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• Use Case

A use case instance defines a sequence of actions performed by the system that yields an observable result of value to a particular actor.

Explanation

There are several key words in this definition:

• "use case instance" The sequence referred to in the definition is really a specific flow of events through the system, or an instance. Many flows of events are possible, and many may be very similar. To make a use-case understandable, you should group similar flows of events into one use case. Identifying and describing a use case really means identifying and describing a group of related flows of events.
• "actions" An action is a computational or algorithmic procedure. It is invoked either when the actor provides a signal to the system or when the system gets a time event. An action may imply signal transmissions to either the invoking actor or other actors. An action is atomic, which means it is performed either entirely or not at all.
• "performed by the system" This means that the system provides the use case. An actor communicates with a use-case instance of the system.
• "an observable result of value" You can put a value on a successfully performed use case. A use case should make sure that an actor can perform a task that has an identifiable value. This is very important in determining the correct level or granularity for a use case. Correct level refers to achieving use cases that are not too small.  
• "a particular actor" The actor is key to finding the correct use case, especially because the actor helps you avoid use cases that are too large. As an example, consider a visual modeling tool. There are really two actors to this application: a developer - someone who develops systems using the tool as support; and a system administrator - someone who manages the tool. Each of these actors has his own demands on the system, and will therefore require his own set of use cases.

The functionality of a system is defined by different use cases, each of which represents a specific goal (observable result of value) for a particular actor. The description of a use case defines what happens in the system when the use case is performed.

In an automated teller machine the client can, for instance, withdraw money from an account, transfer money to an account, or check the balance of an account. These correspond to specific goals that the actor has in using the system.

Each use case has a task of its own to perform. The collected use cases constitute all the possible ways of using the system. You should be able to determine the goal of a use-case task simply by observing its name.

How to Find Use Cases

See the Guideline: Find and Outline Actors and Use Cases for guidance on finding Actors and Use Cases.

How a Use Case Evolves

See the Guideline: Detail Use Cases and Scenarios for guidance on evolving use cases.

Are All Use Cases Described in Detail?

There will often be use cases in your model that are so simple that they do not need a detailed description of the flow of events, a step-by-step outline is quite enough. The criteria for making this decision is that you don't see disagreement among user kind of readers on what the use case means, and that designers and testers are comfortable with the level of detail provided by the step-by-step format. Examples are use cases that describe simple entry or retrieval of some data from the system.

For more information on possible formats and level of detail captured for each use case see Guideline: Use Case Formats.

The Scope of a Use Case

It is often hard to decide if a set of user-system interactions, or dialog, is one or several use cases. Consider the use of a recycling machine. The customer inserts deposit items, such as cans, bottles, and crates, into the recycling machine. When she has inserted all her deposit items, she presses a button, and a receipt is printed. She can then exchange this receipt for money.

Is it one use case to insert a deposit item, and another use case to require the receipt? Or is it all one use case? There are two actions, but one without the other is of little value to the customer. Rather, it is the complete dialog with all the insertions, and getting the receipt, that is of value for the customer (and makes sense to her). Thus, the complete dialog, from inserting the first deposit item, to pressing the button and getting the receipt, is a complete case of use, a use case.

Additionally, you want to keep the two actions together, to be able to review them at the same time, modify them together, test them together, write manuals for them and in general manage them as a unit. This becomes very obvious in larger systems.

How Use Cases Are Realized

A use case describes what happens in the system when an actor interacts with the system to execute the use case. The use case does not define how the system internally performs its tasks in terms of collaborating objects. This is left for the use-case realizations to show.

Example:

In the telephone example, the use case would indicate - among other things - that the system issues a signal when the receiver is lifted and that the system then receives digits, finds the receiving party, rings his telephone, connects the call, transmits speech, and so on.

In an executing system, an instance of a use case does not correspond to any particular object in the implementation model (for example, an instance of a class in the code). Instead, it corresponds to a specific flow of events that is invoked by an actor and executed as a sequence of events among a set of objects. In other words, instances of use cases correspond to communicating instances of implemented objects. We call this the realization of the use case. Often, the same objects participate in realizations of more than one use case. For example, both the use cases Deposit and Withdrawal in a banking system may use a certain account object in their realization. This does not mean that the two use cases communicate, only that they use the same object in their realization.

You can view a flow of events as consisting of several subflows, which taken together yield the total flow of events. You can reuse the description of a subflow in other use cases' flow of events. Subflows in the description of one use case's flow of events may be common to those of other use cases. In the design you should have the same objects perform this common behavior for all the relevant use cases; that is, only one set of objects should perform this behavior, no matter which use case is executing.

Example:

In an automated teller machine system the initial subflow is the same in the flow of events of the use cases Withdraw Money and Check Balance. The flow of events of both use cases start by checking the identity of the card and the client's personal access code.

A Use Case has Many Possible Instances

A use-case instance can follow an almost unlimited, but enumerable, number of paths. These paths represent the choices open to the use-case instance in the description of its flow of events. The path chosen depends on events. Types of events include:

• Input from an actor. For example, an actor can decide, from several options, what to do next.

Example:

In the use case Recycle Items in the Recycling-Machine System the Customer always has two options: hand in still another deposit item or get the receipt of returned items.

• A check of values or types of an internal object or attribute. For example, the flow of events may differ if a value is greater or less than a certain value.

Example:

In the use case Withdraw Money in an automated teller machine system, the flow of events will differ if the Client asks for more money than he has in his account. Thus, the use-case instance will follow different paths.

Concurrency of Use-Case Instances

Instances of several use cases and several instances of the same use case work concurrently if the system permits it. In use-case modeling, you can assume that instances of use cases can be active concurrently without conflict. The design model is expected to solve this problem, because use-case modeling does not describe how things work. One way to view this is to assume that only one use-case instance is active at a time and that executing this instance is an atomic action. In use-case modeling, the "interpreting machine" is considered infinitely fast, so that serialization of use case instances is not a problem.

Name

Each use case should have a name that indicates what is achieved by its interaction with the actor(s). The name may have to be several words to be understood. No two use cases can have the same name.

Example:

These are examples of variations of the name for the use case Recycle Items in the Recycling Machine example:

• Receive Deposit Items
• Receiving Deposit Items
• Return Deposit Items
• Deposit Items

Brief Description

The brief description of the use case should reflect its purpose. As you write the description, refer to the actors involved in the use case, the glossary and, if you need to, define new concepts.

Example:

Following are sample brief descriptions of the use cases Recycle Items and Add New Bottle Type in the Recycling-Machine System:

Recycle Items: The user uses this machine to automatically have all the return items (bottles, cans, and crates) counted, and receives a receipt. The receipt is to be cashed at a cash register (machine).

Add New Bottle Type: New kinds of bottles can be added to the machine by starting it in 'learning mode' and inserting 5 samples just like when returning items. In this way, the machine can measure the bottles and learn to identify them. The manager specifies the refund value for the new bottle type.

Flow of Events - Contents

The Flow of Events of a use case contains the most important information derived from use-case modeling work. It should describe the use case's flow of events clearly enough for an outsider to easily understand it. Remember the flow of events should present what the system does, not how the system is design to perform the required behavior.

Guidelines for the contents of the flow of events are:

• Describe how the use case starts and ends.
• Describe what data is exchanged between the actor and the use case.
• Do not describe the details of the user interface, unless it is necessary to understand the behavior of the system. For example, it is often good to use a limited set of web-specific terminology when it is known beforehand that the application is going to be web-based. Otherwise, your run the risk that the use-case text is being perceived as too abstract. Words to include in your terminology could be "navigate", "browse", "hyperlink" "page", "submit", and "browser". However, it is not advisable to include references to "frames" or "web pages" in such a way that you are making assumptions about the boundaries between them - this is a critical design decision. 
• Describe the flow of events, not only the functionality. To enforce this, start every action with "When the actor ... ".
• Describe only the events that belong to the use case, and not what happens in other use cases or outside of the system.
• Avoid vague terminology.
• Detail the flow of events-all "whats" should be answered. Remember that test designers are to use this text to identify test cases.

If you have used certain terms in other use cases, be sure to use the exact same terms in this use case, and that their intended meaning is the same. To manage common terms, put them in a glossary.

Flow of Events - Structure

The two main parts of the flow of events are basic flow of events and alternative flows of events. The basic flow of events should cover what "normally" happens when the use case is performed. The alternative flows of events cover behavior of optional or exceptional character in relation to the normal behavior, and also variations of the normal behavior. You can think of the alternative flows of events as "detours" from the basic flow of events, some of which will return to the basic flow of events and some of which will end the execution of the use case.

The typical structure of the flow of events. The straight arrow represents the basic flow of events, and the curves represent alternative paths in relation to the normal. Some alternative paths return to the basic flow of events; whereas others end the use case.

Both the basic flow of events and the alternative flows events should be further structured into steps or subflows. In doing this, your main goal should be readability of the text (see also the section Flow of Events - Style below). A rule of thumb is that a subflow should be a segment of behavior within the use case that has a clear purpose, and is "atomic" in the sense that you do either all or none of the actions described. You may need to have several levels of subflows, but if you can you should avoid it since it makes the text more complex and harder to understand.

This type of written text, structured into consecutive subsections, will by its nature imply to the reader that there is a sequence between the subflows. To avoid misunderstandings, you should always point out whether the order of the subflows is fixed or not. Considerations of this kind are often related to:

• Business rules. For example, the user has to be authorized before the system can make certain data available.
• User-interface design. For example, the system should not enforce a certain sequence of behavior that may be intuitive to some but not to other users.

To clarify where an alternative flow of events fits in the structure, you need to describe the following for each "detour" to the basic flow of events:

• Where in the basic flow of events the alternative behavior can be inserted.
• The condition that needs to be fulfilled for the alternative behavior to start.
• How and where the basic flow of events is resumed, or how the use case ends.

Example:

This is an alternative subflow in the use case Return Items in the Recycling-Machine System.

2.1. Bottle Stuck

If in section 1.5, Insert Deposit Items, a bottle gets stuck in the gate, the sensors around the gate and the measuring gate will detect this problem. The conveyer belt is stopped and the machine issues an alarm to call for the operator. The machine will wait for the operator to indicate that the problem has been fixed. The machine then continues in section 1.9 of the basic flow.

In the example above, the alternative flow of events is inserted at a specific location in the basic flow of events. There are also alternative flow of events that can be inserted at more than one location, some can even be inserted at any location in the basic flow of events.

Example:

This is an alternative subflow in the use case Return Items in the Recycling-Machine System.

2.2. Front Panel is Removed

If somebody removes the front panel to the Recycling machine, the can compression is deactivated. It will not be possible to start the can compression with the front panel off. The removal will also activate an alarm to the operator. When the front panel is closed again, the machine resumes operation from the location in the basic flow of events at which it was stopped.

It might be tempting, if the alternative flow of events is very simple, to just describe it in the basic flow of events section (using some informal "if-then-else" construct). This should be avoided. Too many alternatives will make the normal behavior difficult to see. Also, including alternative paths in the basic flow of events section will make the text more "pseudo-code like" and harder to read.

In general, extracting parts of the flow of events and describing these parts separately, can increase the readability of the basic flow of events and improve the structure of the use case and the use-case model. You can model extracted parts as:

• An alternative flow of events within the base use case if it is a simple variant, option, or exception to the basic flow of events.
• As an explicit inclusion in the base use case, if it is something that you wish to encapsulate so that it can be reused by other use cases.
• As an implicit inclusion in the base use case, if the basic flow of events of the base use case is complete, that is, has a defined beginning and end. The nature of the extending flow should be such that you prefer to conceal it in the description of the base use case to render it less complex.
• A subflow in the basic flow of events, possibly as another option, if none of the above alternatives applies. For example, in a Maintain Employee Information use case, there may be separate subflows for adding, deleting and modifying employee information.

Flow of Events - Style

You can describe use cases in many styles. As an example we show the basic flow of events of the use case Administer Order described in three different styles, varying primarily in how formal they are. The first style, shown in example 1 below, is recommended, because it is easy to understand, and the order in which things happen is clearly evident. The text is divided into numbered and named subsections. Numbers are there to make it easy to refer to a subsection. Names of subsections will let the reader get a quick overview of the flow of events by browsing through the text reading only the headers.

In example 2 below, the description of the flow of events fails to clarify the order in which things happen. If you write in this style, you and others might miss important things that concern the system.

Example 3 below shows a yet another style, which can be useful if you find it difficult to express the sequence of events clearly. This pseudo-code style is more precise, but the text is hard to read and absorb for a non-technical person, especially if you want to grasp the flow of events quickly.

Example 1:

Describing a use case: In this style, the text is easy to read and the flow of events is easy to follow. Aim for this style in your descriptions.

Example 2:

Describing a use case: This style is readable, but there is no clear flow of events.

Example 3:

'Administrate order' (User identity) 
REPEAT
    <='Show administer order menu'     IF (=> 'Creating an Order' (Measurement function,
network element, measurement object)) THEN
    The system finds a unique name, default values for when and
how long the measurement should be executed. 
<= 'Show order' (Default attributes) REPEAT     => 'Edit order' (Attribute to change, New value of attribute)
    <= 'Update screen' (New attributes) UNTIL (All attributes are defined) REPEAT     IF (=> 'Edit order' (Attribute to change, New value of  attribute))
THEN <= 'Update screen' (New attributes) ELSIF (=> 'Save order' (Order identity, Attributes)) THEN
    The order is created and initialized in the system with
    the defined attributes, the name of the creator,
    date of creation and the status 'scheduled'.
    <= 'New order created' (The order) ENDIF UNTIL (=> 'Quit') 
    ENDIF 
UNTIL 'Quit administer order'

Describing a use case: Here the writer has chosen a formal style using pseudocode. This style makes it hard to quickly grasp the process steps, but can be useful if the flow of events is difficult to capture precisely.

Flow of Events - Example

The complete description of the flow of events of the use case Administer Order, including its alternative flows, could look as follows:

1. Basic Flow of Events

1.1. Start of Use Case

This use case starts when the actor Operator tells the system to create a measurement order. The system will then retrieve all Network Element actors, their measurement objects and corresponding measurement functions that are available to this particular Operator. Available Network Elements are those that are in operation, and that the Operator has the authority to access. The availability of measurement functions depends on what has been set up for a particular type of measurement object.

1.2. Configure Measurement Order

The system allows the actor Operator to select which Network Elements to measure and then shows which measurement objects are available for the selected Network Elements. The system allows the Operator to select from these measurement objects, and then select which measurement functions to set up for each measurement object.

The system allows the Operator to enter a textual comment on the measurement order.

The Operator tells the system to complete the measurement order. The system will respond by generating a unique name for the measurement order and setting up default values for when, how often, and for how long the measurement should be made. The default values are unique to each Operator. The system then allows the Operator to edit these default values.

1.3. Initialize Order

The Operator tells the system to initialize the measurement order. The system will then record the identity of the creating Operator, the date of creation, and the "Scheduled" status of the measurement order.

1.4. Use Case Ends

The system confirms initialization of the measurement order to the Operator, and the measurement order is made available for other actors to view.

2. Alternative Flows of Events

2.1. No Network Elements Available

If in 1.1, Start of Use Case, it turns out that no Network Elements are available to measure for this Operator, the system will inform the Operator. The use case then ends.

2.2. No Measurement Functions Available

If in 1.2, Configure Measurement Order, no measurement functions are available for the selected Network Elements, the system will inform the Operator and allow the Operator to select other Network elements.

2.3. Cancel Measurement Order

The system will allow the Operator to cancel all actions at any point during the execution of the use case. The system will then return to the state it was in before the use case was started, and end the use case.

Special Requirements

In the Special Requirements of a use case, you describe all the requirements on the use case that are not covered by the flow of events. These are non-functional requirements that will influence the design model. See also the discussion on non-functional requirements in Concept: Requirements. You could organize these requirements in categories such as Usability, Reliability, Performance, and Substitutability, but normally there are so few of them that such grouping is not particularly value-adding.

Example:

In the Recycling-Machine System, a special requirement of the Return Deposit Items use case could be:

The machine has to be able to recognize deposit items with a reliability of more than 95 percent.

Preconditions and Postconditions

It can be useful to use the notion of precondition and postcondition to clarify how the flow of events starts and ends. However, only use it if it is perceived as adding value by the audience of the use case.

A precondition is the state of the system and its surroundings that is required before the use case can be started. A postcondition is the states the system can be in after the use case has ended.

Consider the following:

• The states described by pre- or postconditions should be states that the user can observe. "The user has logged on to the system" or "The user has opened the document" are examples of observable states.
• A precondition is a constraint on when a use case can start. It is not the event that starts the use case.
• A precondition for a use case is not a precondition for only one subflow, although you can define preconditions and postconditions at the subflow level.
• A postcondition for a use case should be true regardless of which alternative flows were executed; it should not be true only for the main flow. If something could fail, you would cover that in the postcondition by saying "The action is completed, or if something failed, the action is not performed", rather than just "The action is completed".
• When you use postconditions together with extend-relationships, you should take care that the extending use case does not introduce a subflow that violates the postcondition in the base use case.
• Postconditions can be a powerful tool for describing use cases. You first define what the use case is supposed to achieve - the postcondition. You can then describe how to reach this condition - the flow of events needed.

Example:

A precondition for the use case Cash Withdrawal in the ATM machine: The customer has a personally-issued card that fits in the card reader, has been issued a PIN number, and is registered with the banking system.

A postcondition for the use case Cash Withdrawal in the ATM machine: At the end of the use case, all account and transaction logs are balanced, communication with the banking system is reinitialized and the customer has been returned his card.

Extension Points

An extension point opens up the use case to the possibility of an extension. It has a name, and a list of references to one or more locations within the flow of events of the use case. An extension point may reference a single location between two behavior steps within the use case. It may also reference a set of discrete locations.

To use named extension points will help you separate the specification of the behavior of the extending use case from the internal details of the base use case. The base use case can be modified or rearranged, as long as the names of the extension points remain the same it will not affect the extending use case. At the same time, you are not loading down the text describing the flow of events of the base use case with details of where behavior might be extended into it.

Example:

In a phone system, the use case Place Call can be extended by the abstract use case Show Caller Identity. This is an optional service, often referred to as "Caller ID", that may or may not have been requested by the receiving party. A description of the extension point in the use case Place Call could look as follows:

Name: Show Identity

Location: After section 1.9 Ring Receiving Party's Telephone.

Use-Case Diagrams

You may choose to illustrate how a use case relates to actors and other use cases in a use-case diagram (in unusual cases, more than one diagram), owned by the use case. This is useful if the use case is involved with many actors, or has relationships to many other use cases. A diagram of this kind is of "local" character, since it shows the use-case model from the perspective of one use case only and is not intended to explain any general facts about the whole use-case model. Refer to Guideline: Use-Case Model for more information.

• Find and Outline Actors and Use Cases
• Detail Use Cases and Scenarios