1. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 1 Architectural Design

2. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 2 Objectives l To introduce architectural design and to discuss its importance l To explain the architectural design decisions that have to be made l To introduce three complementary architectural styles covering organisation, decomposition and control l To discuss reference architectures are used to communicate and compare architectures

3. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 3 Topics covered l Introduction l Architectural design decisions l System organisation l Decomposition styles l Control styles l Reference architectures

4. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 4 The software design process

5. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 5 Software architecture l The design process for identifying the sub- systems making up a system and the framework for sub-system control and communication is architectural design. l The output of this design process is a description of the software architecture.

6. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 6 Architectural design l An early stage of the system design process. l Represents the link between specification and design processes. l Often carried out in parallel with some specification activities. l It involves identifying major system components and their communications.

7. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 7 Subsystem decomposition l Concerned with decomposing the system into interacting sub-systems. l The architectural design is normally expressed as a block diagram presenting an overview of the system structure. l More specific models showing how sub- systems share data, are distributed and interface with each other may also be developed.

8. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 8 Packing robot control system

9. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 9 Box and line diagrams l Very abstract - they do not show the nature of component relationships nor the externally visible properties of the sub-systems. l However, useful for communication with stakeholders and for project planning.

10. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 10 Advantages of explicit architecture l Stakeholder communication Architecture may be used as a focus of discussion by system stakeholders. l Large-scale reuse The architecture may be reusable across a range of systems. l System analysis Means that analysis of whether the system can meet its non-functional requirements is possible.

11. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 11 Architecture and system characteristics l Performance Localise critical operations and minimise communications. Use large rather than fine-grain components. l Security Use a layered architecture with critical assets in the inner layers. l Safety Localise safety-critical features in a small number of sub- systems. l Availability Include redundant components and mechanisms for fault tolerance. l Maintainability Use fine-grain, replaceable components.

12. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 12 Architectural conflicts l Using large-grain components improves performance but reduces maintainability. l Introducing redundant data improves availability but makes security more difficult. l Localising safety-related features usually means more communication so degraded performance.

13. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 13 Topics covered l Introduction l Architectural design decisions l System organisation l Decomposition styles l Control styles l Reference architectures

14. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 14 Architectural design decisions l Architectural design is a creative process so the process differs depending on the type of system being developed. l However, a number of common decisions span all design processes.

15. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 15 Architectural design decisions l Is there a generic application architecture that can be reused? How will the subsystems be distributed? Distributed architectures (ch. 12) Application architectures (ch. 13) Product line architectures (ch. 18) Reference architectures (11.5) l What architectural styles are appropriate? What approach will be used to structure the system? Repository, client-server, layered (11.2) l How will the system be decomposed into modules? Object-oriented decomposition (11.3, ch. 14) Functional/dataflow decomposition (11.3)

16. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 16 Architectural design decisions (con’t) l What control strategy should be used? Centralized, event-driven (11.4) l How will the architectural design be evaluated? l How should the architecture be documented?

17. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 17 Chapter outline l Architecture reuse Application architectures (ch. 13) Reference architectures (11.5) Product line architectures (ch. 18) l Architectural styles Repository, client-server, layered (11.2) l Modular decomposition styles Object-oriented decomposition, pipes and filters (11.3) l Control strategies Centralised, event-driven (11.4) Distributed systems (ch. 12)

18. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 18 Architecture reuse l Systems in the same domain often have similar architectures that reflect domain concepts. l Application product lines are built around a core architecture with variants that satisfy particular customer requirements. l Application architectures are covered in Chapter 13 and product lines in Chapter 18.

19. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 19 Architecture evaluation: “Good” architectures l Well-defined modules. Allocate functionalities based on information hiding and separation of concerns. Parallel-processing systems should have well-defined processes that may not necessarily mirror the static structure. l Separation of concerns. Allow relatively independent module development. Separate modules that produce data from modules that consume data. l Information-hiding. Encapsulate idiosyncracies of platform. Do not depend on a particular version of a product or tool. Make it easy to reallocate processes to different processors. Bass, L., Clements, P., Kazman, R. Software Architecture in Practice

20. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 20 Best practices l The architecture should be the product of a single architect or team. l The architect should have the technical requirements of the system and prioritized list of quality attributes. The resulting architecture should be analyzed against these attributes. The resulting architecture should clearly set resource budgets. l The architecture should be well-documented and circulated to stakeholders. l The architecture should have an infrastructure that can be implemented first. Bass, L., Clements, P., Kazman, R. Software Architecture in Practice

21. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 21 Architecture evaluation techniques l Architecture reviews Useful for evaluating functional completeness, dependability, maintainability. Scenario-based Work through several scenarios to understand system behavior. Questionnaire-based Architects answer questions about how the architecture was derived, level of coupling between subsystems, etc. Checklist-based Derived from past experiences and proven architecture design techniques for a particular domain. l Simulation and prototypes Good for evaluating performance and usability Bass, L., Clements, P., Kazman, R. Software Architecture in Practice

22. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 22 Documenting architectures: Architectural models l Used to document an architectural design. l Static structural model that shows the major system components. l Dynamic process model that shows the process structure of the system. l Interface model that defines sub-system interfaces. l Relationships model such as a data-flow model that shows sub-system relationships. l Distribution model that shows how sub-systems are distributed across computers.

23. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 23 Example: structural model (Linux kernel)

24. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 24 Example: process model (A temperature control system) Thermostat process Sensor process Furnace control process Heater control process 500 Hz 5 Thermostat process 500 Hz Sensor values Switch command Room number

25. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 25 Example: interface model (A data collector component) Provides interfaceRequires interface Data collector addSensor removeSensor startSensor stopSensor testSensor listAll report initialise sensorManagement sensorData

26. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 26 Example: relationship model (Salary payment DFD)

27. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 27 Example: distribution model (An internet banking system)

28. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 28 Topics covered l Introduction l Architectural design decisions l System organisation l Decomposition styles l Control styles l Reference architectures

29. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 29 Architectural styles l The architectural model of a system may conform to a generic architectural model or style. l An awareness of these styles can simplify the problem of defining system architectures. l However, most large systems are heterogeneous and do not follow a single architectural style.

30. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 30 System organisation l Reflects the basic strategy that is used to organize a system. l Three architectural styles are widely used: A shared data repository style; A shared services and servers style; An abstract machine or layered style.

31. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 31 The repository model l Sub-systems must exchange data. This may be done in two ways: Shared data is held in a central database or repository and may be accessed by all sub- systems; Each sub-system maintains its own database and passes data explicitly to other sub-systems. l When large amounts of data are to be shared, the repository model of sharing is most commonly used.

32. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 32 CASE toolset architecture

33. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 33 Repository model characteristics l Advantages Efficient way to share large amounts of data; Sub-systems need not be concerned with how data is produced; Centralised management e.g. backup, security, etc. Sharing model is published as the repository schema. l Disadvantages Sub-systems must agree on a repository data model. Inevitably a compromise; Data evolution is difficult and expensive; No scope for specific management policies; Difficult to distribute efficiently.

34. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 34 Client-server model l Distributed system model which shows how data and processing is distributed across a range of components. l Set of stand-alone servers which provide specific services such as printing, data management, etc. l Set of clients which call on these services. l Network which allows clients to access servers.

35. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 35 Film and picture library Catalogue server Library catalogue Video server Film clip files Picture server Digitised photographs Web server Film and photo info. Client 1Client 2Client 3Client 4 Internet

36. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 36 Client-server characteristics l Advantages Distribution of data is straightforward; Makes effective use of networked systems. May require cheaper hardware; Easy to add new servers or upgrade existing servers. l Disadvantages No shared data model so sub-systems use different data organisation. Data interchange may be inefficient; Redundant management in each server; No central register of names and services - it may be hard to find out what servers and services are available.

37. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 37 Abstract machine (layered) model l Used to model the interfacing of sub-systems. l Organises the system into a set of layers (or abstract machines) each of which provide a set of services. l Supports the incremental development of sub- systems in different layers. When a layer interface changes, only the adjacent layer is affected. l However, often difficult to structure systems in this way. New services may require changes that cut across multiple layers. l Performance can be a problem as a request may have to go through several layers before being processed.

38. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 38 Version management system Configuration management system layer Database system layer Operating system layer Object management system layer

39. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 39 General application layers

40. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 40 Information system model User interface Information retrieval and modification Database Transaction management User communications

41. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 41 Topics covered l Introduction l Architectural design decisions l System organisation l Decomposition styles l Control styles l Reference architectures

42. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 42 Modular decomposition styles l Styles of decomposing sub-systems into modules. l No rigid distinction between system organisation and modular decomposition.

43. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 43 Sub-systems and modules l A sub-system is a system in its own right whose operation is independent of the services provided by other sub-systems. l A module is a system component that provides services to other components but would not normally be considered as a separate system.

44. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 44 Modular decomposition l Another structural level where sub-systems are decomposed into modules. l Two modular decomposition models covered An object model where the system is decomposed into interacting object; A pipeline or data-flow model where the system is decomposed into functional modules which transform inputs to outputs. l If possible, decisions about concurrency should be delayed until modules are implemented.

45. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 45 Object models l Structure the system into a set of loosely coupled objects with well-defined interfaces. l Object-oriented decomposition is concerned with identifying object classes, their attributes and operations. l When implemented, objects are created from these classes and some control model used to coordinate object operations.

46. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 46 Invoice processing system

47. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 47 Object model advantages and disadvantages l Objects are loosely coupled so their implementation can be modified without affecting other objects. l The objects may reflect real-world entities. l OO implementation languages are widely used. l However, object interface changes may cause problems and complex entities may be hard to represent as objects.

48. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 48 Function-oriented pipelining l Functional transformations process their inputs to produce outputs. l May be referred to as a pipe and filter model (as in UNIX shell). l Variants of this approach are very common. When transformations are sequential, this is a batch sequential model which is extensively used in data processing systems. l Not really suitable for interactive systems.

49. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 49 Invoice processing system

50. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 50 Pipeline model advantages and disadvantages l Supports transformation reuse. l Intuitive organisation for stakeholder communication. l Easy to add new transformations. l Relatively simple to implement as either a concurrent or sequential system. l However, requires a common format for data transfer along the pipeline and difficult to support event-based interaction.

51. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 51 Topics covered l Introduction l Architectural design decisions l System organisation l Decomposition styles l Control styles l Reference architectures

52. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 52 Control styles l Are concerned with the control flow between sub-systems. Distinct from the system decomposition model. l Centralised control One sub-system has overall responsibility for control and starts and stops other sub-systems. l Event-based control Each sub-system can respond to externally generated events from other sub-systems or the system’s environment.

53. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 53 Centralised control l A control sub-system takes responsibility for managing the execution of other sub-systems. l Call-return model Top-down subroutine model where control starts at the top of a subroutine hierarchy and moves downwards. Applicable to sequential systems. l Manager model Applicable to concurrent systems. One system component controls the stopping, starting and coordination of other system processes. Can be implemented in sequential systems as a case statement.

54. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 54 Call-return model

55. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 55 Real-time system control System controller User interface Fault handler Computation processes Actuator processes Sensor processes

56. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 56 Event-driven systems l Driven by externally generated events. l Two principal event-driven models Broadcast models. An event is broadcast to all sub- systems. Any sub-system which can handle the event may do so; Interrupt-driven models. Used in real-time systems where interrupts are detected by an interrupt handler and passed to some other component for processing. l Other event driven models include spreadsheets and production-control systems.

57. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 57 Broadcast model l Effective in integrating sub-systems on different computers in a network. l Sub-systems register an interest in specific events. When these occur, control is transferred to the sub- system which can handle the event. l Control policy is not embedded in the event and message handler. Sub-systems decide on events of interest to them. l However, sub-systems don’t know if or when an event will be handled.

58. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 58 Selective broadcasting

59. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 59 Interrupt-driven systems l Used in real-time systems where fast response to an event is essential. l There are known interrupt types with a handler defined for each type. l Each type is associated with a memory location and a hardware switch causes transfer to its handler. l Allows fast response but complex to program and difficult to validate.

60. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 60 Interrupt-driven control

61. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 61 Topics covered l Introduction l Architectural design decisions l System organisation l Decomposition styles l Control styles l Reference architectures

62. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 62 Reference architectures l Architectural models may be specific to some application domain. l Two types of domain-specific model Generic models which are abstractions from a number of real systems and which encapsulate the principal characteristics of these systems. Covered in Chapter 13. Reference models which are more abstract, idealised model. Provide a means of information about that class of system and of comparing different architectures. l Generic models are usually bottom-up models; Reference models are top-down models.

63. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 63 Reference architectures l Reference models are derived from a study of the application domain rather than from existing systems. l May be used as a basis for system implementation or to compare different systems. It acts as a standard against which systems can be evaluated. l OSI model is a layered model for communication systems.

64. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 64 OSI reference model Presentation Session Transport Network Data link Physical 7 6 5 4 3 2 1 Communications medium Network Data link Physical Application Presentation Session Transport Network Data link Physical Application

65. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 65 CASE reference model l Data repository services Storage and management of data items. l Data integration services Managing groups of entities. l Task management services Definition and enaction of process models. l Messaging services Tool-tool and tool-environment communication. l User interface services User interface development.

66. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 66 The ECMA reference model

67. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 67 Key points l The software architecture is the fundamental framework for structuring the system. l Architectural design decisions include decisions on the application architecture, the distribution and the architectural styles to be used. l Different architectural models such as a structural model, a control model and a decomposition model may be developed. l System organisational models include repository models, client-server models and abstract machine models.

68. Modified from Sommerville’s originalsSoftware Engineering, 7th edition. Chapter 11 Slide 68 Key points l Modular decomposition models include object models and pipelining models. l Control models include centralised control and event-driven models. l Reference architectures may be used to communicate domain-specific architectures and to assess and compare architectural designs.