18/41 Uniprocessor Operating Systems Separating applications from operating system code through a microkernel. 1.11

19. Kyung Hee University 19/41 Multicomputer Operating Systems (1) General structure of a multicomputer operating system 1.14

20. Kyung Hee University 20/41 Multicomputer Operating Systems (2) Alternatives for blocking and buffering in message passing. 1.15

21. Kyung Hee University 21/41 Distributed Shared Memory Systems (1) a)Pages of address space distributed among four machines b)Situation after CPU 1 references page 10 c)Situation if page 10 is read only and replication is used

22. Kyung Hee University 22/41 Distributed Shared Memory Systems (2) False sharing of a page between two independent processes. 1.18

23. Kyung Hee University 23/41 Network Operating System (1) General structure of a network operating system. 1-19

24. Kyung Hee University 24/41 Network Operating System (2) Two clients and a server in a network operating system. 1-20

25. Kyung Hee University 25/41 Network Operating System (3) Different clients may mount the servers in different places. 1.21

26. Kyung Hee University 26/41 DOS and NOS v.s DS DOS qualifies as DS?  Computers are not independent  Easy to use and transparent NOS qualifies as DS?  No single coherent view  Scalable and open

27. Kyung Hee University 27/41 Positioning Middleware General structure of a distributed system as middleware. 1-22

28. Kyung Hee University 28/41 Middleware and Openness In an open middleware-based distributed system, the protocols used by each middleware layer should be the same, as well as the interfaces they offer to applications. 1.23

29. Kyung Hee University 29/41 Comparison between Systems A comparison between multiprocessor operating systems, multicomputer operating systems, network operating systems, and middleware based distributed systems. Item Distributed OS Network OS Middleware- based OS Multiproc.Multicomp. Degree of transparencyVery HighHighLowHigh Same OS on all nodesYes No Number of copies of OS1NNN Basis for communication Shared memory MessagesFilesModel specific Resource management Global, central Global, distributed Per node ScalabilityNoModeratelyYesVaries OpennessClosed Open

30. Kyung Hee University 30/41 The Clients and Servers Model

31. Kyung Hee University 31/41 Client and Server -Clients request services. -Servers provide services by replying to the requests. General interaction between a client and a server.

32. Kyung Hee University 32/41 An Example Client and Server (1) The header.h file used by the client and server. Client and server need to agree on a couple things first!

33. Kyung Hee University 33/41 An Example Client and Server (2) A sample server (A basic server loop) -The server continually listens for new requests (receive). -Extracting the request and simply executes it. -It then prepares a response message and sends it back to the requesting client.

34. Kyung Hee University 34/41 An Example Client and Server (3) Sample Client: using the server to copy a file. 1-27 b

35. Kyung Hee University 35/41 Application Layering A client-server application typically consists of three layers: 1.User-Interface level: +Consists of the programs that allow end users to interact with application. 2.Processing level: +Implements the application logic (core functionality) +Typically implemented at the server side 3.Data level +Maintain the actual data on which the application operate. +Also keep data consistent across different application

36. Kyung Hee University 36/41 Application Layering The general organization of an Internet search engine into three different layers 1-28 Web browser Application server Database server

37. Kyung Hee University 37/41 Client-Server Architectures Alternative client-server organizations (a) – (e). 1-29 Two-tiers Architectures

38. Kyung Hee University 38/41 Client-Server Architectures (con.t) An example of a server acting as a client. 1-30 Three-tiers Architectures

39. Kyung Hee University 39/41 Modern Architectures An example of horizontal distribution of a Web service. -Vertical distribution Model : where the tiers corresponded directly with the logical organization of the apps (The previous architectures) -H orizontal distribution Model: where the application layers are physically split up into logically equivalent part -> This is how high-performance web services (e.g., Google, Amazon, E-bay..)

40. Kyung Hee University 40/41 End of Chapter 1