|December 08, 2011|
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Use case analysis is a major technique used to find out the functional requirements of a software system. Use case, an important concept in use case analysis, represents an objective user wants to achieve with a system. It can be in text form, or be visualized in a use case diagram, like this:
The beauty of use case is that it aims at describing a system from external usage viewpoint, rather than from developer's perspective. Therefore, writing use case can be the deciding factor for building a system that meets users' needs. In this tutorial, you will learn how to make use of various functions to write effective use case.
A use case is an objective user(s) wants to achieve with a system. Use cases are named with verb or verb + noun phrase. It is usually short yet descriptive enough to describe a user objective. You are encouraged to use concrete and specific verbs and nouns to avoid ambiguity. Verbs like 'do' and 'perform' and nouns like 'data' and 'information' should be avoided whenever possible.
User performs use case to yield observable goal. Take online hotel reservation system as an example. "Make reservation" is undoubtedly a use case as this is what user wants to achieve with the system. The function of looking up a hotel on an online map can also be what a user needs. However, it is not a use case because it is only a part of the reservation process instead of an objective.
Some analysts try to make use of use case to describe user interface requirements (e.g. support multiple look & feel), performance requirements (e.g. load in background), deployment arrangement (e.g. ready server) and even implementation level (internal) requirements (e.g. construct database). All these are wrong and will not help you in identifying the objectives user want to achieve, thus the functions the system should deliver.
A use case can be a simple title that describe a user goal, such as "Make Reservation" in hotel reservation system. It is intended to provide an overview of what the user want without knowing how to achieve the goal. In order to identify how to achieve a goal, you can also document its scenario and steps (i.e. interaction) involved between user and system, with main flow, exceptional flow, conditional flow, etc.
In VP-UML, there are some other features that helps you leverage the power of use case. You can use the use case grid to tidily list the use cases in model, and to model the other aspects of use case by elaborating use case with sub-diagram of another diagram type (e.g. use a sub-requirement diagram to model the additional functional and performance requirements).
A use case diagram is a kind of Unified Modeling Language (UML) diagram defined by Object Management Group (OMG), created for use case analysis. Use case diagram provides a graphical overview of goals (represented by use cases) users (represented by actors) want to achieve by using the system (represented by system boundary but is often opt out in diagram). Use cases in a use case diagram can be organized and arranged according to their relevance, level of abstraction and impacts to users. They can be connected to show their dependency, inclusion and extension relationships. The main purpose of modeling use case with use case diagram is to establish a solid foundation of the system by identifying what the users want. Base on the result of analysis you can move forward to study how to fulfill those user needs.
A use case diagram is mainly formed by actors, use cases and associations (connectors).
An actor is any person or external system that interacts with the system in achieving a user goal. There are two kinds of actor - primary and secondary. Primary actor is anyone or thing that interacts with the system to gain direct benefit. Secondary actor is anyone or thing that involve in achieving a use case yet, they do not gain direct benefit from the system. Very often, secondary actor is someone who assist the primary actor to achieve a use case.
In this tutorial, we will make use of an online hotel reservation system as an example to demonstrate how to write effective use case with VP-UML. Let's begin by drawing a use case diagram. We will carry on with writing effective use case with the resulting design.
A named use case depicts the 'what' aspect of a use case by telling you what the users need. The 'how' aspect of use case that explains how a user goal can be accomplished can be further analyzed by using flow of events, which is a technique for analyzing the interaction between actor and system in accomplishing a use case.
Flow of events constitutes a high level user-and-system conversation, which aims to find out the intents or actions of actor, known as inputs, and how system react to those actor inputs. You should be concise when deciding what to include in the events flow. Do not include how system process user's input internally, or even implementation detail like an insertion of database record. This is wrong as flow of events, in fact use case analysis, is aimed to view things from actor's perspective. Implementation details is not of flow of events' interest. Implementation detail can, however, be modeled with sequence diagram in form of sub-diagram of use case.
Let's write the flow of events of use case. We will describe some of the important skills in writing flow of events shortly. Now, we go back to our use case diagram first.
|User input||System response|
|Click on a hotel's logo to read its detail|
|Display hotel details|
|Click Book Now|
|Display payment form|
|Enter customer details, billing information and click Submit.|
|Validate billing information and display the result.|
|User input||System response|
|Select Rush Reservation option|
|Display a customer code and telephone number. Ask user to call for a special arrangement.|
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