1. Dr. Kalpakis CMSC621 Advanced Operating Systems Introduction

2. CMSC 621 2 Distributed Systems The development of low cost powerful microprocessors together with the invention of high speed networks enable us to construct computer systems by connecting a large number of computers A distributed system is a collection of independent computers that appears to its users as a single coherent system.

3. CMSC 621 3 Distributed Systems Often a distributed system is organized as a middleware. 1.1 Note that the middleware layer extends over multiple machines.

4. CMSC 621 4 Goals of distributed systems When should we build a distributed system? Make resources easily accessible Provide distribution transparency Should be open Be scalable

5. CMSC 621 5 Forms of transparency TransparencyDescription AccessHide 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 RelocationHide that a resource may be moved to another location while in use ReplicationHide that a resource may be shared by several competitive users 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 Sometimes, due to eg performance/comprehensibility, we prefer to expose distribution rather than hide it

6. CMSC 621 6 Openness An open distributed system is a system that offers services according to standard service syntax/semantic descriptions (interfaces) Interoperability Portability Extensibility Separation of policy and mechanism

7. CMSC 621 7 Scalability Scalability with respect to Size Geographic distribution Administrative domains

8. CMSC 621 8 Scalability Limitations Limitations to size scalability due to centralized solutions - Limitations to geographic scalability due to assumptions on communications ( synchronous vs asynchronous ) - Limitations to administrative scalability due to conflicting usage, management, and security policies ConceptExample Centralized servicesA single server for all users Centralized dataA single on-line telephone book Centralized algorithms Doing routing based on complete information

9. CMSC 621 9 Scaling techniques Use asynchronous communication Move computation from the servers to the clients Use distribution, which is the splitting of a component into smaller parts and spread across the system Use replication Need to handle replica consistency

10. CMSC 621 10 Example scaling techniques: Web forms

11. CMSC 621 11 Example scaling techniques: DNS

12. CMSC 621 12 Pitfalls in developing distributed systems Peter Deutch identifies false assumptions made when building distributed systems The network is reliable The network is secure The network is homogeneous The topology does not change Latency is zero Bandwidth is infinite Transport cost is zero There is one administrator

13. CMSC 621 13 Types of distributed systems Cluster computing systems Beowulf and MOSIX clusters Grid computing systems 4-layered architecture (fabric, connectivity, resource, and collective layers) Transaction processing monitors Pervasive systems Systems with small, battery-powered, mobile, network capable embedded devices Home systems Body-Area networks Sensor networks (in-network data processing)

14. CMSC 621 14 Hardware Concepts 1.6 Different basic organizations and memories in distributed computer systems

15. CMSC 621 15 Multiprocessors A bus-based multiprocessor. 1.7

16. CMSC 621 16 Multiprocessors a) A crossbar switch b) An omega switching network (shuffle exchange network) 1.8

17. CMSC 621 17 Homogeneous Multicomputer Systems a) Grid b) Hypercube 1-9

18. CMSC 621 18 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

19. CMSC 621 19 Uniprocessor Operating Systems Separating applications from operating system code through a microkernel. 1.11

20. CMSC 621 20 Multicomputer Operating Systems General structure of a multicomputer operating system 1.14

21. CMSC 621 21 Multicomputer Operating Systems: communication Alternatives for blocking and buffering in message passing. 1.15

22. CMSC 621 22 Multicomputer Operating Systems Relation between blocking, buffering, and reliable communications. Synchronization pointSend buffer Reliable comm. guaranteed? Block sender until buffer not fullYesNot necessary Block sender until message sentNoNot necessary Block sender until message receivedNoNecessary Block sender until message deliveredNoNecessary

23. CMSC 621 23 Distributed Shared Memory Systems 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

24. CMSC 621 24 Distributed Shared Memory Systems False sharing of a page between two independent processes. 1.18

25. CMSC 621 25 Network Operating System General structure of a network operating system. 1-19

26. CMSC 621 26 Network Operating System Two clients and a server in a network operating system. 1-20

27. CMSC 621 27 Network Operating System Different clients may mount the servers in different places. 1.21

28. CMSC 621 28 Positioning Middleware General structure of a distributed system as middleware. 1-22

29. CMSC 621 29 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

30. CMSC 621 30 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

31. CMSC 621 31 System architectures Architecture= components + connectors + configuration Common architectures Layered Object-based Data –centered Event-based Centralized architectures 2-tier Client-server and application layering Multi-tiered Decentralized architectures Vertical and horizontal distribution Peer-to-peer architectures (structured vs unstructured) Hybrid architectures Adaptive and self-managed systems

32. CMSC 621 32 Clients and Servers General interaction between a client and a server. 1.25

33. CMSC 621 33 An Example Client and Server The header.h file used by the client and server.

34. CMSC 621 34 An Example Client and Server A sample server.

35. CMSC 621 35 An Example Client and Server A client using the server to copy a file. 1-27 b

36. CMSC 621 36 Application layering example The general organization of an Internet search engine into three different layers 1-28

37. CMSC 621 37 Multitiered Architectures Alternative client-server organizations (a) – (e). 1-29

38. CMSC 621 38 Multitiered Architectures An example of a server acting as a client.in a 3-tiered architecture 1-30

39. CMSC 621 39 Modern Architectures An example of horizontal distribution of a Web service. 1-31

40. CMSC 621 40 Peer to Peer Systems Structured P2P: Distributed Hash Tables CHORD and CAN Unstructured P2P Random graphs: exchange of segments of partial views Hybrid + Superpeers