1. Primitive Concepts of Distributed Systems Chapter 1

2. Why Distribution?  More complex computing problems  Grosch’s Law  CPU speed will not increase by time

3. Parallelism vs. Distribution  Sometimes a user’s program which is inherently parallel, can be run on a multiprocessor.  This will happen by a parallelizing compiler, proper operating system and specific hardware.  In a distributed system, this will happen among more than a computer.

4. Distributed System’s Goal  Connecting users and resources  Transparency  Openness  Scalability

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 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

6. Transparency vs. Performance  There is also a trade-off between a high degree of transparency and the performance of a system.  Examples: –Multiple retries for a crashed server –Keeping a group of replicated servers strictly consistent

7. Openness  An open system is such a system that its syntax and semantic is properly defined.  Interoperability and Portability  Separating Policy and Mechanism

8. Scalability  Size  Geographical  Administrative

9. 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

10. Scalability Techniques  Asynchronous Communication  Distribution  Replication

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

12. Hardware Concepts Multicomputer vs. Multiprocessor (global address space) Cable TV Telephone

13. Multiprocessors (1)  A bus-based multiprocessor.  Problem of bus-based systems: scalability –Solution: cache  Cache problem: makes the memory incoherent  What can we do?

14. Multiprocessors (2) a) A crossbar switch b) An omega switching network c) NUMA?

15. Spectrum of HW Systems  MPP (Massively Parallel Processors) –Expensive, need HP networking, simultaneous access to shared memory, single address space, hardwired, tightly couples  COW (Cluster of Workstations) –Cheaper, easy to make, message passing, not hardwired, flexible, loosely coupled

16. Software Concepts  An overview of  DOS (Distributed Operating Systems)  NOS (Network Operating Systems) 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

17. Uniprocessor Operating Systems  Separating applications from operating system code through a microkernel.

18. Which is easier for programming?  Question: Which one is easier for programming? A multicomputer or a multiprocessor?

19. 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

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

21. Network Operating System (1)  General structure of a network operating system.

22. Middleware DOS (Transparency) NOS (Openness and Scalability) Middleware

23. Positioning Middleware  General structure of a distributed system as middleware.

24. Middleware Problems  An application developed on top of a middleware system may not work on another one.  Using an upperware may solve the problem.

25. 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

26. Processing Level

27. Multitiered Architectures (1)  Alternative client-server organizations (a) – (e).

28. Questions  A complex algorithm is going to be executed. Which one is cheaper? Using a multicomputer or a multiprocessor?  What’s the idea behind the DSM?  What’s the idea behind a middleware?  What’s the problems of using a middleware system?  What’s the difference of policy and mechanism?  Why open distributed systems are interoperable?  What’s false sharing?  Discuss about COW and MPP for OS, cost, EoP, scalability, speed, model of programming,  Why microkernel OS? Pros and cons?