Lectures Software Engineering - Chapter 7: System models

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System models l Abstract descriptions of systems whose requirements are being analysed ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 1
Objectives l To explain why the context of a system should be modelled as part of the RE process l To describe behavioural modelling, data modelling and object modelling l To introduce some of the notations used in the Unified Modeling Language (UML) l To show how CASE workbenches support system modelling ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 2
Topics covered l Context models l Behavioural models l Data models l Object models l CASE workbenches ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 3
System modelling l System modelling helps the analyst to understand the functionality of the system and models are used to communicate with customers l Different models present the system from different perspectives • External perspective showing the system’s context or environment • Behavioural perspective showing the behaviour of the system • Structural perspective showing the system or data architecture ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 4
Structured methods l Structured methods incorporate system modelling as an inherent part of the method l Methods define a set of models, a process for deriving these models and rules and guidelines that should apply to the models l CASE tools support system modelling as part of a structured method ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 5
Method weaknesses l They do not model non-functional system requirements l They do not usually include information about whether a method is appropriate for a given problem l The may produce too much documentation l The system models are sometimes too detailed and difficult for users to understand ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 6
Model types l Data processing model showing how the data is processed at different stages l Composition model showing how entities are composed of other entities l Architectural model showing principal sub- systems l Classification model showing how entities have common characteristics l Stimulus/response model showing the system’s reaction to events ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 7
Context models l Context models are used to illustrate the boundaries of a system l Social and organisational concerns may affect the decision on where to position system boundaries l Architectural models show the a system and its relationship with other systems ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 8
Process models l Process models show the overall process and the processes that are supported by the system l Data flow models may be used to show the processes and the flow of information from one process to another ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 10
Behavioural models l Behavioural models are used to describe the overall behaviour of a system l Two types of behavioural model are shown here • Data processing models that show how data is processed as it moves through the system • State machine models that show the systems response to events l Both of these models are required for a description of the system’s behaviour ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 12
Data-processing models l Data flow diagrams are used to model the system’s data processing l These show the processing steps as data flows through a system l Intrinsic part of many analysis methods l Simple and intuitive notation that customers can understand l Show end-to-end processing of data ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 13
Data flow diagrams l DFDs model the system from a functional perspective l Tracking and documenting how the data associated with a process is helpful to develop an overall understanding of the system l Data flow diagrams may also be used in showing the data exchange between a system and other systems in its environment ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 15
State machine models l These model the behaviour of the system in response to external and internal events l They show the system’s responses to stimuli so are often used for modelling real-time systems l State machine models show system states as nodes and events as arcs between these nodes. When an event occurs, the system moves from one state to another l Statecharts are an integral part of the UML ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 17
Statecharts l Allow the decomposition of a model into sub- models (see following slide) l A brief description of the actions is included following the ‘do’ in each state l Can be complemented by tables describing the states and the stimuli ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 21
Semantic data models l Used to describe the logical structure of data processed by the system l Entity-relation-attribute model sets out the entities in the system, the relationships between these entities and the entity attributes l Widely used in database design. Can readily be implemented using relational databases l No specific notation provided in the UML but objects and associations can be used ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 23
Data dictionaries l Data dictionaries are lists of all of the names used in the system models. Descriptions of the entities, relationships and attributes are also included l Advantages • Support name management and avoid duplication • Store of organisational knowledge linking analysis, design and implementation l Many CASE workbenches support data dictionaries ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 25
Object models l Object models describe the system in terms of object classes l An object class is an abstraction over a set of objects with common attributes and the services (operations) provided by each object l Various object models may be produced • Inheritance models • Aggregation models • Interaction models ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 27
Object models l Natural ways of reflecting the real-world entities manipulated by the system l More abstract entities are more difficult to model using this approach l Object class identification is recognised as a difficult process requiring a deep understanding of the application domain l Object classes reflecting domain entities are reusable across systems ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 28
Inheritance models l Organise the domain object classes into a hierarchy l Classes at the top of the hierarchy reflect the common features of all classes l Object classes inherit their attributes and services from one or more super-classes. these may then be specialised as necessary l Class hierarchy design is a difficult process if duplication in different branches is to be avoided ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 29
The Unified Modeling Language l Devised by the developers of widely used object- oriented analysis and design methods l Has become an effective standard for object- oriented modelling l Notation • Object classes are rectangles with the name at the top, attributes in the middle section and operations in the bottom section • Relationships between object classes (known as associations) are shown as lines linking objects • Inheritance is referred to as generalisation and is shown ‘upwards’ rather than ‘downwards’ in a hierarchy ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 30
Library class hierarchy
User class hierarchy
Multiple inheritance l Rather than inheriting the attributes and services from a single parent class, a system which supports multiple inheritance allows object classes to inherit from several super-classes l Can lead to semantic conflicts where attributes/services with the same name in different super-classes have different semantics l Makes class hierarchy reorganisation more complex ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 33
Object aggregation l Aggregation model shows how classes which are collections are composed of other classes l Similar to the part-of relationship in semantic data models ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 35
Object behaviour modelling l A behavioural model shows the interactions between objects to produce some particular system behaviour that is specified as a use-case l Sequence diagrams (or collaboration diagrams) in the UML are used to model interaction between objects ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 37
CASE workbenches l A coherent set of tools that is designed to support related software process activities such as analysis, design or testing l Analysis and design workbenches support system modelling during both requirements engineering and system design l These workbenches may support a specific design method or may provide support for a creating several different types of system model ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 39
Analysis workbench components l Diagram editors l Model analysis and checking tools l Repository and associated query language l Data dictionary l Report definition and generation tools l Forms definition tools l Import/export translators l Code generation tools ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 41
Key points l A model is an abstract system view. Complementary types of model provide different system information l Context models show the position of a system in its environment with other systems and processes l Data flow models may be used to model the data processing in a system l State machine models model the system’s behaviour in response to internal or external events ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 42
Key points l Semantic data models describe the logical structure of data which is imported to or exported by the systems l Object models describe logical system entities, their classification and aggregation l Object models describe the logical system entities and their classification and aggregation l CASE workbenches support the development of system models ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 7 Slide 43