Purpose

Use cases and scenarios describe how a person or system interacts with the solution being modeled to achieve a goal.

Business Analysis Body of Knowledge^® (BABOK^®)

Use cases are used during the analysis phase of a project to identify and partition the system functionality. They separate the uses cases into actors and use cases. Actors represent the roles that are played by the users of the system. The users can be humans, other computers, pieces of hardware, or even other software systems. Use cases represent the activities that actors perform with the help of the system in achieving the goal. Use cases reflect the user needs and goals that should be initiated by the actor. It describes the system behavior under various conditions as it responds to a request of the stakeholders called as the primary actor. Use cases specifies the flow of events in the system. It is more concerned with what is performed by the system in order to perform the sequence of actions.

Use cases and scenarios describe the interaction between the primary actor, the solution , and any secondary actor needed to achieve the goal. It describe the possible outcome of attempt to accomplish a particular goal that the solution will support. It details with the different paths that can be followed by defining the primary and the alternative flows. The primary flow represents the most direct way to accomplish the goal of the use case. They are written from the point of view from an actor and avoid describing the internal workings of the solution. They are the graphical representation of the relationships between the actors or one or more use cases supported by the solution. The scenario describe just one way that an actor can accomplish a particular goal. They are written as the series of steps performed by the actors or by the solution that enable an actor to achieve a goal.   

Some of the articles related to Use cases and scenario techniques are as follows,

• Use Cases and Scenarios - https://www.inflectra.com/Ideas/Topic/Use-Cases.aspx
• Use Cases and Scenarios Model - https://www.sparxsystems.com/downloads/quick/writing-structured-use-case-scenarios-mdd.pdf
• Use Case Diagrams - https://www.researchgate.net/publication/50365823_Formal_Analysis_Of_Use_Case_Diagrams
• Generating Use Cases and Scenarios - https://www.ibm.com/developerworks/rational/library/content/RationalEdge/jun01/GeneratingTestCasesFromUseCasesJune01.pdf
• Use Case Modelling - https://www.oracle.com/technetwork/testcontent/gettingstartedwithusecasemodeling-133857.pdf
• Use Cases and Scenarios Technique - https://www.safeadapt.eu/content/dam/esk/dokumente/SafeAdapt_D2-1_Use_cases_and_scenarios_public.pdf
• Requirements of Use Cases - http://bredemeyer.com/pdf_files/functreq.pdf
• Use Cases for Agile Development - https://www.visual-paradigm.com/guide/agile-software-development/user-story-use-case/
• Overview of Use Cases and Scenarios - http://www.cse.msu.edu/~cse870/Lectures/Notes/03b-use-cases-notes.pdf
• Defining Use Case and Scenario - https://www.safeadapt.eu/content/dam/esk/dokumente/SafeAdapt_D2-1_Use_cases_and_scenarios_public.pdf
• Use Cases and Requirements - http://ict-citypulse.eu/page/sites/default/files/citypulse_d2.1_requirements_v1.0_0.pdf

The use case template is provided as a starting point from which to develop the project specific use case requirement. It ties the business needs of a system to the design and the implementation of the system. It helps to ensure that the system being developed meets the business requirements of the system. The following elements are frequently captured in a use case description.

• Use Case Diagram - It visually depicts the scope of the solution, by showing the actors who interact with the solution, which use cases they interact with, and any relationships between the use cases.
• Relationships - Relationship between the actors and use cases are called as associations. An association indicated that an actor has an access to the functionality represented by the use case. There are two commonly used relationship between the use cases. They are,

1.  Extend - It allow for the insertion of additional behavior in to the use case. This is used to show that an alternate flow has been added to an existing use case.
2. Include - It allows for the use case to make use of functionality present in another use case.  

• Use Case Description

1. Name - The use case ha a unique name. It generally includes the verb that describes the action taken by the actor and a noun that describes either what is being done or the target of the action.
2. Goal - It is a brief of a successful outcome of the use case from the perspective of the primary actor.
3. Actors - It is any person or system external to the solution that interacts with that solution. The use case is started by an actor referred to as an primary actor of that use case.
4. Preconditions - It is any face that must be true before the use case can begin. It is not tested in the use case bu acts as a constraints on its execution.
5. Trigger - It is an event that initiates the flow of events for a use case. The most common trigger is an action taken by the primary actor.
6. Flow of Events - It is the set of steps performed by the actor and the solution during the execution of the use case. It may also include the alternate or the exceptional flows.
7. Post Condition - It is any fact that must be true when the use case is complete. It must be true for all possible flows through the use case, including both the primary and the alternate flows.

The benefits of Use cases and Scenario technique are,

• It is an easy means of capturing the functional requirements with the focus on value added to the user.
• They are relatively easy to read and write.
• It forces the developers to think from the end user perspective.
• They engage the user in the requirement process.

Common Example of Use Case and Scenario is given below for reference,

The Actors - Check-in Representative, Customs of destination airport. Passenger, Baggage Transportation.

Relationship - It is between the actors and the use cases.

Use cases - It describes how the actor uses a system accomplish a particular goal.

System Boundary - It define the system of interest in relation to the world around it.

Some of the books for Use cases and scenario techniques are,

• Use Case Scenarios: A Clear and Concise Reference | Gerardus Blokdyk | https://www.amazon.com/Use-Case-Scenarios-Clear-Concise-Reference/dp/0655435107/ref=sr_1_28?dchild=1&keywords=use+cases+and+scenario+books&qid=1592976375&sr=8-28
• Use Case and Scenario Modelling | Kurt Bittner | https://www.amazon.com/Use-Case-Modeling-Kurt-Bittner/dp/0201709139/ref=sr_1_7?dchild=1&keywords=use+cases+books&qid=1592976578&sr=8-7
• Use Case and Scenario: Development Life Cycle | Neil Maiden | https://www.amazon.com/Scenarios-Use-Cases-Development/dp/0470841990/ref=sr_1_1?dchild=1&keywords=use+cases+and+scemnariop+books&qid=1532956692&sr=8-1-spell