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Requirement Analysis for Embedded System. Nien-Lin Hsueh. Outline. Topic 1: Requirements Analysis of Real-Time Systems Topic 2: Analysis: Object Domain Analysis Topic 3: Analysis: Defining Object Behavior Reference: - PowerPoint PPT Presentation
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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
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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
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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
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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.
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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
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Conti.
Advantage:– To capture a broad view of the primary functionalit
y 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
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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
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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
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Decomposition of Deliver Anesthesia Use Case
legacy systems
The system-level use case is decomposed into 12 included use cases
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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
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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
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Characterization of Use Cases
optional and need only be entered if it is otherwise impossible to disambiguate the use
case in question
provides a location for a high-level statement as to the user purpose for the capability of t
he use case
detailed textual requirements may be state
d.
conditions that must be true before the use
case begins
conditions that are guaranteed to be true by the system after the u
se case is finished
commonly used to hold (QoS requirements
for the use case)
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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.
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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
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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.
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1.2.3 Use Case Relations
Using <<includes>>: Common capability is required for both these use case
s
Generalization relation
Using <<extend>>: the scheduled downlink can optionally compress images, either using lossy or non-lossy compression algorithms.
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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 e
xtracted 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.
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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».
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1.2.4 Using Use Cases
Phase Application of Use Cases
Analysis Suggest large-scale partitioning of the domainProvide structuring of analysis objectsClarify system and object responsibilitiesCapture and clarify new features as they are added during developmentValidate analysis model
Design Validate the elaboration of analysis models in the presence of design objects
Coding Clarify purpose and role of classes for codersFocus coding efforts
Testing Provide primary and secondary test scenarios for system validation
Deployment Suggest iterative prototypes for spiral development
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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 activity diagram at the highest level and from there determine how these might be mapped into use cases.
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Conti.
The analyst can sit with the customer and ask probing questions, such as these:
– What are the primary functions of the system?– What are the secondary functions 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 sequence of events and data needed to realize the scenario– The variations on the scenario that are possible (other related sce
narios)
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Use Cases in Development
Use cases are used primarily during requirements analysis Once the system is broken down into its primary subsystems, u
se 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 objects collaborating within the system to realize the specific use case
The need for additional use cases having to do with the concurrency and component models is normally uncovered during architectural design as well
In testing, the use cases and their associated scenarios form the key set of tests to be applied to the system.
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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
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Conti.
Problems of the use case “Set Ventilator Tidal Volume”– What is the maximum value that can be selected? What is the minimum val
ue that can be selected?– What is the accuracy of the delivery of tidal volume with respect to its set v
alue? +/- 10 ml? +/- 5%?– Are there different ranges, such as one range for adults, another for pediatr
ics, 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, su
ch as setting patient age or weight?
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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 mo
difiers of the primary requirements.
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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
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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
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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
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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.
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Relating Text and Scenarios Internal «trace» stereotyped dependency relations can provide traceability inside the
model.
38 An example sequence diagram from the anesthesia machine
descriptive note names the sequence diagram, a brief description, and the preconditions and postconditions of t
he secnario
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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
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Deliver Anesthesia Collaboration
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?"
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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
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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.
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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 req
uirements model is possible The semantics and syntax of statecharts was describ
ed in UML introduction
44 Alarm On Critical Event Requirements
Even the requirements are relatively small, but they are still nonetheless nontrivial
to understand.
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Statecharts and Sequence Diagrams
Statecharts can be related to scenarios as well
Different operational scenarios take different paths through the statechart
State
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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
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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.
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1.3.4 Timing Diagrams
Similar in some ways to sequence diagrams, timing diagrams also represent scenarios
– Timing diagram: emphasize change in value or state over time – Sequence diagram: emphasize sequences of message exchange– isomorphic and able to represent the same information, but their p
urpose is different Timing diagrams focus on the qualities of service having to do
with time, such as – execution time – jitter – deadlines – periodicity – how they affect the state of the system
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Exercise 1.1
For your mobile phone application, following the guideline of chapter to build the analysis model and documentation
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Exercise 1.2
For your robot application, following the guideline of chapter to build the analysis model and documentation
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The object discovery process Connecting the object model with the use case model Key strategies for object identification Identify object association Object attributes Discovering candidate classes Class diagram
– Associative classes– Generalization relationships
Exercise
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Object behavior– Simple behavior– State behavior– Continuous behavior
Defining object state behavior– Cardiac pacemaker example– Calculator example– Event hierarchies
Interactions– Sequence diagrams
Defining operations– Type of operations– Strategies for defining operations
Exercise
TOPIC 3: Analysis: Defining Object Behavior
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Use cases
//introduce the following topics Actors Use case and text Use case relations Using use cases
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Detailing the use cases
//introduce the following topics Scenarios for use cases Statecharts Activity diagrams Time diagrams (This is special for real time
systems)