1. 1 Introduction Chapter 1

2. 2 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.

3. 3 Definition of a Distributed System (2) A distributed system organized as middleware. Note that the middleware layer extends over multiple machines. 1.1

4. 4 Examples of DS 1.The Internet 1.Vast interconnected collection of computer network of many different types 2.Intranets 1.Portion of Internet that is separately administrated and has a boundary that can be configured to enforce local security policy. 3.Mobile and ubiquitous computing 1.Small and portable computing device. 2.Laptops computers. 3.PDA, mobiles phone, pagers, video cameras and digital cameras. 4.Wearable device, such as smart watches 5.Devices embedded in appliances- hi-fi+washing machine using one “universal remote control”.

5. 5 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 ReplicationHide 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

6. 6 Scalability yA system is described scalable if remain effective when there is a significant increase in the number of resources and users Date Computers Web servers 1979, Dec. 1880 1989, July130,0000 1999, July56,218,0005,560,866

7. 7 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. 8 Scaling Techniques (1) 1.4 The difference between letting: a)a server or b)a client check forms as they are being filled

9. 9 Scaling Techniques (2) 1.5 An example of dividing the DNS name space into zones.

10. 10 Lecture 2 Hardware Concepts Multiprocessors Homogenous Multicomputer System Software Concepts Distributed OS Uniprocessor Operating Systems Multiprocessor Operating System Multicomputer Operating System Distributed Shared Memory Network OS Middleware

11. 11 Hardware Concepts 1.6 Different basic organizations and memories in distributed computer systems

12. 12 Multiprocessors (1) A bus-based multiprocessor. 1.7 a)Coherent b)Incoherent – while using the cache

13. 13 Multiprocessors (2) a)A crossbar switch b)An omega switching network 1.8

14. 14 Homogeneous Multicomputer Systems a)Grid b)Hypercube 1-9

15. 15 Software Concepts 1.DS as resource manager 2.Provide Virtual Machine to hide the heterogeneous nature of underlying hardware

16. 16 Software Concepts An overview of 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

17. 17 DOS (Distributed Operating Systems) Multiprocessor Operating System –Manage the resource of multiprocessor Multicomputer Operating System –Develop for homogenous multicomputer

18. 18 Uniprocessor Operating Systems The goal- user and application sharing resources User mode & kernel mode- Controlled by OS. Separating applications from operating system code through a microkernel-code for device register, switching between process etc.

19. 19 Multicomputer Operating Systems (1) General structure of a multicomputer operating system 1.14

20. 20 Multicomputer Operating Systems (2) Alternatives for blocking and buffering in message passing. Leading to 4 possible synchronization point. 1.15

21. 21 Multicomputer Operating Systems (3) Relation between blocking, buffering, and reliable communications. Synchronization pointSend buffer Reliable comm. guaranteed? Block sender until buffer not full (s1)YesNot necessary Block sender until message sent (s2)NoNot necessary Block sender until message received (s3)NoNecessary Block sender until message delivered (s4)NoNecessary

22. 22 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

23. 23 Distributed Shared Memory Systems (2) False sharing of a page between two independent processes. 1.18

24. 24 Network Operating System(1) Heterogeneous. –Variety and difference in terms of: Networks Computer hardware Operating system Programming language Implementation by different developers Constructed from a collection of uniprocessor system using own OS. Connected to each other using computer network.

25. 25 Network Operating System (2) General structure of a network operating system. 1-19

26. 26 Network Operating System (3) Two clients and a server in a network operating system. 1-20 Generally maintain hierarchical file system. Client can import/mount these file system.

27. 27 Network Operating System (3) Different clients may mount the servers in different places. 1.21 #showmount –e #mount /mygames /private/games

28. 28 Positioning Middleware –Programs written by different developers cannot communicate unless use common standard –Middleware applied to a software masking the heterogeneity of hardware & software 1-22 General structure of a distributed system as middleware.

29. 29 Middleware Services 1.Communication facilities – message passing eg: RPC & Distributed Object Invocation. 2.Naming- URL still works even the server has been moved. 3.Persistence – special facilities for storage. 4.Distributed Transactions- multiple read/write operations atomicaly. 5.Security – cannot rely on OS only.

30. 30 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

31. 31 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

32. 32 Clients and Servers –General interaction between a client and a server. –Request and reply behavior. –Connectionless vs connection oriented protocol.

33. 33 An Example Client and Server (1) The header.h file used by the client and server.

34. 34 An Example Client and Server (2) A sample server.

35. 35 An Example Client and Server (3) A client using the server to copy a file. 1-27 b

36. 36 Processing Level The general organization of an Internet search engine into three different layers 1-28

37. 37 Multitiered Architectures (1) 2 types of machines –two-tiered architecture » client just a dumb terminal(user interface) »Server is handle everything Alternative client-server organizations (a) – (e). 1-29

38. 38 Multitiered Architectures (2) An example of a server acting as a client. eg: transaction processing – next chap 1-30

39. 39 Modern Architectures Multitier Client Server Architecture- –Divide application into 3 components – vertical distribution Modern Architecture is more consent about the distribution of client and the servers. –An example of horizontal distribution of a Web service.

40. 40 Summary (INTRODUCTION) DS: autonomous computer+single coherent system. Goals Hardware Concept Software Concept Client Server model-application layering »Vertical distribution vs horizontal distribution

41. 41 Exercise Do the problems on page 54 and 55. Eg: #4,#6,#15,#17 Ask the student in a group to find a journal about the distributed system. Work into group, (5- 6 people) each group. Make a synopsis and give a critique.