1. Introducing … Distributed Systems Paul Barry Muhammed Cinsdikici

2. 1. Definition of a Distributed System (1) “A distributed system is a collection of independent computers that appear to the users of the system as a single computer.” [Tanenbaum] “A system in which hardware or software components located at networked computers communicate and coordinate their actions only by message passing.” [Coulouris] “A system that consists of a collection of two or more independent computers which coordinate their processing through the exchange of synchronous or asynchronous message passing.” “A distributed system is a collection of autonomous computers linked by a network with software designed to produce an integrated computing facility.”

3. 1.2 Goals of Distributed Systems Easily Connect Users/Resources Exhibit Transparency Support Openness Be Scalable –in size –geographically –administratively Looking at these goals helps use answer the question: “Is building a distributed system worth the effort?”

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

5. 1.2.1 Connect Users & Resources -Easy access of users to remote resources -Geographically distributed users can be build groupware -Exchange of various types of data (voice, data, video..) -Share resources with other users in an appropriate way -Limited resources can be assigned to multiple users -Cost optimization is done -BUT ! Security is important aspect that should be observed and implemented carefuly.

6. 1.2.2 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. Degree of Transparency Hide blindly all distribution aspects from users is not always a good idea. Ex1_Content: Morning paper. When you request your countrie’s morning newspaper at 7:00 am from other side of the world, time should not be transparent. If time is accepted as transparent, then requested paper should’nt be the expected one. Ex2_Performance: Replication of Failed Server When a data is searched, a failed server is tried several times. In order to getting rid of dealing with failed server, user can cancel the searching that server.

8. 1.2.3 Openness -Offers services according to standard rules that describe the syntax and semantics of those services. -Interoperability -Portability -Flexibility of configuration by different developers -IDL(Interface Definition Language) is used to describe standard interfaces.

9. Seperating Policy Mechanism (Flexibility) -System is organized as a collection of relatively small and easily replaceable or adaptable components -In Monolithic approach, system components are only logically seperated but implemented as huge program. -In Monolithic approach, changing one of the logical components effects the whole. -Ex: Web Caching. Browsers should provide basic facilities of storing documents and at the same time users can decide which docs are stored and how long etc…

10. 1.2.4 Scalability Distributed Systems Scalability can be summurized as; - In Size: Add more users and resources - Geographically: Users and resources lie far apart - Administratively: Many Independent administrative organizations.

11. Scalability Problems (1) Decentralization has the following specs; 1.No machine has complete info about system state 2.Machines make decisions based on local info 3.Failure of one machine does not ruin the algorithm 4.There is no implicit assumption that a global clock exists ConceptExample Centralized services A single server for all users (single Windows Logon server) Centralized data A single on-line telephone book (single DNS server data) Centralized algorithms Doing routing based on complete information (Making all routing in a single routing process)

12. Scalability Problems (2) BUT!!! Decentralization has also consider the following limitations (out of the LAN); 1.Synchronization should be done in great deal for geographicaly far aparts. 2.Communication is should be guaranteed on unreliable connections. 3.Scaling distributed systems across multiple independent administrative domains. 4.System security issues should be provided carefully.

13. Scaling Techniques (Asynchronous Communication) 1.4 The difference between letting (asynchronous comm): a)a server or b)a client check forms as they are being filled

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

15. Scaling Techniques (Caching & Replication) Considering that scalability problems often appear in the form of performance degradation. So, - Replicate components (it increases availability and also balance the load) - Caching is a special form of replication (in contrast to replication, caching is a decision made by client of a resource and not by the owner of a resource) BUT !!! CONSISTENCY OF THE DATA SHOULD BE PROVIDED

16. Modeling Distributed Systems When building distributed applications, system builders have often looked to the non-distributed systems world for models to follow (… inspiration?) Consequently, distributed systems tend to exhibit certain characteristics that are already familiar to us This applies equally to hardware concepts as it does to software concepts

17. 1.3 Modeling Hardware Concepts 1.6 Different basic organizations and memories in distributed systems

18. 1.3.1 Multiprocessors (1) A bus-based multiprocessor. 1.7

19. Multiprocessors (2) A crossbar switch An omega switching network 1.8

20. 1.3.2 Homogeneous Multicomputer Systems Grid Hypercube 1-9

21. 1.4 Modeling 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

22. 1.4.1 Distributed Operating Systems (DOS) a) Uniprocessor Operating Systems Separating applications from operating system code through a “microkernel” – can provide a good base upon which to build a distributed operating system (DOS). 1.11

23. Distributed Operating Systems (DOS) b) Multiprocessor Operating Systems (1) 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;} }

24. Distributed Operating Systems (DOS) 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; }

25. Distributed Operating Systems (DOS) Multicomputer Operating Systems (1) General structure of a (DOS) multicomputer operating system – all the systems are of the same type: homogeneous 1.14

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

27. Distributed Operating Systems (DOS) Multicomputer Operating Systems (3) 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

28. Distributed Operating Systems (DOS) Distributed Shared Memory Systems (1) Pages of address space distributed among four machines Situation after CPU 1 references page 10 Situation if page 10 is read only and replication is used

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

30. 1.4.2 Network Operating System (1) General structure of a network operating system – all the systems are of different types: heterogeneous 1-19

31. Network Operating System (2) Two clients and a server in a network operating system – relatively primitive set of services provided. 1-20

32. Network Operating System (3) Different clients may mount the servers in different places – difficult to maintain a consistent “view” of the system. 1.21

33. The Best of Both Worlds? DOS: too inflexible (all systems of the same type) NOS: too primitive (lowest common demoninator – too much diversity) “Middleware” – best possible compromise? Middleware = NOS + additional software layer

34. 1.4.3 Positioning Middleware General structure of a distributed system as middleware. 1-22

35. 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. This is a much higher level of abstraction than (for instance) the NOS Socket API. 1.23

36. Middleware Models/Paradigms Distributed File Systems The Remote Procedure Call (RPC) Distributed Objects Distributed Documents [All of which we return to in detail later in this course … ]

37. Comparing DOS/NOS/Middleware 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

38. The Classic DS Model How are “processes” organised within a Distributed System? General agreement/concensus: “Client/Server” Model Multi-tiering: User Interface Level, Processing Level, Data Level.

39. 1.5 Clients and Servers General interaction between a client and a server. 1.25

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

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

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

43. 1.5.2 Application Layering In the general organization of an Internet search engine into three different layers – often referred to as “tiers”. 1-28

44. 1.5.3 Client-Server Multitiered Architectures (1) Alternative client-server organizations (a) – (e). 1-29

45. Client-Server Multitiered Architectures (2) An example of a server acting as a client – this is a very common vertical distribution model for distributed systems. 1-30

46. Client-Server Example Modern Architecture An example of horizontal distribution of a Web service (often also referred to as “clustering”). 1-31

47. Examples of Distributed Systems

48. Summary (Introduction) Distributed Systems … autonomous computers working together to give the appearance of a single, coherent system. They are transparent, scalable and open. Unfortunately, they also tend to be complex. Types of DS: DOS, NOS, Middleware. Processes within DSs conform to the “client/server model”. Architectures included vertical and horizontal arrangements, often into many levels/tiers.