Concept of unified use case model for software requirements specification

Use case is a specification of actions sequence executed by system for achieving a significant result for any or all actors. The use cases model is used for capturing both structural and behavioral parts of product's functional requirements in current developing processes. Unified Modelling Language offers several graphical representations of use cases scenarios, such as activity diagrams, rather than textual formats. Nevertheless, there are many different textual formats used for use cases scenarios specifications, e.g. Wirfs-Brock's two-column table style and Cockburn style. Different stakeholders may prefer a different specification format, e.g. customer may prefer textual formats, while developers may prefer graphical formats. In addition, use cases can be used for subsequent development stages (e.g. analysis, design, testing). For use cases to be automatically traced into these stages they must be formally defined. Such definition is usually possible in the graphical representation but not in textual. When several formats are required, following approaches can be applied: 1. Use case should be created manually in every required formats and every change should be to be synchronized between used formats to keep requirements specifications up to date. Such synchronization is time consuming and error-prone process due to the human factor. To simplify this process, the development of the unified use cases model that will support different formats of use cases scenarios is proposed. 2. Use case is specified in a single format. Then transformation into required formats is performed by special tools. Any changes in the use case are made in the primary format of use case thus limiting an ability to edit derived formats (subsequent transformation will cause a loss of derived formats edits and these edits will not affect primary format). 3. An approach proposed in this paper is to create the unified use case model that will support different formats of use cases scenarios and not restricting scenario editing to one of them. There are the following use cases features: 1. Use case has multiple scenarios that can either succeed or fail. 2. Each scenario is described by actions sequence. 3. Each action is executed by a specific actor. 4. A set of these scenarios can be infinite due to loops and cannot be applied. To make scenarios more useful, a main success scenario can be selected and other scenarios can be defined as extensions to specific actions of a main success scenario. The model proposed in this paper is an extension of UML 2.4.1 (formally published as ISO/IEC 19505-1 and ISO/IEC 19505-2). It is based on the following packages: Classes::Kernel and CommonBehaviors::BasicBehaviors and consists of the following: 1. Scenarios::Kernel package that contains the unified model kernel. In this package use case scenarios, scenario nodes and flows are defined. 2. Scenarios: :Nodes package that contains specific scenario nodes, such as scenario actions, triggers, handlers, decisions, etc. 3. Scenarios::Partitions package that defines partitions used for scenario nodes grouping. 4. Scenarios::EventTypes package that define events types, e.g. generic errors, system events, user events, time events, etc. As a result, the unified use case model that has its own graphical notation is developed. In addition, prototype of a tool that supports editing use cases in the developed model and transformation between Wirfs-Brock's textual format and model's graphical notation is implemented.

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