Presentation on theme: "SEG3101 (Fall 2010) Structural Modeling"— Presentation transcript:
1 SEG3101 (Fall 2010)Structural ModelingBased on Powerpoint slides by Gunter Mussbacher, Gregor v. Bochmann, with material from:K.E. Wiegers, D. Leffingwell & D. Widrig, M. Jackson, I.K. Bray, B. Selic, Volere, Telelogic, D. Damian, S. Somé 2008, and D. Amyot
2 Outline Entity-Relationship modeling (concepts and notations) Object-oriented modeling (concepts and notations)Methodology for Object-Oriented Analysis (OOA)A case study: A library system
3 Entity-relationship modeling (ERM) Entity-Relationship modeling (originally proposed by Peter Chen in 1976)Concepts:Entity: represents a type of entity instances, defines the properties that hold for all such instances.Relationship: represents relationship instances that hold between certain pairs of entity instances.The related entity types are also called roles.Multiplicity information indicate how many instances of the “other” side may be related to a given instance of “this” side.Attribute: An entity or a relationship may have one or several attributes. Each attribute is identified by a name and its type, where such a type is usually some simple data type such as integer or character string. Note: An entity type is normally not used as the type of an attribute, because such a situation is rather represented by a relationship between the given entity and the attribute type.
8 Object Oriented Analysis Background Model the requirements in terms of objects and the services they provide Grew out of object oriented design But applied to modeling the application domain rather than the program Motivation OO is (claimed to be) more ‘natural’ As a system evolves, the functions (processes) it performs tend to change, but the objects tend to remain unchanged Hence a model based on functions/processes will get out of date, but an object oriented model will not… …hence the claim that object-oriented designs are more maintainable OO emphasizes importance of well-defined interfaces between objects compared to ambiguities of dataflow relationshipsNOTE: OO applies to requirements engineering because it is a modeling tool. But we are modeling domain objects, not the design of the new system10
9 Nearly anything can be an object… External Entities Organizational Units …that interact with the system being modeled that are relevant to the applicationE.g. division, group, team, etc.E.g. people, devices, other systems Places Things …that establish the context of the problem being modeled …that are part of the domain being modeledE.g. manufacturing floor, loading dock, etc.E.g. reports, displays, signals, etc. Occurrences or Events Structures …that occur in the context of the system that define a class or assembly of objectsE.g. transfer of resources, a control action, etc.E.g. sensors, four-wheeled vehicles, computers, etc. RolesSome things cannot be objects: played by people who interact with the system procedures (e.g. print, invert, etc) attributes (e.g. blue, 50Mb, etc)11
11 Unified Modeling Language (UML) de facto OO method Booch, Rumbaugh & Jacobson are principal authors Still in development Attempt to standardize the proliferation of OO variants Is purely a notation No modeling method associated with it (RUP) Is primarily owned by Rational Corp./IBM (who sell lots of UML tools and services) Has a standardized meta-model (designed by committee; standard is managed by OMG) Use case diagrams Class diagrams Message sequence charts Activity diagrams State Diagrams (uses Harel’s statecharts) Module Diagrams Platform diagrams …14
12 Evaluation of OOA (serve as Summary) Advantages of OO analysis for RE Fits well with the use of OO for design and implementation Transition from OOA to OOD ‘smoother’ (but is it?) Removes emphasis on functions as a way of structuring the analysis Avoids the fragmentary nature of structured analysis object-orientation is a coherent way of understanding the world Disadvantages Emphasis on objects brings an emphasis on static modeling although later variants have introduced dynamic models Not clear that the modeling primitives are appropriate are objects, services and relationships really the things we need to model in RE? Strong temptation to do design rather than problem analysis Fragmentation of the analysis e.g. reliance on use-cases means there is no “big picture” of the user’s needs Too much marketing hype! and false claims - e.g. no evidence that objects are a more natural way to think15
13 Object-oriented modeling (OOM) – (1) Object-oriented modeling is essentially ERM with certain additional concepts. An Entity is called a ClassAdditional Concepts:Inheritance: This is the idea that some entity B inherits all properties that are defined for another entity A. This means that B is a specialization of A. One also says that B is a refinement of A or B extends A.Important note: Inheritance is not a relationship as defined above, since it does not define relationship instances between the instances of the two entities.Internal state of entity instances and methods for interacting with it: An important issue of object-orientation is information hiding. In particular, certain internal attributes are not directly accessible from outside the entity instance. An entity is characterized by its interface which includes the list of accessible attributes and the list of methods that can be called for manipulating the internal state of the instance.Note: The methods have behavior – the rest is structure.
14 Object-oriented modeling (OOM) – (2) Refinement of ERM Concepts:Composite structure: A subtype of Relationship (with special notation) is used to show that one entity is part of another (composed) entity.Calling relationship: Another subtype of Relationship has the meaning that an entity instance playing one role can access attributes and call methods on an instance playing the other role to which it is related.Disadvantages of OOMDifficulties of modeling two-way communication : an interface defines only one direction of communication. For event-based systems, one often needs two-way communication relationships. Note: one can use two one-way relationships.The encapsulation of the behavior inside the objects (the information hiding approach) is often not suitable for modeling the problem domain during requirements engineering (see examples below).
15 Methodology for Object-Oriented Analysis (OOA) Five main stepsIdentify core classes within problem domainModel relationships between classesClass diagramDefine the attributes associated with each classDetermine relevant operations for each classDefine the messages that may be passed between objectsInteraction diagram, state machine diagram
16 OOA Methodology – Library Example (1) A library system is intended to provide its users with the ability to automate the process of:Acquiring library itemsCataloguing library itemsBrowsing library itemsLoaning library itemsLibrary items comprise published and recorded materialThe system will be administered by a member of the library staffUsers must register with the system administrator before they can borrow library itemsSource: Sommerville & Kotonya
17 OOA Methodology – Library Example (2) Library users are drawn from three primary groupsStudents, Members of staff, and External usersAll library users have as part of their registrationName, Library number, Address, AccountIn addition the following information is also required for registrationStudents – Degree programme and admission numberStaff – Staff numberExternal users – Employer detailsSource: Sommerville & Kotonya
19 OOA Methodology – Library Example – Step 2 Identify relationships between classes from the partial requirements(i) A library user borrows a library item(ii) A library item is recorded or published(iii) The system administrator registers the library user(iv) Library users are students, staff, and external users(v) The system administrator catalogues the library items(vi) The library assistant issues the library items
20 OOA Methodology – Library Example – Step 2 (2) Show attributes and relationships in basic model
21 OOA Methodology – Library Example – Step 2 (3) Identify inheritance relationships for library user
22 OOA Methodology – Library Example – Step 2 (4) Identify inheritance relationships for library item
23 OOA Methodology – Library Example – Step 3 Identify attributes and populate model with themAttributes can be revealed by the analysis of the system requirementsFor example, it is a requirement that all library users must be registered before they can use the libraryThis means that we need to keep registration data about library usersLibrary users are also provided with an account to keep track of the items loaned to themLibrary items may have the attributes title, description, and classmarkLibrary users may have the attributes name, address, and library id
24 OOA Methodology – Library Example – Step 4 Identify object operationsThis step is intended to describe operations to be performed on the objectsCertain operations are implicit from the object structureCRUD operations (create – read – update – delete)Operations for accessing and modifying the attribute values (getters and setters)These operations are assumed and we need not show them explicitly in the modelOne way of identifying operations is by modeling the messages that may be passed between the objects
25 OOA Methodology – Library Example – Step 4 (2) Identify messages between objectsFind required messages for each scenario (play out the scenario), then take union of all messages
26 OOA Methodology – Library Example – Step 4 (3) Populate model of library user with discovered operations
27 OOA Methodology – Library Example – Step 4 (4) Populate model of library item with discovered operationsNote: This makes no sense as a model of the problem domain. How can a book (library item) perform the method acquire or return ?It may, however, make sense as the internal design of the system-to-be. In this case the objects are instances within the computer system that should reflect the objects in the real world.
28 OOA Methodology – Library Example – Step 5 Define the messages that may be passed between objects
29 OO Analysis – Problems (1) Caution: Not really analysisMost OOA approaches actually address high-level designAssume a pre-existing requirements documentClass diagrams can however be used for analysis, especially for the description of domain conceptsUse case analysis supplements OOA, filling in some gapsFurther composition and decomposition problemsRelated requirements cannot all be assigned to a single component or a single classOne scenario may affect several classes at onceOO modularization is not perfect either... Scattering and tangling effects - Motivation for aspect-oriented analysis and design
30 OO Analysis – Problems (2) Scattering: design elements to support R1 in many componentsRequirement1 (R1)Requirement2 (R2)Requirement3 (R3)RequirementN (RN)…ComponentAR1 elementsComponentFR2 elementsR3 elementsRN elementsComponentEComponentDComponentBComponentCTangling: single component haselements for many requirements
31 A partial solution – Aspects intertype declarationAspectClassAR1 elementsF.R1R1 elementsClassGR1 elementsTriggered behavior (code)R1 elementsadviceClassCR1 elementsPredicateR1 elementsClassBR1 elementspointcutR1 elementsClassFR1 elementsR2 elementsR3 elementsRN elementsR1 elements(identifies joinpoints where advice is executed)Terminology based on AspectJ: