1. 1 Distributed Systems Alexis Delis ad@di.uoa.gr Monday 6:00-9:00 Spring 2002 www.di.uoa.gr/~ad/MDE519.html

2. 2 Grades - Evaluation Final Examination: 35% In-class Presentations & Homeworks: 30% Project(s): 35% Book: A.S. Tanenbaum, M. van Steen, Distributed Systems, Prentice Hall, 2002. Other: Papers (available from the course’s WWW site)

3. 3 Definition of a Distributed System A distributed system is: A collection of independent computers that appears to its users as a single coherent system.

4. 4 Definition of a Distributed System A distributed system is organized as middleware. Note that the middleware layer may extend over multiple machines. Examples: Workflow Systems, WWW-services (URL) 1.1

5. 5 Goals of Distributed Systems connecting users with resources transparency openess scalability

6. 6 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

7. 7 Openness Open DS: a system that offer services according to Standard Rules. Such rules are described in terms of protocols IDLs help in the description of such protocols.  Interoperability?  Portability?  Difference between the two?

8. 8 Scalability Numerous “definitions” of scalability  system can be scalable in terms of size  Add more users/resources into the system  Geographically Scalable system  Users/resources may lie far apart  Administratively Scalable  Easy to manage even if it spans multiple orgs. GOLDEN RULE : as system becomes scalable in one of these three dimension exhibits loss in performance.

9. 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. 10 Scaling Techniques  asynchronous communication  filling (database) forms.  distribution (take a component, break into smaller pieces, spread these components across the system Example: DNS – Domain Name Service.  Division in (non-overlapping) zones – domains

11. 11 Scaling Techniques 1.4 The difference between letting: a)a server (character/string-based communication) or b)a client check forms as they are being filled (bulk uploading).

12. 12 Scaling Techniques 1.5 An example of dividing the DNS name space into zones.

13. 13 Scalability - Caching Generally Good Idea: Replicate (data/services) Replication increases availability (of systems) Caching (difference from replication?) Main problem with caching/replication?  Is this a serious problem?  CDNs (how do they do it?)

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

15. 15 Multiprocessors A bus-based multiprocessor. 1.7

16. 16 Multiprocessors a)A crossbar switch (n*n crosspoint switches needed) b)An omega switching network (low latency problem) c)How to avoid latency? use hierarchical schemes: NUMA (NonUniform Memory Access) 1.8

17. 17 Homogeneous Multicomputer Systems ( building is “easy”-problem:interconnection network ) a)Bus-based ( FDDI/ Fast-Ethernet /Gigabit network ) b)Grid ( switched based; messages are routed via the interconnection network ) c)Hypercube( switched based; four dimensional (b); two ordinary cubes with 8 vertices and 12 edges ) 1-9 Homogeneous Multi-computers: System Area Networks (SANs)

18. 18 Switched Multi-computers Can vary significantly… From… Massively Parallel Processors ($$ MPP – CM5, IBM SP2 etc.) To.. Clusters/Networks of Workstations (cheap$ - COWs or NOWs)

19. 19 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

20. 20 Uniprocessor Operating Systems Separating applications from operating system code through a microkernel. 1.11

21. 21 Multiprocessor Operating Systems A monitor to protect an integer against concurrent access. monitor Counter { private: int count = 0; public: int value() { return count;} void incr () { count = count + 1;} void decr() { count = count – 1;} }

22. 22 Multiprocessor Operating Systems (2) A monitor to protect an integer against concurrent access, but blocking a process. monitor Counter { private: int count = 0; int blocked_procs = 0; condition unblocked; public: int value () { return count;} void incr () { if (blocked_procs == 0) count = count + 1; else signal (unblocked); } void decr() { if (count ==0) { blocked_procs = blocked_procs + 1; wait (unblocked); blocked_procs = blocked_procs – 1; } else count = count – 1; }

23. 23 Distributed Shared Memory Systems (DSM) 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 Trick: replicate pages that are referenced frequently! Another: replicate all pages (??) – problems? Solutions?

24. 24 Distributed Shared Memory Systems Always an issue: size of a page in DSM – why? Large sized page are potentially good but they may entail false sharing Having data belonging to two independent processes in the same page…

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

26. 26 Network Operating System Two clients and a server in a network operating system. Services allowed to users… rlogin machineA rcp machineA:file1 machineB:file2 ftp, http, sftp, ssh, etc. 1-20

27. 27 Network Operating System Different clients may mount the servers in different places. 1.21 Issue of FS mount-ing

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

29. 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. 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. 31 Clients and Servers General interaction between a client and a server. 1.25

32. 32 An Example Client and Server The header.h file used by the client and server.

33. 33 An Example: The Server A sample server.

34. 34 An Example: The Client A client using the server to copy a file. 1-27 b

35. 35 Three Processing Levels The general organization of an Internet Search Engine into three different layers 1-28

36. 36 Multitiered Architectures Alternative client-server organizations (a) – (e). 1-29

37. 37 Multitiered Architectures An example of a server acting as a client. 1-30

38. 38 Modern Architectures An example of horizontal distribution of a Web service. 1-31

39. 39 Ongoing (Research) Work DSs based on Horizontal Distribution(of data/services) DSs based on Horizontal and/or Vertical Distribution (of data/services) Peer-to-Peer –Distributions of data –Services (indexing, querying, TP processing) – Self-organizing systems