Adding Soft Real-Time Requirements in a Step-Wise Development Process

ion, depending on the purpose of the particular specification. Figure 1 Sending SMS messages from a web portal. The user enters the message text and address to the web portal (1), which calls the web service provided by the telephone company (2). The SMS message is then sent to the mobile phone (3) 69 Telektronikk 1.2009 ISSN 0085-7130 © Telenor ASA 2009 70 Telektronikk 1.2009 informally illustrated in Figure 1; in Section 2 we explain the scenario further and show how to express it with a UML sequence diagram. For specifying telecom systems, UML sequence diagrams and similar notations are very well suited, as they convey communication scenarios in an easily understandable manner. Sequence diagram specifications can be used for a number of purposes. They can be used for verifying and validating requirements with clients, as a system description for programmers during development, for testing or verification of the system, or as maintenance documentation [1]. Depending on how far a development process has progressed and the purpose of the specification, different levels of abstraction/refinement may be suitable. Hence, we need to understand what it means for one specification to be a refinement of another specification. The approach we propose utilizes probabilistic STAIRS (pSTAIRS) [2], [3], which is a specification language based on UML 2.x sequence diagrams that allows probabilistic requirements to be expressed. Unlike UML [4], pSTAIRS is supported by a formal semantics and definitions of refinement. This reduces ambiguity and facilitates mathematical proof of properties. In order to ensure that the proposed procedure is useful in practice, as well as theoretically sound, we have ensured that the procedure has a number of important properties that we motivate and identify below. First of all, few developers will have detailed knowledge of the formal semantics and definitions of pSTAIRS. Therefore, an intuitive understanding of pSTAIRS should suffice to be able to apply the procedure. Property 1 The procedure is based on a simple syntactic manipulation of the sequence diagram, without requiring detailed knowledge of the formal semantics and definitions of pSTAIRS. When working with a sequence diagram, considering only a part of a diagram is clearly simpler than having to consider the whole diagram. Furthermore, ensuring that only a part of the diagram needs to be manipulated allows different developers to work on different parts of the diagram independently of each other. Property 2 The procedure is local in the sense that only the part of the diagram to which the soft realtime requirement applies needs to be modified by the application of the procedure. Introducing new soft real-time requirements to a specification means adding more constraints on the specified system. Hence, if a system complies with the new specification, then it clearly also complies with the original specification that did not include the additional constraints. Therefore, introduction of new soft real-time requirements should be a refinement step. Property 3 The specification obtained by applying the procedure is a refinement of the original specification. The rest of this article is organized as follows: In Section 2 we explain the example that will be used to demonstrate the procedure and show how it can be expressed by a UML sequence diagram. An introduction to pSTAIRS is given in Section 3, before the procedure for adding soft real-time requirements is presented and demonstrated in Section 4. Section 5 explains refinement and states some general refinement principles related to real-time requirements. Section 6 discusses the approach, before we conclude in Section 7. 2 Example case We now explain the example that will be used to demonstrate the approach, and show how the scenario is expressed by a sequence diagram. The example involves a service where a user can send SMS messages to mobile phones from a web portal. This functionality is facilitated by the cooperation between a telephone company and the company providing the web portal through a so-called