Concepts: Conceptual Data Modeling
Conceptual data modeling represents the initial stage in the development of the design of the persistent data and persistent data storage for the system. In many cases, the persistent data for the system are managed by a relational database management system (RDBMS). The business and system entities identified at a conceptual level from the business models and system requirements will be evolved through the use-case analysis, use-case design, and database design activities into detailed physical table designs that will be implemented in the RDBMS. Note that the Conceptual Data Model discussed in this concept document is not a separate artifact. Instead it consists of a composite view of information contained in existing Business Modeling, Requirements, and Analysis and Design Disciplines artifacts that is relevant to the development of the Data Model.
The Data Model typically evolves through the following three general stages:
This stage involves the identification of the high level key business and system entities and their relationships that define the scope of the problem to be addressed by the system. These key business and system entities are defined using the modeling elements of the UML profile for business modeling included in the Business Analysis Model and the Analysis Class model elements of the Analysis Model
This stage involves the refinement of the conceptual high level business and system entities into more detailed logical entities. These logical entities and their relationships can be optionally defined in a Logical Data Model using the modeling elements of the UML profile for database design as described in Guidelines: Data Model. This optional Logical Data Model is part of the Artifact: Data Model and not a separate RUP artifact.
This stage involves the transformation of the logical class designs into detailed and optimized physical database table designs. The physical stage also includes the mapping of the database table designs to tablespaces and to the database component in the database storage design.
The activities related to database design span the entire software development lifecycle, and the initial database design activities might start during the inception phase. For projects that use business modeling to describe the business context of the application, database design may start at a conceptual level with the identification of Business Actors and Business Use Cases in the Business Use-Case Model, and the Business Workers and Business Entities in the Business Analysis Model. For projects that do not use business modeling, the database design might start at the conceptual level with the identification of System Actors and System Use Cases in the Use-Case Model, and the identification of Analysis Classes in the Analysis Model from the Use-Case Realizations.
The figure below shows the set of Conceptual Data Model elements that reside in the Business Models, Requirements Models, and the Analysis Model.
The following sections describe the elements of the Business Models, Use-Case Model, and Analysis Model that can be used to define the initial Conceptual Data Model for persistent data in the system.
2. Conceptual Data Modeling Elements
Business Use-Case Model
The Business Use-Case Model consists of Business Actors and Business Use Cases. The Business Use Cases represent key business processes that are used to define the context for the system to be developed. Business Actors represent key external entities that interact with the business through the Business Use Cases. The figure below shows a very simple example Business Use-Case Model for an online auction application.
As entities of significance to the problem of space for the system, Business Actors are candidate entities for the Conceptual Data Model. In the example above, the Buyer and Seller Business Actors are candidate entities for which the online auction application must store information.
Business Analysis Model
The Business Analysis Model contains classes that model the Business Workers and Business Entities identified from analysis of the workflow in the Business Use Case. Business Workers represent the participating workers that perform the actions needed to carry out that workflow. Business Entities are "things" that the Business Workers use or produce during that workflow. In many cases, the Business Entities represent types of information that the system must store persistently.
The figure below shows an example sequence diagram that depicts Business Workers and Business Entities from one scenario of the Business Use Case titled "Provide Online Auction" for managing an auction.
In this simplified example, the Auction Manager object represents a Business Worker role that will likely be performed by the online auction management system itself. The Auction and Auction Item objects are Business Entities that are used or produced by the Auction Manager worker acting as an agent for the Seller and Buyer Business Actors. From a database design perspective, the Auction and Auction Item Business Entities are candidate entities for the Conceptual Data Model.
Requirements and Analysis Models
For projects that do not perform business modeling, the Requirements (System Use Case) and Analysis Models contain model elements that can be used to develop an initial Conceptual Data Model. For projects that use business modeling, the business entities and relationships identified in the Business Analysis Models are refined and detailed in the Analysis Model as Entity Classes.
System Use-Case Model
The System Use-Case Model contains System Actors and System Use Cases that define the primary interactions of the users with the system. The System Use Cases define the functional requirements for the system.
From a conceptual data modeling perspective, the System Actors represent entities external to the system for which the system might need to store persistent information. This is important in cases where the System Actor is an external system that provides data to and/or receives data from the system under development. System Actors can be derived from the Business Actors in the Business Use-Case Model and the Business Workers in the Business Analysis Model.
The figure below depicts the Business Use-Case Model for the online auction system. In this model, the Buyer and Seller Business Actors are now derived from a generic User Business Actor. A new System Actor named Credit Service Bureau has been added to reflect the need to process payments through an external entity. This new System Actor is another candidate entity for the Conceptual Data Model.
The Analysis Model contains the Analysis Classes identified in the Use-Case Realizations for the System Use Cases. The types of Analysis Classes that are of primary interest from a conceptual data modeling perspective are the Entity Analysis Classes. As defined in Guidelines: Analysis Class, Entity Analysis Classes represent information managed by the system that must be stored in a persistent manner. The Entity Analysis
Classes and their relationships form the basis of the initial Data Model for the application.
The conceptual Entity Analysis Classes in the Analysis Model might be refined and detailed into logical Persistent Design Classes in the Design Model. These design classes represent candidate tables in the Data Model. The attributes of the classes are candidate columns for the tables and also represent candidate keys for them. See Guidelines: Forward-Engineering Relational Databases for a description of how elements in the Design Model can be mapped to Data Model elements.
3. Conceptual Class Category List
A conceptual class is a real-world concept or thing; a conceptual or essential perspective. At the noun filtering stage we are looking for conceptual classes. As we move through the design process we will start to design software classes that represent an implementation perspective of a software component but we will not get into language specific classes in 466. A conceptual class is not an implementation class, such as a class that can be implemented in an OO language such as Java or C++
4. Types of Classes
During use case realization, we identify mainly four "types" of classes, boundary classes, data store classes and control classes.
The entity classes represent the information that the system uses. Examples of entity classes are: Customer, Product, and Supplier. Entity classes are essential to the system as the expected functionality of the system includes maintaining information about them or retrieving information from them.
The boundary classes represent the interaction between the system and its actors. A GUI form is an example of a boundary class.
Data store classes encapsulate the design decisions about data storage and retrieval strategies. This provides us flexibility to move a n application from database platform to another.
The control classes represent the control logic of the system. They implement the flow of events as given in a use case.
Entity classes are the abstractions of the keys concepts of the system being modelled. If the steps of the Architectural Analysis have been carried out, many of the entity classes may have already been identified during those steps.
The Core functionality and logic of the system are encapsulated in the various entity classes. For example, if interest is to be calculated and paid to savings account holders, a savings Account entity class may be responsible for computing and returning the interest.
You can normally look for the following types of things as potential entity classes:
1. Roles played by people or organizations about which information is required to be maintained by the system. For Example, Student in a Library Management System, Vendor in a Purchase Ordering System.
2. Other physical, tangible things. For example, Book in a Library Management System.
3. Events that requires remembrance. For example, Reservation and Issue in a Library Management System.
The logical data structures (attributes and relationships) of the entity classes would be designed to hold and manipulate the data according to the system's requirements. Values of the attributes and their relationships of the entity class objects are often given by actors. The entity classes are responsible for storing and managing information in the system.
Entity class objects are usually persistent, having attributes and relationships that need to be retained for a long time, sometimes even before the life of system. An entity class is usually not specific to one use cause realization. Objects of most entity classes would required in multiple use cases. Sometimes, an entity object may not be specific to the system itself.
Boundary classes represent the interaction between the system and its actors. They insulate the system from changes in the surroundings of the system, such as user interfaces, and interfaces to other systems.
There may be various types of boundary classes in a system:
1. User Interfaces classes: Classes for encapsulating the human user interface of the system, such as GUI forms.
2. System Interface Classes: Classes that encapsulate the interaction of the system with other systems.
3. Device Interface Classes: Classes that provide the interface to devices that detect external events.
An important objective of identifying boundary classes is to ensure that the entity classes and the control classes are not affected by any changes to the boundary classes.
Actors interact with the system only through the boundary classes
User Interface Classes
A user interface class represents the interaction between a use case and its initiating actor. This class has the responsibility of coordinating the interaction with the actor. A boundary class may have various subsidiary classes to which some of its responsibilities are delegated. For example, in a GUI application, there may be multiple forms within a use case.
During use case analysis, you should use the boundary classes as just place-holders for the GUI forms. Detailed GUI design is an activity of Class Design. During Analysis, the emphasis should be only on isolating all environment-dependent behaviour as boundary classes. These classes will get refined or replaced in the later stages.
System Interface Classes
A system interface class is responsible for interfacing with an external system. The interface offered by the external system would have been identified by the developers of that system. Thus, the behaviour of a system interface class should be derived directly from the interface specifications of the external system.
System interface classes achieve the purpose of isolating the internal details of the external systems, which may change over a period of time. Our system should not get affected by such changes in the internal details of the external systems.
Device Interface Classes
Device interface classes are responsible for interacting with the external devices that the system may depend upon for receiving inputs or handling outputs. Examples of such external devices would be: bar code reader, system clock, printer, etc.
A device may already have a well-defined interface, which could be used by you later during design. Therefore, a note of such interface should be made in the model documentation.
Data Store Classes
Data Store classes encapsulate our design decisions about the database structures which are used to store entity class objects, and to retrieve them later. For each entity class that required persistence, we create a corresponding data store class. A data store class typically receives an object of an entity class, and make it persistence (for example, by inserting a row in a table). At a later point of time, we may ask the data store class to return the entity class object.
Encapsulating the database design designs in data store classes, makes the entity classes independent of the database structure, and thus provides us greater flexibility to move an application from one database platform to another.
Controller classes provide co-ordinating behaviour in the system. A typical example would be a controller class implementing the logic and flow of events of a use case.
Controller Classes isolates the entity classes and boundary classes from each other, making the system independent of the changes to the system boundary. They also isolate the use case specific behaviour from the entity class objects, thus making them re-usable across use cases and even across systems.
Simple use cases may be performed without using controller classes, with direct flow of data between boundary objects and entity objects. However, more complex use cases usually require and benefit from such controller classes. The characteristics of controller classes are:
1. They define the order of events and transactions within a use case. In other words, they encapsulate the use case-specific behaviour of the system.
2. They are relatively independent of the changes to the internal structure or behaviour of the entity classes.
3. They are nearly independent of changes to the boundary classes.
4. They may use or set several entity classes, thus coordinating the behaviour of these entity classes. However, this coordination can be invisible to the participating entity classes.
Though most of the times a control class correspond to a single use case, some times a single controller class may be use to control several use cases. Some tines there may even be multiple controller classes with in a single use case. As mentioned earlier, there may be use cases that do not require controller classes.