1. 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.

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

3. 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 may be shared by several competitive users 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

4. 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 Scalability Size Geography Administrative organizations

5. Scaling Techniques (1) 1.4 Moving part of the computation to the client… The difference between letting: a)a server or b)a client check forms as they are being filled

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

7. 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

8. Uniprocessor Operating Systems Separating applications from operating system code through a microkernel. 1.11

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

10. Multicomputer Operating Systems (2) Alternatives for blocking and buffering in message passing. 1.15

11. 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

12. Distributed Shared Memory (DSM) 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

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

14. Network Operating System (1) General structure of a network (not distributed) operating system. 1-19

15. Network Operating System (2) evolution : file server Two clients and a server in a network operating system. File servers generally maintain hierarchical file system. 1-20

16. Network Operating System (3) Different clients may mount the servers in different places. Different clients have different views of the file systems 1.21

17. With a basic network operating system we loose in transparency Explicit remote login Independent accounting Independent access permission Low protection

18. Positioning Middleware General structure of a distributed system as middleware. Middleware doesn’t manage an individual node 1-22

19. Middleware Models Distributed objects – each object can be located on a single machine, each object hides internal details including communications Plan 9 – file oriented paradigm (Unix like) Distributed documents - WWW Models based on RPCs – hide network communications Distributed file system – transparency supported only for traditional files

20. Middleware Services Naming allows entities to be shared and looked up. Persistence for storage, i.e. databases or interface to connect to databases Distributed transactions allow multiple read and write operations to occur atomically. Security Access transparency by communication facilities hiding message passing through network

21. 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

22. Comparison between 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

23. Clients and Servers General interaction between a client and a server. Request-reply behavior 1.25

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

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

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

27. Processing Level for client server model The general organization of an Internet search engine into three different layers 1-28

28. Multitiered Architectures (1) Alternative client-server organizations 1-29

29. Multitiered Architectures (2) An example of a server acting as a client( vertical distribution). 1-30

30. Modern Architectures An example of horizontal distribution of a Web service. 1-31