Requirement Analysis for Embedded System

1. Requirement Analysis for Embedded System Nien-Lin Hsueh

2. Outline • Topic 1: Requirements Analysis of Real-Time Systems • Topic 2: Analysis: Object Domain Analysis • Topic 3: Analysis: Defining Object Behavior • Reference: • B.P. Douglass, Real Time UML- Advances in the UML for Real-time Systems (3rd), Addison Wesley, 2004

3. 編輯說明（薛念林） • 主要參考 Douglass的書籍編輯 • 目前圖形是暫時劃上去的，將來會換成 Together 的圖 • UML 語法說明部分不在此章說明，而是另外投影片介紹 • Douglass對UML的說明是放在 chapter 2-3 • 每個 TOPIC 都有 exercise，以手機與機器人為主 • 圖片說明用『動畫』呈現，如果自有檔到請用『播放模式』觀看

4. TOPIC 1: Requirements analysis of real-time systems • Requirements • Use cases • Actors • Use cases and text • Use case relations • Identifying use cases • Detailing the use cases • Scenarios for use cases • Statechart diagrams • Activity diagrams • Timing diagrams • Exercise

5. Rapid Object-Oriented Process for Embedded Systems (ROPES) Process

6. Here we are

7. 1.1 Requirements • Specifications of what a system must do independently of how the system is designed • Specified in UML profile for system engineering • Requirement taxonomy • helps us understand the relation of requirements to the system and its test, as well as understand how requirements tend to be represented.

8. Requirement Taxonomy in UML 2.0 Three types of requirements

9. 1.2 Use Cases • A named capability of a structural entity in a model • Use case define a system-level capability without revealing or implying any particular implementation or design of that capability • Functional view of the system • Are implemented by collaborations of classes • Use case exist within a structural context, the context consists of the system and actors • To be a use case • The capability must return a result visible to one or more actors

10. Use case diagram constraint actor Use case boundary Use case relation

11. Conti. • Advantage: • To capture a broad view of the primary functionality of the system in a manner easily grasped by non-technical users • Become a centralized roadmap of the system usage scenarios for people specifying the requirements of the system

12. More about use cases • Use case are not themselves requirements • They organize requirements into chunks, based on the organizational principle of common operational capability • They organize requirements into chunks, based on the organizational principle of common operational capability • This principle can be used regardless of the more detailed representation of the requirements themselves, whether it is text, sequence diagrams, state machines, or activity diagrams

13. 1.2.1 Actor • An actor is an object outside that scope of the system under consideration that has significant interactions with it • Mis-concept: an actor must be human users of the system

14. Air Traffic Control System Use Cases only Controller is a human user legacy systems

15. Decomposition of Deliver Anesthesia Use Case The system-level use case is decomposed into 12 included use cases legacy systems

16. 1.2.2 Use case and text • Developers too familiar with the "Victorian novel" approach to capturing requirements • the generation of hundreds or thousands of pages of text specifying requirements • Using text alone to capture requirements is problematic because text is difficult to make simultaneously precise, unambiguous, and understandable • Textural requirements have different interpretations to arise • Textural requirements documents are often conflicting, having requirements mismatched in different parts of large documents • It is possible to employ a use case approach and specify requirements entirely in text

17. Use case and text (conti.) • UML provides more formal languages (statecharts, activity diagrams, and sequence diagrams) for capturing the details of requirements • text is still useful in conjunction with these more formal approaches • Different authors have defined different contents and formats for such textual characterizations

18. optional and need only be entered if it is otherwise impossible to disambiguate the use case in question Characterization of Use Cases detailed textual requirements may be stated. provides a location for a high-level statement as to the user purpose for the capability of the use case conditions that are guaranteed to be true by the system after the use case is finished conditions that must be true before the use case begins commonly used to hold (QoS requirements for the use case)

19. 1.2.3 Use case relations • The UML defines three distinct relationships among use cases • Generalization • one use case is a more specialized or refined version of another. • For example, the Validate User use case can be specialized into Check Password, Check Fingerprint Scan, and Check Retinal Scan use cases.

20. Conti. • «include» is used when the capability described in the client use case uses the capability described in another use case. • only be used when the behavior is shared among two or more use cases or • is mapping the "part" use case to a system architectural component and is required for all of the client use case scenarios

21. Conti. • «extend» is used when one use case provides an optional additional capability within a client use case. • This optional capability is inserted at a named extension point.

22. 1.2.3 Use Case Relations Using <<extend>>: the scheduled downlink can optionally compress images, either using lossy or non-lossy compression algorithms. Using <<includes>>: Common capability is required for both these use cases Generalization relation

23. Conti. • Spacecraft turns in order to achieve two capabilities in the Spacecraft system: • taking a picture (under the premise that you must point at something to take its picture) and • executing a scheduled downlink of information. • This common capability is required for both these use cases, it is extracted out and put into its own use case. The two means by which the spacecraft can be turned are specialized use cases of the Adjust Attitude base use case. • In one case, rockets can be fired to turn the spacecraft, and in the other, reaction wheels are activated. Finally, the scheduled downlink can optionally compress images, either using lossy or non-lossy compression algorithms. Because this is an option, it is shown as an «extends» relation.

24. A caution in building use case diagram • Too often, beginners overuse the use case relations and use them to capture the wrong things. • Remember that you can model the requirements of systems without using generalization, «extends», or «includes».

25. 1.2.4 Using Use Cases

26. 1.2.5 Identifying Use Cases • Four primary approaches to identifying use cases: • List the primary capabilities of the system, then identify the actors and scenarios within each use case. • Identify the actors to the system and the messages they send or receive (the scenarios), and then group them into use cases. • Start with system scenarios, identify the actors that participate in them, and then lump them into use cases. • Identify a system workflow with an activitydiagram at the highest level and from there determine how these might be mapped into use cases.

27. Conti. • The analyst can sit with the customer and ask probing questions, such as these: • What are the primaryfunctions of the system? • What are the secondaryfunctions of the system? • Why is this system being built? What is it replacing and why? • The analyst must then identify the following for each use case: • The role the actors and system play in each scenario • The interactions (flows) necessary to complete the scenario • The sequenceofevents and data needed to realize the scenario • The variations on the scenario that are possible (other related scenarios)

28. Use Cases in Development • Use cases are used primarily during requirements analysis • Once the system is broken down into its primary subsystems, use cases may be applied to each of the subsystems in turn to define its requirements with respect to the other elements of the system • As the object model becomes fleshed out, the system- and subsystem- level use cases may be refined in more detail, replacing the system with the objectscollaborating within the system to realize the specific use case • The need for additional use cases having to do with the concurrency and componentmodels is normally uncovered during architecturaldesign as well • In testing, the use cases and their associated scenarios form the key set of tests to be applied to the system.

29. Use Cases in Development

30. 1.3 Detailing the Use Cases • A name alone isn't enough to understand what a use case means • Use case “Set Ventilator Tidal Volume” is not clear • Brief description: the user turns a knob and sets the amount of mixed breathing gas pumped out per breath for the ventilator

31. Conti. • Problems of the use case “Set Ventilator Tidal Volume” • What is the maximum value that can be selected? What is the minimum value that can be selected? • What is the accuracy of the delivery of tidal volume with respect to its set value? +/- 10 ml? +/- 5%? • Are there different ranges, such as one range for adults, another for pediatrics, and another for neonates? • What happens if the knob is turned accidentally—does tidal volume change directly or is an explicit confirmation required? • If there is a confirmation, can the user cancel the operation? • What happens if the user tries to set a different value, say respiration rate, before confirmation? • How does the user know whether a value is currently being set (waiting for confirmation)? • Does anything have to either precede or come after setting tidal volume, such as setting patient age or weight?

32. Conti. • Two categories of approaches are possible. • A specification can be written for the requirements. • This specification can be either informal (text) or formal using a formal or semi-formal language such as statecharts or activity diagrams. • To provide examples of operational usage • Three kinds of requirements and their representation • Functional requirements are best captured in specifications. • Operational requirements are best captured in scenarios or activity diagram workflows. • QoS requirements are added to both representations as modifiers of the primary requirements.

33. 1.3.1 Scenarios for Use Cases • A scenario is a particular actor-system interaction corresponding to a use case • it is a specific example of a use case execution in the system's operational environment • it models order-dependent message sequences among object roles collaborating to produce system behavior in its operational environment • Each use case will have infinite set of scenarios • but it is only necessary to capture the ones that are interestingly different

34. Conti. • Use cases are realized by collaborations of objects inside the system working together • In the earlier phase, internal object are skipped • if the use case diagram has two actors and the system, only three objects can appear in the scenario • Later, once the system is opened up and is under design, internal objects are identified • Building and analyzing scenarios is a creative process of discovery

35. Conti. • Three primary scenario representations exist within the UML: • Sequence diagrams • emphasize messages and their sequence • In use case analysis, sequence diagrams are preferred over communication diagrams. • Communication diagrams • are less popular and tend to stress the system object structure • Communication diagrams are not used until the object model of the system stabilizes (and even then, many people prefer sequence diagrams anyway). • Timing diagrams • are best applied when the requirements are highly time-sensitive • less applied, but are useful when timing is crucial. • We will primarily focus on sequence diagrams in this chapter

36. 1.3.1.1 Sequence Diagrams for Requirements Capture • Even after adopting the UML, many organizations continue using text as an adjunct to scenarios and statecharts to capture requirements in a more formal way.

37. Internal «trace» stereotyped dependency relations can provide traceability inside the model. Relating Text and Scenarios

38. An example sequence diagram from the anesthesia machine descriptive note names the sequence diagram, a brief description, and the preconditions and postconditions of the secnario

39. Conti. • A measure of goodness of the architecture or object model is that the design can realize the operational scenarios defined at the system level • If it can realize all of the scenarios defined at the system level, then the architecture or object model is good

40. Shows three subsystems working together to realize the Deliver Anesthesia use case A very relevant question to ask of this collaboration is "Is this good?" That is the same as asking, "Does this collaboration meet its requirements?" Deliver Anesthesia Collaboration

41. 1.3.1.2 Capturing QoS Requirements on Sequence Diagrams • The single most differentiating characteristic of real-time systems is their concern and treatment of time • However, most timing requirements are derived rather than primary requirements • Because these requirements are derived, it is all too common for them to be missed by systems designers, leading to unstable system performance. • It is vital that these time constraints be captured as part of the system model so that they can be treated appropriately

42. Conti. • A number of time values can be captured. • Time values that are QoS requirements • can be captured as constraints applied against the actions or messages. • Time values that are estimates, used for the purpose of analysis, can be captured as tagged values • Tagged values are shown as { property = value} pairs in constraints.

43. 1.3.2 Statecharts • Statecharts are a formal behavioral language that lends itself to the specification of use case behavior • The use case formal language has a number of advantages over text: • It is verifiable, through mathematical analysis or execution. • It is precise, and not nearly as likely to be misinterpreted. • It is generative, meaning that creation of an executable requirements model is possible • The semantics and syntax of statecharts was described in UML introduction

44. Alarm On Critical Event Requirements Even the requirements are relatively small, but they are still nonetheless nontrivial to understand.

45. Alarm On Critical Event Statechart It is more easier to understand

46. Statechart and text Statecharts can be related to the text in a straightforward fashion

47. Statecharts can be related to scenarios as well Different operational scenarios take different paths through the statechart Statecharts and Sequence Diagrams State

48. 1.3.3 Activity Diagrams • In UML 1.x, activity diagrams are isomorphic with statecharts. • In UML 2.0 they are a superset, since their semantic basis is now token flow semantics, which represent Turing machines

49. Conti. • The most common use of activity diagrams in the development of real-time and embedded systems will still most likely be in their use as concurrent flowcharts • Activity diagrams are most commonly used when a behavior can be specified as a set of control flows with operators (sequence, alternative, loop, fork, and join) • Activity diagrams are most commonly used to represent algorithms that, once initiated, proceed inexorably to their conclusion • Statecharts can represent algorithms as well by using null-triggered (anonymous) events connecting states although their most common use is with explicit triggering events.

50. Display Waveform Activity Diagram