Use case


In software and systems engineering, a use case is a list of actions or event steps typically defining the interactions between a role and a system to achieve a goal. The actor can be a human or other external system. In systems engineering, use cases are used at a higher level than within software engineering, often representing missions or stakeholder goals. The detailed requirements may then be captured in the Systems Modeling Language or as contractual statements.
Use case analysis is an important and valuable requirement analysis technique that has been widely used in modern software engineering since its formal introduction by Ivar Jacobson in 1992. Use case-driven development is a key characteristic of many process models and frameworks such as ICONIX, the Unified Process, the IBM Rational Unified Process, and the Oracle Unified Method. With its inherent iterative, incremental, and evolutionary nature, use case also fits well for agile development.

History

In 1986, Ivar Jacobson first formulated textual, structural, and visual modeling techniques for specifying use cases. In 1992 his co-authored book Object-Oriented Software Engineering - A Use Case Driven Approach helped to popularize the technique for capturing functional requirements, especially in software development. Originally he had used the terms usage scenarios and usage case – the latter a direct translation of his Swedish term användningsfall – but found that neither of these terms sounded natural in English, and eventually he settled on use case.
Since then, other experts have also contributed a great deal to the technique, notably Alistair Cockburn, Larry Constantine, Dean Leffingwell, Kurt Bittner and Gunnar Overgaard.
In 2011, Jacobson published an update to his work, called Use Case 2.0, with the intention of incorporating many of his practical experiences of applying use cases since the original inception of the concept.

Templates

There are many ways to write a use case in text, from use case brief, casual, outline, to fully dressed etc., and with varied [|templates]. Writing use cases in templates devised by various vendors or experts is a common industry practice to get high-quality functional system requirements.

Cockburn style

The template defined by Alistair Cockburn in his popular book Writing Effective Use Cases has been one of the most widely used writing styles of use cases.

Design scopes

Cockburn suggests annotating each use case with a symbol to show the "Design Scope", which may be black-box or white-box. Five symbols are available:
ScopeIcon
Organization Filled House
Organization Unfilled House
System Filled Box
System Unfilled Box
ComponentScrew or Bolt

Other authors sometimes call use cases at Organization level "Business use cases".

Goal levels

Cockburn suggests annotating each use case with a symbol to show the "Goal Level"; the preferred level is "User-goal".
Goal LevelIconSymbol
Very High SummaryCloud++
SummaryFlying Kite+
User GoalWaves at Sea!
SubfunctionFish-
Too LowSeabed Clam-Shell--

Sometimes in text writing, a use-case name followed by an alternative text symbol is a more concise and convenient way to denote levels, e.g. place an order!, login-.

Fully dressed

Cockburn describes a more detailed structure for a use case, but permits it to be simplified when less detail is needed. His fully dressed use case template lists the following fields:
In addition, Cockburn suggests using two devices to indicate the nature of each use case: icons for design scope and goal level.
Cockburn's approach has influenced other authors; for example, Alexander and Beus-Dukic generalize Cockburn's "Fully dressed use case" template from software to systems of all kinds, with the following fields differing from Cockburn:
Cockburn recognizes that projects may not always need detailed "fully dressed" use cases. He describes a Casual use case with the fields:
states "There is no standard way to write the content of a use case, and different formats work well in different cases." He describes "a common style to use" as follows:
The Fowler style can also be viewed as a simplified variant of the Cockburn template.

Actors

A use case defines the interactions between external actors and the system under consideration to accomplish a goal. Actors must be able to make decisions, but need not be human: "An actor might be a person, a company or organization, a computer program, or a computer system—hardware, software, or both." Actors are always stakeholders, but not all stakeholders are actors, since they "never interact directly with the system, even though they have the right to care how the system behaves." For example, "the owners of the system, the company's board of directors, and regulatory bodies such as the Internal Revenue Service and the Department of Insurance" could all be stakeholders but are unlikely to be actors.
Similarly, a person using a system may be represented as different actors because of playing different roles. For example, user "Joe" could be playing the role of a Customer when using an Automated Teller Machine to withdraw cash from his own account, or playing the role of a Bank Teller when using the system to restock the cash drawer on behalf of the bank.
Actors are often working on behalf of someone else. Cockburn writes that "These days I write 'sales rep for the customer' or 'clerk for the marketing department' to capture that the user of the system is acting for someone else." This tells the project that the "user interface and security clearances" should be designed for the sales rep and clerk, but that the customer and marketing department are the roles concerned about the results.
A stakeholder may play both an active and an inactive role: for example, a Consumer is both a "mass-market purchaser" and a User. In turn, a User is both a "normal operator" and a "functional beneficiary". For example, when user "Joe" withdraws cash from his account, he is operating the Automated Teller Machine and obtaining a result on his own behalf.
Cockburn advises to look for actors among the stakeholders of a system, the primary and supporting actors of a use case, the system under design itself, and finally among the "internal actors", namely the components of the system under design.

Business use case

In the same way that a use case describes a series of events and interactions between a user and a system, in order to produce a result of value, a business use case describes the more general interaction between a business system and the users/actors of that system to produce business results of value. The primary difference is that the system considered in a business use case model may contain people in addition to technological systems. These "people in the system" are called business workers. In the example of a restaurant, a decision must be made whether to treat each person as an actor or a business worker. If a waiter is considered an actor, as shown in the example below, then the restaurant system does not include the waiter, and the model exposes the interaction between the waiter and the restaurant. An alternative would be to consider the waiter as a part of the restaurant system, while considering the client to be outside the system. depicts a model of several business use cases which represents the interactions between a restaurant and its primary stakeholders.

Visual modeling

Use cases are not only texts, but also diagrams, if needed. In the Unified Modeling Language, the relationships between use cases and actors are represented in use case diagrams originally based upon Ivar Jacobson's Objectory notation. SysML uses the same notation at a system block level.
In addition, other behavioral UML diagrams such as activity diagrams, sequence diagrams, communication diagrams and state machine diagrams can also be used to visualize use cases accordingly. Specifically, a System Sequence Diagram is a sequence diagram often used to show the interactions between the external actors and the system under design, usually for visualizing a particular scenario of a use case.
Use case analysis usually starts by drawing use case diagrams. For agile development, a requirement model of many UML diagrams depicting use cases plus some textual descriptions, notes or use case briefs would be very lightweight and just enough for small or easy project use. As good complements to use case texts, the visual diagram representations of use cases are also effective facilitating tools for the better understanding, communication and design of complex system behavioral requirements.

Examples

Below is a sample use case written with a slightly-modified version of the Cockburn-style template. Note that there are no buttons, controls, forms, or any other UI elements and operations in the basic use case description, where only user goals, subgoals or intentions are expressed in every step of the basic flow or extensions. This practice makes the requirement specification clearer, and maximizes the flexibility of the design and implementations.

Use Case: Edit an article
Primary Actor: Member '
Scope: a Wiki system
Level: ! '
Brief: '
Stakeholders
...
Postconditions
Preconditions:
Triggers:
Basic flow:
  1. The system provides a new editor area/box filled with all the article's relevant content with an informative edit summary for the member to edit. If the member just wants to edit a section of the article, only the original content of the section is shown, with the section title automatically filled out in the edit summary.
  2. The member modifies the article's content till satisfied.
  3. The member fills out the edit summary, tells the system if they want to watch this article, and submits the edit.
  4. The system saves the article, logs the edit event and finishes any necessary post processing.
  5. The system presents the updated view of the article to the member.
Extensions''':
2–3.
4a. Timeout:
...

Advantages

Since the inception of the agile movement, the user story technique from Extreme Programming has been so popular that many think it is the only and best solution for agile requirements of all projects. Alistair Cockburn lists five reasons why he still writes use cases in agile development.
  1. The list of goal names provides the shortest summary of what the system will offer. It also provides a project planning skeleton, to be used to build initial priorities, estimates, team allocation and timing.
  2. The main success scenario of each use case provides everyone involved with an agreement as to what the system will basically do and what it will not do. It provides the context for each specific line item requirement, a context that is very hard to get anywhere else.
  3. The extension conditions of each use case provide a framework for investigating all the little, niggling things that somehow take up 80% of the development time and budget. It provides a look ahead mechanism, so the stakeholders can spot issues that are likely to take a long time to get answers for. These issues can and should then be put ahead of the schedule, so that the answers can be ready when the development team gets around to working on them.
  4. The use case extension scenario fragments provide answers to the many detailed, often tricky and ignored business questions: "What are we supposed to do in this case?" It is a thinking/documentation framework that matches the if...then...else statement that helps the programmers think through issues. Except it is done at investigation time, not programming time.
  5. The full use case set shows that the investigators have thought through every user's needs, every goal they have with respect to the system, and every business variant involved.
In summary, specifying system requirements in use cases has these apparent benefits comparing with traditional or other approaches:
User focused
Use cases constitute a powerful, user-centric tool for the software requirements specification process. Use case modeling typically starts from identifying key stakeholder roles interacting with the system, and their goals or objectives the system must fulfill. These user goals then become the ideal candidates for the names or titles of the use cases which represent the desired functional features or services provided by the system. This user-centered approach ensure that what has the real business value and the user really want is developed, not those trivial functions speculated from a developer or system perspective.
Use case authoring has been an important and valuable analysis tool in the domain of User-Centered Design for years.
Better communication
Use cases are often written in natural languages with structured templates. This narrative textual form, understandable by almost everyone, complemented by visual UML diagrams foster better and deeper communications among all stakeholders, including customers, end-users, developers, testers and managers. Better communications result in quality requirements and thus quality systems delivered.
Quality requirements by structured exploration
One of the most powerful things about use cases resides in the formats of the use case templates, especially the main success scenario and the extension scenario fragments. Analyzing a use case step by step from preconditions to postconditions, exploring and investigating every action step of the use case flows, from basic to extensions, to identify those tricky, normally hidden and ignored, seemingly trivial but realistically often costly requirements, is a structured and beneficial way to get clear, stable and quality requirements systematically.
Minimizing and optimizing the action steps of a use case to achieve the user goal also contribute to a better interaction design and user experience of the system.
Facilitate testing and user documentation
With content based upon an action or event flow structure, a model of well-written use cases also serves as an excellent groundwork and valuable guidelines for the design of test cases and user manuals of the system or product, which is an effort-worthy investment up-front. There is obvious connections between the flow paths of a use case and its test cases. Deriving functional test cases from a use case through its scenarios is straightforward.

Limitations

Limitations of use cases include:
Common misunderstandings about use cases are:
User stories are agile; use cases are not.
Agile and Scrum are neutral on requirement techniques. As the Scrum Primer states,
Product Backlog items are articulated in any way that is clear and sustainable. Contrary to popular misunderstanding, the Product Backlog does not contain "user stories"; it simply contains items. Those items can be expressed as user stories, use cases, or any other requirements approach that the group finds useful. But whatever the approach, most items should focus on delivering value to customers.

Use cases are mainly diagrams.
Craig Larman stresses that "use cases are not diagrams, they are text".
Use cases have too much UI-related content.
As some put it,
Use cases will often contain a level of detail which make it not well suited for capturing the requirements for a new system from scratch.

Novice misunderstandings. Each step of a well-written use case should present actor goals or intentions, and normally it should not contain any user interface details, e.g. naming of labels and buttons, UI operations etc., which is a bad practice and will unnecessarily complicate the use case writing and limit its implementation.
As for capturing requirements for a new system from scratch, use case diagrams plus use case briefs are often used as handy and valuable tools, at least as lightweight as user stories.
Writing use cases for large systems is tedious and a waste of time.
As some put it,
The format of the use case makes it difficult to describe a large system in less than several hundred pages. It is time consuming and you will find yourself spending time doing an unnecessary amount of rework.

Spending much time in writing tedious use cases which add no or little value and result in a lot of rework is a bad smell indicating that the writers are not well skilled and have little knowledge of how to write quality use cases both efficiently and effectively. Use cases should be authored in an iterative, incremental and evolutionary way. Applying use case templates does not mean that all the fields of a use case template should be used and filled out comprehensively from up-front or during a special dedicated stage, i.e. the requirement phase in the traditional waterfall development model.
In fact, the use case formats formulated by [|those popular template styles], e.g. the RUP's and the Cockburn's etc., have been proved in practice as valuable and helpful tools for capturing, analyzing and documenting complex requirements of large systems. The quality of a good use case documentation should not be judged largely or only by its size. It is possible as well that a quality and comprehensive use case model of a large system may finally evolve into hundreds of pages mainly because of the inherent complexity of the problem in hand, not because of the poor writing skills of its authors.

Tools

Text editors and/or word processors with template support are often used to write use cases. For large and complex system requirements, dedicated use case tools are helpful.
Some of the well-known use case tools include:
Most UML tools support both the text writing and visual modeling of use cases.