1. Lecturer: Sebastian Coope Ashton Building, Room G.18 E-mail: coopes@liverpool.ac.uk COMP 201 web-page: http://www.csc.liv.ac.uk/~coopes/comp201 Lecture 16 – Distributed System Architectures

2. Distributed Systems Architectures Architectural design for software that executes on more than one processor l2l2 l COMP201 - Software Engineering

3. Distributed Systems Virtually all large computer-based systems are now distributed systems Information processing is distributed over several computers rather than confined to a single machine Distributed software engineering is now very important l3l3 l COMP201 - Software Engineering

4. System Types Personal systems that are not distributed and that are designed to run on a personal computer or workstation. Embedded systems that run on a single processor or on an integrated group of processors. Distributed systems where the system software runs on a loosely integrated group of cooperating processors linked by a network. l4l4 l COMP201 - Software Engineering

5. Distributed System Characteristics Resource sharing Openness Concurrency Scalability Fault tolerance Transparency Distributed system disadvantages : l Complexity l Security l Manageability l Unpredictability l5l5 l COMP201 - Software Engineering

6. Distributed Systems Architectures Client-server architectures Distributed services which are called on by clients. Servers that provide services are treated differently from clients that use services Distributed object architectures No distinction between clients and servers. Any object on the system may provide and use services from other objects l6l6 l COMP201 - Software Engineering

7. Middleware Software that manages and supports the different components of a distributed system. In essence, it sits in the middle of the system Middleware is usually off-the-shelf rather than specially written software Examples Transaction processing monitors Data converters Communication controllers l7l7 l COMP201 - Software Engineering

8. 1. Multiprocessor Architectures Simplest distributed system model System composed of multiple processes which may (but need not) execute on different processors Architectural model of many large real-time systems Distribution of process to processor may be pre-ordered or may be under the control of a dispatcher l8l8 l COMP201 - Software Engineering

9. A Multiprocessor Traffic Control System l9l9 l COMP201 - Software Engineering

10. 2. Client-Server Architectures The application is modelled as a set of services that are provided by servers and a set of clients that use these services Clients know of servers but servers need not know of clients Clients and servers are logical processes The mapping of processors to processes is not necessarily 1 : 1 l 10 l COMP201 - Software Engineering

11. A Client-Server System l 11 l COMP201 - Software Engineering

12. Computers in a C/S Network l 12 l COMP201 - Software Engineering

13. Layered Application Architecture Presentation layer Concerned with presenting the results of a computation to system users and with collecting user inputs Application processing layer Concerned with providing application specific functionality e.g., in a banking system, banking functions such as open account, close account, etc. Data management layer Concerned with managing the system databases l 13 l COMP201 - Software Engineering

14. Application Layers l 14 l COMP201 - Software Engineering

15. Thin and Fat Clients Thin-client model In a thin-client model, all of the application processing and data management is carried out on the server. The client is simply responsible for running the presentation software. Fat-client model In this model, the server is only responsible for data management. The software on the client implements the application logic and the interactions with the system user. l 15 l COMP201 - Software Engineering

16. Thin and Fat Clients l 16 l COMP201 - Software Engineering

17. Thin Client Model Used when legacy systems are migrated to client server architectures. The legacy system acts as a server in its own right with a graphical interface implemented on a client A major disadvantage is that it places a heavy processing load on both the server and the network l 17 l COMP201 - Software Engineering

18. Fat Client Model More processing is delegated to the client as the application processing is locally executed Most suitable for new client-server systems where the capabilities of the client system are known in advance More complex than a thin client model especially for management. New versions of the application have to be installed on all clients l 18 l COMP201 - Software Engineering

19. A Client-Server ATM System l 19 l COMP201 - Software Engineering

20. Three-Tier Architectures In a three-tier architecture, each of the application architecture layers may execute on a separate processor Allows for better performance than a thin-client approach and is simpler to manage than a fat-client approach A more scalable architecture - as demands increase, extra servers can be added to the data management or application processing layers. l 20 l COMP201 - Software Engineering

21. A 3-Tier Client-Server Architecture l 21 l COMP201 - Software Engineering

22. An Internet Banking System l 22 l COMP201 - Software Engineering

23. Use of Client-Server Architectures l COMP201 - Software Engineering l 23 ArchitectureApplications Two-tier C/S with thin clients Legacy system applications where separating application processing and data management is impractical. Computationally-intensive applications such as compilers with little or no data management. Data-intensive applications (browsing/querying) with little or no application processing. Two-tier C/S with fat clients Applications where processing uses off-the-shelf software (eg. Microsoft Excel) on the client. Applications with relatively stable end-user functionality used in an environment with well-established system management. Three-tier or multi-tier C/S architecture Large-scale applications with hundreds or thousands of clients. Applications where both the data and applications are volatile.

24. 3. Distributed Object Architectures There is no distinction in a distributed object architectures between clients and servers Each distributable entity is an object that provides services to other objects and receives services from other objects Object communication is through a middleware system called an object request broker (software bus) However, they can be more complex to design than client- server systems l 24 l COMP201 - Software Engineering

25. Distributed Object Architecture l 25 l COMP201 - Software Engineering

26. Advantages of Distributed Object Architecture It allows the system designer to delay decisions on where and how services should be provided Service-providing objects can execute on any node of the network and thus the distinction between thin/fat-client models becomes irrelevant. It is a very open system architecture that allows new resources to be added to it as required Object communication standards have been developed allowing objects written in different languages to communicate with each other. l 26 l COMP201 - Software Engineering

27. Advantages of Distributed Object Architecture The system is flexible and scalable New objects can be added as the load on the system increases without disrupting the other system objects. Replicated object can be created to cope with load. It is possible to reconfigure the system dynamically with objects migrating across the network as required This may be important when there is fluctuating patterns of demand on services. A service-providing object can migrate to the same processor as service-requesting objects, thus improving performance. l 27 l COMP201 - Software Engineering

28. Lecture Key Points Client-server systems are distributed systems where the system is modelled as a set of services provided by servers to client processes. In a client-server system, the user interface always runs on a client and data management is always provided by a shared server. Application functionality may be implemented on the client computer or the server. l COMP201 - Software Engineering l 28

29. Lecture Key Points In a distributed object architecture, there is no distinction between clients and servers; objects provide general services that may be called on by other objects. Distributed object systems require middleware to handle object communications and to allow objects to be added or removed from the system. l COMP201 - Software Engineering l 29