Computer Science, UML

UML – Requirements Elicitation

 

 

The client, the developers, and the users identify a problem area and define a system that addresses the problem. Such a definition is called a requirements specification and serves as a contract between the client and the developers…The requirements specification is structured and formalized during analysis to produce an analysis model…(August 7, 2017).

The first step of requirements elicitation is the identification of actors. This serves both to define the boundaries of the system and to find all the perspectives from which the developers need to consider the system…(September 18, 2017).

 

 

Actividad WBS (Overview)
 

Fuente:

  1. Object-Oriented Software Engineering
    1. Requirement Elicitation concepts – Functional requirements
    2. Requirement Elicitation activities -Identifying actors – Identifying scenarios – Identifying Use Cases – Identifying relationships between actors and Use Cases – Identifying Initial Analysis Objects -Identifying Non-functional requirements – Documenting Requirements Elicitation- (p 152).
A requirement is a feature that the system must have or a constraint that it must satisfy to be accepted by the client. Requirements engineering aims at defining the requirements of the system under construction. Requirements engineering includes two main activities; Requirements Elicitation, which results in the specification of the system that the client understands, and analysis, which results in an analysis model that the developers can unambiguously interpret. Requirements elicitation is the more challenging of the two because it requires the collaboration of several groups of participants with different backgrounds. On the one hand, the client and the users are experts in their domain and have a general idea of what the system should do, but they often have little experience in software development. On the other hand, the developers have experience in building systems, but often have little knowledge of the everyday environment of the users.

Requirement elicitation

Overview

 

A requirement is a feature that the system must have or a constraint that it must satisfy to be accepted by the client. Requirements engineering aims at defining the requirements of the system under construction. Requirements engineering includes two main activities; Requirements Elicitation, which results in the specification of the system that the client understands, and analysis, which results in an analysis model that the developers can unambiguously interpret. Requirements elicitation is the more challenging of the two because it requires the collaboration of several groups of participants with different backgrounds. On the one hand, the client and the users are experts in their domain and have a general idea of what the system should do, but they often have little experience in software development. On the other hand, the developers have experience in building systems, but often have little knowledge of the everyday environment of the users.

Scenarios and use cases provide tools for bridging this gap. A scenario describes an example of system use in terms of a series of interactions between the user and the system. A use case is an abstraction that describes a class of scenarios. Both scenarios and use cases are written in natural language, a form that is understandable to the user. In this chapter, we focus on scenario-based requirements elicitation.,,Requirements elicitation is about communication among developers, clients, and users to define a new system…Requirements elicitation methods aim at improving communication among developers, clients, and users…Developers construct a model of the application domain by observing users in their environment. Developers select a representation that is understandable by the clients and users (e.g., scenarios and use cases). Developers validate the application domain model by constructing simple prototypes of the user interface and collecting feedback from potential users…Requirements elicitation focuses on describing the purpose of the system. The client, the developers, and the users identify a problem area and define a system that addresses the problem. Such a definition is called a requirements specification and serves as a contract between the client and the developers…The requirements specification is structured and formalized during analysis to produce an analysis model:

Both requirements specification and analysis model represent the same information. They differ only in the language and notation they use; the requirements specification is written in natural language, whereas the analysis model is usually expressed in a formal or semiformal notation.

Requirements elicitation and analysis focus only on the user’s view of the system. For example, the system functionality, the interaction between the user and the system, the errors that the system can detect and handle, and the environmental conditions in which the system functions are part of the requirements. The system structure, the implementation technology selected to build the system, the system design, the development methodology, and other aspects not directly visible to the user are not part of the requirements.

 

Requirements elicitation includes the following activities:

  1. Identifying actors
  2. Identifying scenarios
  3. Identifying use cases
  4. Refining use cases
  5. Identifying relationships among use cases
  6. Identifying nonfunctional requirements

Elicitation concepts

Functional requirements describe the interactions between the system and its environment independent of its implementation. The environment includes the user and any other external system with which the system interacts… The functional requirements focus only on the possible interactions between the system and its external world. This description does not focus on any of the implementation details.

Nonfunctional requirements describe aspects of the system that are not directly related to the functional behavior of the system. Nonfunctional requirements include a broad variety of requirements that apply to many different aspects of the system, from usability to performance. The FURPS+ model2 used by the Unified Process [Jacobson et al., 1999] provides the following categories of nonfunctional requirements:

Elicitation Activities

Identifying Actors: Actors represent external entities that interact with the system. An actor can be human or an external system. In the SatWatch example, the watch owner, the GPS satellites, and the Webify Watch serial device are actors (see Figure 4-4). They all exchange information with the SatWatch.

Figure 4-4 Actors for the SatWatch system. WatchOwner moves the watch (possibly across time zones) and consults it to know what time it is. SatWatch interacts with GPS to compute its position. Webify Watch upgrades the data contained in the watch to reflect changes in time policy.

The first step of requirements elicitation is the identification of actors. This serves both to define the boundaries of the system and to find all the perspectives from which the developers need to consider the system. When the system is deployed into an existing organization (such as a company), most actors usually exist before the system is developed: they correspond to roles in the organization. During the initial stages of actor identification, it is hard to distinguish actors from objects. For example, a database subsystem can at times be an actor, while in other cases it can be part of the system. Note that once the system boundary is defined, there is no trouble distinguishing between actors and such system components as objects or subsystems. Actors are outside of the system boundary; they are external. Subsystems and objects are inside the system boundary; they are internal.

Thus, any external software system using the system to be developed is an actor. When identifying actors, developers can ask the following questions:

Questions for identifying actors:

• Which user groups are supported by the system to perform their work?

• Which user groups execute the system’s main functions?

• Which user groups perform secondary functions, such as maintenance and administration?

• With what external hardware or software system will the system interact?

Identifying Scenarios: A scenario is “a narrative description of what people do and experience as they try to make use of computer systems and applications” [Carroll, 1995]. A scenario is a concrete, focused, informal description of a single feature of the system from the viewpoint of a single actor. Scenarios cannot (and are not intended to) replace use cases, as they focus on specific instances and concrete events (as opposed to complete and general descriptions). However, scenarios enhance requirements elicitation by providing a tool that is understandable to users and clients…

Figure 4-6 is an example of scenario for the FRIEND system, an information system for incident response. In this scenario, a police officer reports a fire and a Dispatcher initiates the incident response.

Scenarios can have many different uses during requirements elicitation and during other activities of the life cycle. Below is a selected number of scenario types taken from [Carroll, 1995]:

As-is scenarios describe a current situation. During reengineering, for example, the current system is understood by observing users and describing their actions as scenarios.

Visionary scenarios describe a future system. Visionary scenarios are used both as a point in the modeling space by developers as they refine their ideas of the future system and as a communication medium to elicit requirements from users.

Evaluation scenarios describe user tasks against which the system is to be evaluated. The collaborative development of evaluation scenarios by users and developers also improves the definition of the functionality tested by these scenarios.

Training scenarios are tutorials used for introducing new users to the system. These are step-by-step instructions designed to hand-hold the user through common tasks.

In requirements elicitation, developers and users write and refine a series of scenarios in order to gain a shared understanding of what the system should be. Initially, each scenario may be high level and incomplete, as the warehouseOnFire scenario is. The following questions can be used for identifying scenarios.

Questions for identifying scenarios

• What are the tasks that the actor wants the system to perform?

• What information does the actor access? Who creates that data? Can it be modified or removed? By

whom?

• Which external changes does the actor need to inform the system about? How often? When?

• Which events does the system need to inform the actor about? With what latency?

Identifying Use Cases: A scenario is an instance of a use case; that is, a use case specifies all possible scenarios for a given piece of functionality. A use case is initiated by an actor. After its initiation, a use case may interact with other actors, as well. A use case represents a complete flow of events through the system in the sense that it describes a series of related interactions that result from its initiation…Figure 4-7 depicts the use case ReportEmergency of which the scenario warehouseOnFire (see Figure 4-6) is an instance

Generalizing scenarios and identifying the high-level use cases that the system must support enables developers to define the scope of the system. Initially, developers name use cases, attach them to the initiating actors, and provide a high-level description of the use case as in Figure 4-7. The name of a use case should be a verb phrase denoting what the actor is trying to accomplish. The verb phrase “Report Emergency” indicates that an actor is attempting to report an emergency to the system (and hence, to the Dispatcher actor). This use case is not called “Record Emergency” because the name should reflect the perspective of the actor, not the system. It is also not called “Attempt to Report an Emergency” because the name should reflect the goal of the use case, not the actual activity

Attaching use cases to initiating actors enables developers to clarify the roles of the different users. Often, by focusing on who initiates each use case, developers identify new actors that have been previously overlooked.

Describing the flow of events of a use case enables developers and clients to discuss the interaction between actors and system. This results in many decisions about the boundary of the system, that is, about deciding which actions are accomplished by the actor and which actions are accomplished by the system.

Identifying relationships between actors and Use Cases: Communication relationships between actors and use cases represent the flow of information during the use case. The actor who initiates the use case should be distinguished from the other actors with whom the use case communicates. By specifying which actor can invoke a specific use case, we also implicitly specify which actors cannot invoke the use case. Similarly, by specifying which actors communicate with a specific use case, we specify which actors can access specific information and which cannot. Thus, by documenting initiation and communication relationships among actors and use cases, we specify access control for the system at a coarse level.

Identifying Initial Analysis Objects: To establish a clear terminology, developers identify the participating objects for each use case. Developers should identify, name, and describe them unambiguously and collate them into a glossary.3 Building this glossary constitutes the first step toward analysis, which we discuss in the next chapter. The glossary is included in the requirements specification and, later, in the user manuals. Developers keep the glossary up to date as the requirements specification evolves. The benefits of the glossary are manyfold: new developers are exposed to a consistent set of definitions, a single term is used for each concept (instead of a developer term and a user term), and each term has a precise and clear official meaning. The identification of participating objects results in the initial analysis object model. The identification of participating objects during requirements elicitation only constitutes a first step toward the complete analysis object model.

During requirements elicitation, participating objects are generated for each use case. If two use cases refer to the same concept, the corresponding object should be the same. If two objects share the same name and do not correspond to the same concept, one or both concepts are renamed to acknowledge and emphasize their difference. This consolidation eliminates any ambiguity in the terminology used.

Identifying Non-functional requirements: Nonfunctional requirements describe aspects of the system that are not directly related to its functional behavior. Nonfunctional requirements span a number of issues, from user interface look and feel to response time requirements to security issues. Nonfunctional requirements are defined at the same time as functional requirements because they have as much impact on the development and cost of the system.

Documenting Requirements Elicitation: The results of the requirements elicitation and the analysis activities are documented in the Requirements Analysis Document (RAD). This document completely describes the system in terms of functional and nonfunctional requirements.Figure 4-16 is an example template for a RAD:

Written by: Larry Francis Obando – Technical Specialist

Escuela de Ingeniería Eléctrica de la Universidad Central de Venezuela, Caracas.

Escuela de Ingeniería Electrónica de la Universidad Simón Bolívar, Valle de Sartenejas.

Escuela de Turismo de la Universidad Simón Bolívar, Núcleo Litoral.

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