Object-Oriented Analysis and Design By DBMS, May 1996
Michael
Gora
The good, the bad, and the ugly of OOAD methodologies, and various approaches to using them.
Most new client/server application development tools emphasize object-oriented (OO) features. This implies that an OO analysis and design (OOAD) methodology should be an effective guide to applying these tools to business problems. But with the numerous methodologies available for OOAD, how do you choose the correct one? How do you get started, and how do you ascertain whether your approach is as efficient as it should be? And how do you avoid the pitfalls? OOAD can provide wonderful benefits, but believing that a new methodology will solve all your problems is like believing in Utopia. It's time for a reality check. In this article, I review different OOAD methodologies and various approaches to using them. I explain why I disagree with a number of mainstream concepts, I discuss what has worked and not worked in a number of projects, and I discuss how client/server applications that are developed with 4GL Windows tools may require a nonstandard approach to OOAD. My most common approach to OO methodology involves fairly strict adherence to OO design and construction guidelines as appropriate to a particular language, but a very loose and flexible approach to OO analysis. For me, OO analysis has become just another screwdriver in my toolkit -- useful for some projects but not for others. I spent all of 1988 and some of 1989 working with Richard Brown, one of the top OO developers in the U.S., building an OO EIS/DSS system under OS/2, using C as the language. Brown is a former Mitre Corp. principal scientist who, in an earlier life, was best known as the architect of the Kurzweil reading machine, which reads books to the blind, is still found in some libraries, and is still considered a high-water mark in OCR technology. The architecture of the EIS/DSS system we designed was based on LISP-based OO and artificial intelligence (AI) systems that Brown created for NASA and the Department of Defense while he was at Mitre. The project gave me good early exposure to OO concepts that had been proven in the field. Since 1989, my focus has been on Windows client/server development, although for a while, objects played no significant role in my work. For the last three years, however, almost all my technical work (that is, some of the analysis and all of the design and development) has been object oriented. I wouldn't do it any other way, but I'm still not comfortable with OOAD in the analysis phase. Although I currently teach three different OOAD courses, and have led custom seminars on OOAD, and one on managing OO projects, I'm still not completely happy with what I'm teaching. Is my lack of OO analysis understanding the problem, or is something else wrong? Every developer I know who has worked with OOAD and OO construction for more than a few months is extremely reluctant to go back to any other way of developing systems. The development speed, degree of organization, robustness, and code reuse have all been enhanced so much that going back to any other way of doing things is completely unthinkable. So why do I continue to squirm? A survey of available literature shows that the applications competing for "object honors" are written mostly in C++ or Smalltalk, and have a heavy manufacturing, engineering, aerospace, or scientific focus. That the major OOAD methodologies are best for such problems, however, does not guarantee that they are also best for typical business client/server applications. You need to determine which methodology is best suited for your
particular application, and what to do if no methodology matches your requirements precisely. (Using OOAD often requires adaptations to methodologies.)
The Role of OOAD in the Software Life Cycle To understand what's right and wrong with OOAD, you need to know where OO methodologies fit into the software life cycle. These methodologies do not replace traditional approaches (such as data flow, process flow, and state transition diagrams); they are important new additions to the toolkit. According to Donald Firesmith in his book Dictionary of Object Technology (SIGS Books, 1995), analysis is "the development activity consisting of the discovery, modeling, specification and evaluation of requirements," while OO analysis is "the discovery, analysis and specification of requirements in terms of objects with identity that encapsulate properties and operations, message passing, classes, inheritance, polymorphism and dynamic binding." Firesmith also states that OO design is "the design of an application in terms of objects, classes, clusters, frameworks and their interactions." In comparing the definition of traditional analysis with that of OOAD, the only aspect that is really new is thinking of the world or the problem in terms of objects and object classes. A class is any uniquely identified abstraction (that is, model) of a set of logically related instances that share the same or similar characteristics. An object is any abstraction that models a single thing, and the term "object" is synonymous with instance. Classes have attributes and methods. For an object class named Customer, attributes might be Name and Address, and methods might be Add, Update, Delete, and Validate. The class definition defines the Customer class attributes and methods, and a real customer such as "XYZ Corp." is an instance of the class. If you have different kinds of customers, such as residential customers and commercial customers, you can create two new classes of customers that are descendants of the Customer class. These descendants use inheritance to gain access to all of the Customer class attributes and methods, but can override any of the ancestor attributes and methods, as well as contain any required new attributes and methods. There are three types of relationships between classes: inheritance, aggregation, and association. Inheritance (also referred to as generalization/specialization) is usually identified by the phrase "is a kind of." For example, Student and Faculty are both a kind of Person and are therefore inherited from the Person class. Aggregation is identified by the phrase "is a part of," as with a product that contains parts. If neither of the first two relationships applies, but the objects are clearly related (for example, an employee is associated with a company), then the relationship is association. An abstract class is a class that has no instances, and is used only for inheritance. A concrete class is a class that can be instantiated, that is, that can have direct instances. Figure 1 shows some class relationships using Rumbaugh notation. All of the major OOAD methodologies have a similar basic view of objects, classes, inheritance, and relationships. The drawing notation is slightly different in each; the real differences in the methodologies are more subtle. Figure 2 illustrates the object relationships typically used in a single window. The student window class w_student in the lower left is the window designed in the application. This window can have any number of instances at runtime. Each instance of the window is associated with an instance of the sheet menu, as well as with an instance of a nonvisual class that can contain the business rules and data buffers that allow student data to be accessed in any window. Table 1 (page 64) summarizes the major OOAD methodologies. When you're choosing a methodology, it is important to consider not only the methodology's features, but also the cost of using it, the types of problems to which it is best suited, its limitations, and the training available. When used in typical initial attempts to develop
client/server applications using OOAD methodologies, all of the methodologies suffer from the same basic flaws: * an overemphasis on the OO approach in general, even though another approach might be better for some parts of the problem * an overemphasis on the problem domain object model during the analysis phase * analysis diagrams and output formats that end users may find difficult to understand * difficulty in the methodology's ability to describe complex analysis problems * a lack of emphasis on the underlying system architecture * an inability to understand the limitations of either 4GL OO languages or of beginning OO developers Every object methodology tells you to start with the object model, not the data model; there are at least four problems with this approach: * The data model often exists before the object model. * The analyst may rightly be more comfortable building the data model before the object model. * A good object model should be able to map to any data model. For me, it is usually a requirement in complex systems that an object's attributes can map to one or more tables in one or more databases. * A good abstract object model of the problem domain may not be easy to implement in the chosen language or development tool. The last problem in this list is the only one that is disappearing rapidly. With PowerBuilder 5.0 and new versions of other tools, the limitations on the problem domain object model have essentially disappeared. In PowerBuilder 4.0, a good conceptual object model of an order-entry system simply could not be implemented easily. SQLWindows (Centura Software), Delphi (Borland International Inc.), and other tools all have or had similar limitations. For example, in SQLWindows 5.0, my company built a good three-tiered object architecture into one application to aid in managing complexity, and we had to strip it out for performance reasons; the overhead of the additional layers was unacceptable. These types of problems are now on their way out. Despite this, your choice of development tool does affect the object architecture. Each major 4GL supports a slightly different flavor of OO, and the object model that you create during analysis may require significant adaptation going into the design phase. This should not continue to be the case, but neither the tools nor the methodologies are perfect. Another aspect of the problem is that for most developers learning about objects in typical development projects using OO client/server tools such as PowerBuilder, Delphi, SQLWindows, NewEra (Informix Software Inc.), Smalltalk, and so on, the problem domain is the least intuitive place to start. To build a client/server application, OO developers must first learn how to inherit windows, menus, controls, system error objects, and so on. Learning to describe the problem domain, that is, the accounting problem or order-entry problem, in object terms is much more difficult. In my experience, most developers using 4GL OO tools spend a year or more becoming comfortable with window classes, menu classes, and window control classes before they even start to experiment seriously with problem domain classes such as a Customer class. Of course, there are exceptions to
this. And, although Smalltalk developers typically begin to deal with abstract classes very quickly, I don't expect Smalltalk to become a major force in the client/server world.
OOAD Methodologies OOAD methodologies fall into two basic types. The ternary (or three-pronged) type is the natural evolution of existing structured methods and has three separate notations for data, dynamics, and process. The unary type asserts that because objects combine processes (methods) and data, only one notation is needed. The unary type is considered to be more object-like and easier to learn from scratch, but has the disadvantage of producing output from analysis that may be impossible to review with users. Dynamic modeling is concerned with events and states, and generally uses state transition diagrams. Process modeling or functional modeling is concerned with processes that transform data values, and traditionally uses techniques such as data flow diagrams. In the following sections, I describe the methodologies of Booch, Coad and Yourdon, Fusion, Jacobson, LBMS, Rumbaugh, and Shlaer and Mellor. There are several other methodologies that I don't discuss, and if you are interested in learning more about them, I strongly recommend Ian Graham's book, Object-Oriented Methods (Addison-Wesley, 1994), which does an excellent job of both describing and comparing available methodologies.
Booch Grady Booch's approach to OOAD (see his book Object-Oriented Design with Applications, Benjamin/Cummings, 1994) is one of the most popular, and is supported by a variety of reasonably priced tools ranging from Visio to Rational Rose. Booch is the chief scientist at Rational Software, which produces Rational Rose. (Now that James Rumbaugh and Ivar Jacobson have joined the company, Rational Software is one of the major forces in the OOAD world.) Booch's design method and notation consist of four major activities and six notations, as shown schematically in Table 2. While the Booch methodology covers requirements analysis and domain analysis, its major strength has been in design. However, with Rumbaugh and Jacobson entering the fold, the (relative) weaknesses in analysis are disappearing rapidly. I believe that Booch represents one of the better developed OOAD methodologies, and now that Rational Rose is moving away from its previous tight link with C++ to a more open approach that supports 4GLs such as PowerBuilder, the methodology's popularity should increase rapidly. For systems with complex rules, state diagrams are fine for those with a small number of states, but are not usable for systems with a large number of states. Once a single-state transition diagram has more than eight to 10 states, it becomes difficult to manage. For more than 20 states, state transition diagrams become excessively unwieldy.
Coad and Yourdon Coad and Yourdon published the first practical and reasonably complete books on OOAD (Object-Oriented Analysis and Object-Oriented Design, Prentice-Hall, 1990 and 1991, respectively). Their methodology focuses on analysis of business problems, and uses a friendlier notation than that of Booch, Shlaer and Mellor, or the others that focus more on design. In Coad and Yourdon, analysis proceeds in five stages, called SOSAS:
* Subjects: These are similar to the levels or layers in data-flow diagrams and should contain five to nine objects. * Objects: Object classes must be specified in this stage, but Coad and Yourdon provide few guidelines for how to do this. * Structures: There are two types: classification structures and composition structures. Classification structures correspond to the inheritance relationship between classes. Composition structures define the other types of relationships between classes. Coad and Yourdon do not deal as well as Rumbaugh, Jacobson, and several other methodologies do with these structures. * Attributes: These are handled in a fashion very similar to that in relational analysis. * Services: The identification of what other methodologies call methods or operations. In design, these five activities are supplanted by and refined into four components: * problem domain component: classes that deal with the problem domain; for example, Customer classes and Order classes * human interaction component: user-interface classes such as window classes * task management component: system-management classes such as error classes and security classes * data management component: database access method classes and the like Although Coad and Yourdon's methodology is perhaps one of the easiest OO methodologies to learn and get started with, the most common complaint is that it is too simple and not suitable for large projects. However, if you adhere to a premise that you should use those pieces of a methodology that work, and add other parts from other methodologies as required, Coad and Yourdon's methodology is not as limiting as its critics claim.
Fusion I have never used Fusion, but everything I read and hear about it places it at or near the head of the pack. In 1990, Derek Coleman of Hewlett-Packard led a team in the U.K. to develop a set of requirements for OOAD, and conducted a major survey of methods in use at HP and elsewhere. The chief requirement was a simple methodology with an effective notation. The result was Fusion, which Coleman and others developed by borrowing and adapting ideas from other methodologies. They incorporated some major ideas from Booch, Jacobson, Rumbaugh, and others, and explicitly rejected many other ideas from these methodologies. Articles that Fusion practitioners have written have been some of the most pragmatic and useful that I have seen about OOAD, but, unless you conduct a significant research effort, you generally hear much more about other methodologies than about Fusion. Coleman did not use some of the major components of Rumbaugh and Shlaer and Mellor in Fusion, because the components were not found to be useful in practice. Some writers have called this encouraging and remarkable, and consider it indirect proof that excessive emphasis on state models comes from Rumbaugh and Shlaer and Mellor's telecommunications and realtime system backgrounds. Fusion's pragmatic approach seems to hold considerable potential for client/server applications, but this methodology is not being marketed as aggressively as most of the other methodologies.
Jacobson: Objectory and OOSE Although Jacobson's full OOAD methodology, Objectory, is proprietary (to use it you must buy consulting services and a CASE tool, OrySE, from Rational Software), it is probably the most serious attempt by an OOAD tool vendor to support the entire software development life cycle. Jacobson is considered to be one of the most experienced OO experts for applying OO to business problems such as client/server applications. Jacobson's Object-Oriented Software Engineering (OOSE) is a simplified version of Objectory, which Jacobson himself has declared inadequate for production applications. According to Jacobson: "You will need the complete ... description which, excluding large examples, amounts to more than 1200 pages" (Object-Oriented Systems Engineering, Addison-Wesley, 1992). Object modeling and many other OO concepts in Objectory and OOSE are similar to OO concepts in other methodologies. The major distinguishing feature in Jacobson is the use case. A use-case definition consists of a diagram and a description of a single interaction between an actor and a system; the actor may be an end user or some other object in the system. For example, the use-case description of an order-entry application would contain a detailed description of how the actor (the user) interacts with the system during each step of the order entry, and would include descriptions of all the exception handling that might occur. According to Jacobson, a use case is any description of a single way to use a system or application, or any class of top-level usage scenario, that captures how actors use their black-box applications. An actor is an interface to the system, that is, something with which the system communicates, and may be a person or another program. Jacobson adds that a use case is any behaviorally related sequence of transactions that a single actor performs in a dialog with a system, in order to provide some measurable value to the actor. Generally, you employ use cases to document user requirements in terms of user dialogs with a system. Use cases appear first in the requirements model, and are then used to generate a domain object model with objects drawn from the entities of the business as mentioned in the use cases. This is then converted into an analysis model by classifying the domain objects into three types: interface objects, entity objects, and control objects. The big danger with OOSE is the assumption that you can express all sequences and business in use cases. One of my best object analysts ran into major difficulties while trying to force a large, complex client/server application into the Jacobson methodology. Several major authors have subsequently declared that for many complex systems and almost all expert systems, it cannot be done. However, the use-case descriptions and their corresponding interaction diagrams can provide a very useful view of many parts of a system, and Objectory and OOSE have a good, simple notation. For client/server applications with rules of typical, rather than extreme, complexity, Jacobson provides a sound approach. The use-case analyses are easier to review with end users than is much of the output from other OO analysis methodologies; and use cases use narrative descriptions in plain English and are much easier to review with individuals who are not OO experts than are object models with object interaction diagrams.
LBMS SEOO Systems Engineering OO (SEOO) is a proprietary methodology and toolkit from the U.K.-based company LBMS, which has its U.S. headquarters in Houston. SEOO is tightly integrated with Windows 4GLs such as PowerBuilder, and is perceived to be a very pragmatic and useful tool, but this perception may be due in part to a stronger marketing effort than is often made for nonproprietary methodologies. LBMS focuses on selling CASE tools that support its methodology, and SEOO is the only methodology described in this article that is not documented in a published book.
Because SEOO is proprietary, there is not as much detailed information available about it as there is about other methodologies, and it is somewhere between difficult and impossible to try it out just to compare it with the others. The four major components of the SEOO methodology are: * work-breakdown structures and techniques * an object modeling methodology * GUI design techniques * relational database linkages to provide ER modeling and 4GL-specific features Of all the major OOAD approaches, only SEOO gives the feeling of having started with non-OO approaches and then adapting to OO. A very positive aspect of this is the heavy focus on data management and data modeling. SEOO is intended to be object oriented while retaining the advantages of traditional data modeling. This makes the methodology well-suited for client/server database applications. SEOO is unique in treating data, triggers, and referential-integrity rules as a set of shared objects in a database. It treats a data model as a view of the shared objects, which also include constraints, rules, and dynamics (state transitions and so on). SEOO draws a clear line between shared objects and other objects, and regards the shared objects as important interfaces between subsystems. This technique allows a distinction, for example, between customer behavior shared by all applications and customer object behavior unique to a single application. It is a technique with which a purist would quibble, but which is eminently practical.
Rumbaugh OMT It offers one of the most complete descriptions yet written of an OO analysis methodology. Although it is somewhat lacking in OO design and construction, it contains a large number of ideas and approaches that are of significant use to analysts and designers. Rumbaugh starts by assuming that a requirements specification exists. Analysis consists of building three separate models: * the Object Model (OM): definition of classes, together with attributes and methods; the notation is similar to that of ER modeling with methods (operations) added * the Dynamic Model (DM): state transition diagrams (STDs) for each class, as well as global event-flow diagrams * the Functional Model (FM): diagrams very similar to data flow diagrams Because Rumbaugh's notation (as well as that of Booch and Shlaer and Mellor) is supported by low-end drawing tools such as Visio, I have used it not only for OO analysis, but for a number of diagrams in recent proposals. For example, I recently drew a diagram of project deliverables (in a proposal for a large OO development project) using Visio and Rumbaugh notation to show the object hierarchy of deliverables which, at the first level of inheritance, included hardware, software, and printed deliverables. Of the major methodologies, Rumbaugh's is one of those with which I feel most comfortable. It supports many traditional diagrams from structured methodologies, and contains a much richer object modeling notation than
Coad and Yourdon's. Its weakness at this time is that is it less useful for client/server application design and construction, but experienced analysts can overcome this with enough experience with class libraries for their development tool, if they have successfully completed other OO projects.
Shlaer and Mellor OO Analysis When Shlaer and Mellor OO analysis first came out in 1988, it represented one of the earliest examples of OO methodology and it has evolved very positively since then. (See Shlaer and Mellor's books, Object-Oriented Systems Analysis -- Modeling the World in Data and Object Lifecycles: Modeling the World in States, Prentice-Hall, 1988 and 1992, respectively.) Originally an object-based extension of data modeling, the Shlaer and Mellor methodology starts with an information model describing objects, attributes, and relationships. (Note that this is more like a data model than an object model.) Next, a state model documents the states of objects and the transitions between them. Finally, a data-flow diagram shows the process model. This methodology seems to be influenced strongly by relational design, but I have not seen it used for client/server development. This does not mean that it is not usable for such work, but the applications occasionally cited as examples of its use seem to be in the areas of real-time or process control. This may have to do with the fact that an earlier version, the Ward/Mellor approach, is widely used in the realtime world.
Using, Not Abusing, Objects Upon reflection, my discomfort with OOAD may be merely a side effect of the enormous rate of change in our industry. The object architectures that I used in PowerBuilder 3.0 were no longer optimum for PowerBuilder 4.0, and PowerBuilder 5.0 suggests yet another new class library architecture. The same is true of all other OO languages. How do we deal with methodologies and development tools that continue to evolve rapidly? How do we train analysts that may be less versed in the new approaches than the developers are? And how do we assure that the analysts can deal with what I call the object problem, that is, the fact that using objects requires more attention to systems architecture than in the past? The answers lie in training, commitment, experience, discipline, and pragmatism. Of these, the latter two may require the most attention. Discipline requires us to question what we are doing every day, assure that we are working on the right objectives every hour, and assure that we are doing this efficiently. Pragmatism in the OO world, to me, means being certain that my object guru is not an object bigot. It seems that there are too many object bigots who build excessively complex object architectures and don't realize that objects can be overused just as easily as the go to statement in Cobol can be abused. I am astounded at the number of gurus who "ooh" and "aah" (or should I say "om") at every new feature in every new OO development tool, believing each feature to be the solution to all their problems. But it never is. No one feature will make all software trivial to write, and no one architecture will be ideal for all problems. Creating good software will continue to be hard work. One of my object experts, considered a guru by many, was formerly an object bigot. That changed when he spent several months trying to force a complex set of business rules into an OO methodology that didn't support them (at least not without significant adaptation). While he was smart enough to learn from his mistake, the experience was very expensive. You must be able to recognize quickly if the object methodology isn't working for your particular project, and you must find another approach that does work, whether it is object oriented or not.
I have no doubt that OOAD methodologies can deliver faster, better code that is more reliable and reusable than ever before, but the combination of turmoil and immaturity in the object world requires a great deal of caution and pragmatism. After my first two years of working in Windows, that is, a GUI world without objects, I saw that there were more ways to do things right than ever before, but just as many ways to do things wrong. I have recently come to believe that the same is probably true of objects. If you do not have staff with the right combination of training, skill, and experience, the object world will not be a better world.
Table 1. Major OOAD Methodologies Analysis Methodology
Proprietary
Type
Scope
Strengths
Primary Cited Applications / Markets
Booch
No
Ternary
Complex, Rich, Pragmatic
Design
All
Coad and Yourdon
No
Unary
Simple, Limited, Pragmatic
Analysis
Client/Server
Fusion
No
Ternary
Complex, Rich, Pragmatic
Full Life Cycle
All
Jacobson Objectory
Yes
Ternary
Complex, Rich
Full Life Cycle
All
LBMS SEOO
Yes
Ternary
Middle of the Road, Pragmatic
Full Life Cycle
Client/Server
Rumbaugh OMT
No
Ternary
Complex, Rich, Somewhat Pragmatic
Analysis & Design
All, but heavily Embedded, Real-Time
Shlaer and Mellor OOA
No
Ternary
Complex, Rich
Design
Embedded, Real-Time
Table 2. The Steps in Booch's Methodology Steps
Notations
Logical structure
Class Diagrams Object Diagrams
Physical structure
Module Diagrams Process Diagrams
Dynamics of Classes Dynamics of Instances
State Transition Diagrams Timing Diagrams
