Extracting Quality Scenarios from Functional Scenarios
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Dr. Amir Tomer's presentation discusses the importance of defining quality attributes (non-functional requirements) alongside functional requirements in software systems, particularly in the context of extracting quality scenarios from functional use cases. It emphasizes how system architecture is heavily influenced by quality attributes such as performance and security, which may necessitate architectural changes or additional functionality. The document outlines a process for identifying quality scenarios through functional analysis, using a car navigation system as a practical example.
Extracting Quality Scenarios from Functional Scenarios
- 1. Dr. Amir Tomer, PMP, CSEP Head of Department Software and Information Systems Engineering amir@amirtomer.com
- 2. Quality Attributes Quality Attributes & Constraints (Non-functional Requirements) • Define the solution quality – How well does the system carry out its functional requirements? • Performance • Availability • Security • Interoperability • … Functional Requirements • Define the solution contents – What is the system required to do? Quality Attributes & Constraints Design / Implementation Functional Requirements Solution Domain Dr. Amir Tomer, ISQ Conference, 2014 2 Extracting Quality Scenarios from Functional Scenarios
- 3. The Conflict • System specifications concentrate mainly on functional requirements – They describe the purpose of the system – They are conceptual (implementation-free) – They remain valid even when quality attributes change Whereas… • The system architecture is driven mainly by quality attributes, e.g. – Using redundancy to improve availability – Using load-balancing to improve performance – Using fire walls to improve security – … These do not affect the basic functionality, but may impose additional functionality to provide quality Dr. Amir Tomer, ISQ Conference, 2014 Extracting 3 Quality Scenarios from Functional Scenarios
- 4. Car Navigation System Define Trip Locate Self Send Report Driver Navigate along Route Maps Provider Get Map Get Road Status GPS Deviation from Route Reports Provider Calculate Route «include» «include» «include» «include» «include» «include» «extend» Use Cases as Functional Scenarios • Functional Scenarios are often specified as Use Cases • A Use Case is a service (function) performed by a system through interaction with a set of external Actors, of two kinds – Primary Actors, who initiate Use Cases to achieve their goals – Supporting Actors, with whom the system interacts for accomplishing the Primary Actors’ goal • UML’s Use Case Diagram depicts the set of Use Cases within a system and their associated Actors • A single Use Case may have various scenarios, but these are not depicted in the Use Case Diagram Example: Use Case Diagram of a Car Navigation System Dr. Amir Tomer, ISQ Conference, 2014 Extracting 4 Quality Scenarios from Functional Scenarios
- 5. Use Case Specifications (Functional Scenarios) • The various courses of interaction through a certain Use Case are captured in a Use Case Specification, which comprises the following ingredients: UC-# UC Name Actors & Goals Primary actors and their stated goals Supporting actors and their roles Other Stakeholders Stakeholders that are not actors and their interests (business benefits, quality attributes) in this specific UC Pre-conditions Conditions/presumptions which must hold to enable the use case Post-Conditions Desirable (success) results after use case completion Trigger The event (either by a primary actor or by the system itself) that “starts-up” the UC Main Success Scenario (MSS) A set of steps which describe the “most straightforward” interaction to achieve the post-conditions Branches Other possible scenarios which may lead either to success (alternatives) or failure (exceptions) in fulfilling the post-conditions Dr. Amir Tomer, ISQ Conference, 2014 Extracting 5 Quality Scenarios from Functional Scenarios
- 6. Functional Analysis and Functional Architecture • From the system’s point of view, a functional scenario is a sequence of applying system functions • Functional Analysis is the process of identifying system functions and allocating each function to one or more functional components • The set of functional components, their internal and external interface can be considered as Functional Architecture • For example, let us consider a simple car navigation system, arranged in a client-server architecture, as follows: Car Unit Maps & Reports Server GPS Satellite Dr. Amir Tomer, ISQ Conference, 2014 Extracting 6 Quality Scenarios from Functional Scenarios
- 7. Functional Scenario – Example • A typical Main Success Scenario (MSS) of the “Navigate Along Route” use-case might look as follows: Trigger The driver presses the “Start Navigation” button Main Success Scenario (MSS) 1. The system enters “Navigate” Mode 2. The system retrieves the pre-defined route (See “Define Trip” UC) 3. The system retrieves a relevant map area 4. If the locally stored map data is missing – the system downloads map data from the server 5. The system displays the map and route 6. The system gets the current location from the GPS 7. If the location is off route – the system re-calculates the route (See “Calculate Route” UC) 8. Back to step 4 Dr. Amir Tomer, ISQ Conference, 2014 Extracting 7 Quality Scenarios from Functional Scenarios
- 8. Functional Analysis and Functional Architecture – Example • Applying the Functional Analysis process to the navigation scenario might yield the following functional architecture: GPS Signals User Commands User Displays Locator GUI Map Download Nav igation App Map Serv ices Allocated Function (Extracted from the functional scenario): “download map data from the server” Maps Location Commands Displays Static Binding Dr. Amir Tomer, ISQ Conference, 2014 Extracting 8 Quality Scenarios from Functional Scenarios
- 9. Quality Scenarios* • Quality Scenarios specify how to prevent a fault from becoming a failure *Bass, L., Clements, P. and Kazman, R., Software Architecture in Practice, 3rd Edition, Addison-Wesley, 2012 Dr. Amir Tomer, ISQ Conference, 2014 Extracting 9 Quality Scenarios from Functional Scenarios
- 10. Relations between Functional Scenarios and Quality Scenarios • Quality scenarios do not stand by themselves – They usually occur during functional scenarios – Therefore it would be beneficial to associate the ingredients of quality scenarios with those of functional scenarios (use cases) as follows: • Actors • Pre-conditions • Trigger • Main Success Scenario • Branches – Alternatives – Exceptios • Post-conditions Source • Environment • Stimulus • Response • Response Measurement • Artifact • Any system element To which the relevant Functionality is allocated Dr. Amir Tomer, ISQ Conference, 2014 Extracting 10 Quality Scenarios from Functional Scenarios
- 11. Discovering Quality Attributes in the Functional Scenario • Quality attributes can be discovered in the functional scenarios by applying the “how well?” questions to the functions in the scenario – For example: Trigger The driver presses the “Start Navigation” button Main Success Scenario (MSS) Is the driver aware? (usability) 1. The system enters “Navigate” Mode 2. The system retrieves the pre-defined route (See “Define Trip” UC) 3. The system retrieves a relevant map area 4. If the locally stored map data is missing – the system downloads map data from the server 5. The system displays the map and route 6. The system gets the current location from the GPS 7. If the location is off route – the system re-calculates the route (See “Calculate Route” UC) 8. Back to step 4 Does the server respond within TBD seconds? (availability) Is the location precise? (performance) Dr. Amir Tomer, ISQ Conference, 2014 Extracting 11 Quality Scenarios from Functional Scenarios
- 12. Applying Response Tactics • Regarding the server availability issue of the last example – A stimulus (fault) occurs by getting no response from the server within TBD seconds – One of the tactics for handling availability is redundancy • So the server may be multiplied – This may add a branch (alternative) to the scenario, as follows: Trigger The driver presses the “Start Navigation” button Main Success Scenario (MSS) 1. The system enters “Navigate” Mode 2. The system retrieves the pre-defined route (See “Define Trip” UC) 3. The system retrieves a relevant map area 4. If the locally stored map data is missing – the system downloads map data from the server 5. The system displays the map and route 6. The system gets the current location from the GPS 7. If the location is off route – the system re-calculates the route (See “Calculate Route” UC) 8. Back to step 4 Branch A Alternative in step 4 of MSS: The server did not respond within TBD seconds 4A1. The system suggests an alternative server 4A2. Back to step 4 A new function! Dr. Amir Tomer, ISQ Conference, 2014 Extracting 12 Quality Scenarios from Functional Scenarios
- 13. Back to Functional Analysis • The newly discovered function should now be allocated to a functional component – If none of the components can handle this function, a new functional component may be added • The following modified architecture applies 3 architecture techniques – Redundancy – Mediation (“broker”) pattern – Late binding GPS Signals User Commands User Displays Locator GUI Map Download Nav igation App Map Broker Status 1..* Map Serv ices Static Binding Map Requests Dynamic Binding Maps Displays Location Commands Mediation Redundancy Dynamic Binding Dr. Amir Tomer, ISQ Conference, 2014 Extracting 13 Quality Scenarios from Functional Scenarios
- 14. Conclusion • Quality Scenarios can be extracted from Functional (use-case) scenarios – They are added as branches to the existing scenario – They may impose architectural changes • This process is applied iteratively during architectural design Dr. Amir Tomer, ISQ Conference, 2014 Extracting 14 Quality Scenarios from Functional Scenarios
- 15. References 1. Jacobson I., Christerson M., Jonsson P., Övergaard G., Object- Oriented Software Engineering - A Use Case Driven Approach, Addison-Wesley, 1992 2. Cockburn, A., Writing Effective Use Cases. Addison-Wesley, 2001. 3. Writing Use Case Scenarios for Model Driven Development, http://www.sparxsystems.com/downloads/quick/writing-structured- use-case-scenarios-mdd.pdf 4. Bass, L., Clements, P. and Kazman, R., Software Architecture in Practice, 3rd Edition, Addison-Wesley, 2012. 5. Bachmann, F., Use Cases Quality Attribute Scenarios, SEI, Carnegie- Mellon University, http://www.cac.cornell.edu/VW/usecases/default.aspx Dr. Amir Tomer, ISQ Conference, 2014 Extracting 15 Quality Scenarios from Functional Scenarios
- 16. Any questions? amir@amirtomer.com 052-8890202 16 Extracting Dr. Amir Tomer, ISQ Conference, 2014 Quality Scenarios from Functional Scenarios