Lecture 2Introductory Case Studies
TopicsTopics Architectural Styles Key Word In Context (KWIC) Other Cases Studies Evolution of Software Engineering
January 15, 2009
CSCE 742 Software Architectures
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OverviewLast TimeLast Time
What is Software Architecture? What do you already know? Architectural styles
On last Time’s Slides(what we didn’t get to)On last Time’s Slides(what we didn’t get to) KWIC case study
NewNew Other Case studies
What do you know?What do you know? What is the waterfall Model? What is the spiral model?
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Architectural Styles Dataflow SystemsDataflow Systems
Batch- sequential Pipes and filters
Call-and-Return SystemsCall-and-Return Systems Main program and subroutine OO systems Hierarchical layered systems
Independent ComponentsIndependent Components Communicating processes Event driven systems
MachinesMachines Interpreters Rule-based systems
Repositories - Data-centered systemsRepositories - Data-centered systems Databases Hypertext Systems Blackboards
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Architectural Case Studies Key word in contextKey word in context Instrumentation Software Instrumentation Software CompilersCompilers Layered Design with Different Styles for the LayersLayered Design with Different Styles for the Layers Interpreter using Different Idioms for ComponentsInterpreter using Different Idioms for Components A Blackboard A Blackboard
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Case Study: Key word in contextIn 1972, Parnas proposed the following problem KWIC:The KWIC [Key Word in Context] index system:1. Accepts an ordered set of lines, each line is an
ordered set of words, and each word is an ordered set of characters.
2. Any line may be ``circularly shifted'' by repeatedly removing the first word and appending it at the end of the line.
3. The KWIC index system outputs a listing of all circular shifts of all lines in alphabetical order.
Reference: “On the Criteria for Decomposing Systems into Modules,” David Parnas. CACM, 1972
http://www-2.cs.cmu.edu/People/ModProb/KWIC.htmlhttp://www-2.cs.cmu.edu/People/ModProb/KWIC.html
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Who is David Parnas anyway?He is an ACM Fellow and a leader in the development of He is an ACM Fellow and a leader in the development of
the field of Software engineering.the field of Software engineering.
http://www.acm.org/sigs/sigsoft/SEN/parnas.html
First work experience in industry (1969) led him to First work experience in industry (1969) led him to realize that the company was breaking things up into realize that the company was breaking things up into modules incorrectly; thus leading to greater modules incorrectly; thus leading to greater complexitycomplexity
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Case Study: Key word in contextExample: SC Clerk of Courts (1985 project)One line
Assault and battery with intent to kill
Permuted yieldsPermuted yields Assault and battery with intent to kill. and battery with intent to kill. Assault battery with intent to kill. Assault and with intent to kill. Assault and battery intent to kill. Assault and battery with to kill. Assault and battery with intent kill. Assault and battery with intent to
Then sorted yields and battery with intent to kill. Assault Assault and battery with intent to kill. battery with intent to kill. Assault and …
So “assault and battery” could be found by looking up either keyword “assault” or“battery”
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Case Study: Decomposition in KWICParnas used the problem to contrast different criteria
for decomposing a system into modules:
1. Functional decomposition with shared access to data representations, and
2. A decomposition that hides design decisions.
Examples: permuted index of the Unix man
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KWIC: Design Considerations Changes in algorithm:
Changes in data representation
Have the system eliminate circular shifts that start with certain noise words (such as "a", "an", "and", etc.).
Make the system interactive, and allow the user to delete lines from the lists.
Finally, considering differences in architectural solutions based on considerations of:
Performance: Both space and time.
Reuse
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KWIC: Software Arch. ConsiderationsChanges in processing algorithm
Changes in data representation
Enhancement to system function
Performance: Both space and time.
Reuse: To what extent can the components serve as reusable entities.
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Architectural Approaches to KWICSolution 1: Main Program/Subroutine with Shared Data
Solution 2: Abstract Data Types
Solution 3: Implicit Invocation
Solution 4: Pipes and Filters
The first two of these were from Parnas 1972
Solution 3 was from Garlan, Kaiser and Notkin 1992
Solution 4 inspired by Unix command.
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KWIC: Main Program/Subroutine with Shared Data
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KWIC: Main / SubroutineNotes:
1. Data is shared, common storage. This is + and –
2. Serious drawbacks: Changes in data storage format affects all modules Changes in algorithm not well supported Enhancements not easily encorporated Not supportive of reuse
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KWIC: Abstract Data Types
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KWIC: Abstract Data TypesNotes:
1. Could be called object-oriented (Parnas 1972)
2. Data not accessed directly, but through accessor functions
3. More easily modified than solution 1 Data Algorithm
4. Reuse better supported because modules make fewer assumptions about other modules.
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KWIC: Implicit Invocation
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KWIC: Implicit InvocationNotes:1. Shared data similar to solution 1.2. Two differences in access model:
1. Data accessed abstractly i.e., “as a list, or set”2. Computations are implicitly invoked; an “Active data” model
E.g., Adding a line causes an event to be sent to the line shift module
3. Because data is accessed abstractly changes in storage format can be localized
4. Supports functional enhancements New modules easily added by registering the data events that
should caused them to be invoked
5. On the negative side it is difficult to control computation order.
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KWIC: Pipes and Filters
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KWIC: Pipe and FiltersNotes:
1. Inspired by the old UNIX permuted index
2. Cat data | permuteLines | sort
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KWIC: Comparison
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KWIC: ComparisonNotes:Notes:
Shared DataShared Data Not good at change in data or algorithm; efficient
ADT/OOADT/OO Good at change in data and in reuse; efficient also
Implicit InvocationImplicit Invocation Good at change in algorithm; just register the new functions
Pipe and FilterPipe and Filter Good at reuse and change in algorithm, modularity; however
stuck with lowest level data transmission involving reparsing overhead
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Case Studies Key word in contextKey word in context Instrumentation SoftwareInstrumentation Software CompilersCompilers Layered Design with Different Styles for the LayersLayered Design with Different Styles for the Layers Interpreter using Different Idioms for ComponentsInterpreter using Different Idioms for Components A Blackboard A Blackboard
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Case Study: Instrumentation Software
Software architecture developed at Tektronix to Software architecture developed at Tektronix to develop a “reusable system architecture” for develop a “reusable system architecture” for oscilloscopes.oscilloscopes.
What is an oscilloscope?What is an oscilloscope?Once simple analog device, now complex digital Once simple analog device, now complex digital
technology with complex software.technology with complex software.Problems faced by Tektronix:Problems faced by Tektronix:
1. Little reuse of software across different products2. Both hardware and interface requirements were rapidly
changing3. Performance problems increasing because of configuration
limitations
Goal: Develop new architecture for oscilloscopesGoal: Develop new architecture for oscilloscopes
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Instrumentation Software: OO Model
Focused on producing object-oriented model of the Focused on producing object-oriented model of the domaindomain
This produced a good model of the data involvedThis produced a good model of the data involved
Oscilloscope Object
Waveform
Max-min Wvfm X-Y Wvfm Accumulate Wvfm
…
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Instrum. Software: OO Model LimitationsNo overall model of how the types fit togetherNo overall model of how the types fit together
Led to confusion about partitioning the functionalityLed to confusion about partitioning the functionality Should measurements be associated with data type of
what is being measured? Or represented externally Which objects should the interface interact with?
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Instrum. Software: A Layered Model
Hardware
Digitization
Visualization
User Interface
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Instrum. Software: A Layered ModelThis model was intuitively appealing since it partitioned This model was intuitively appealing since it partitioned
up the functionality into well defined groups.up the functionality into well defined groups.
However, wrong model:However, wrong model:
main problem was that the boundaries of abstraction main problem was that the boundaries of abstraction enforced by the layers conflict with what was really enforced by the layers conflict with what was really needed.needed.
user interactions with the visualizations but real user interactions with the visualizations but real oscilloscopes must interact at several levelsoscilloscopes must interact at several levels
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Instrum. Software: Pipe and Filter Model
Couple – condition the signalAcquire – derive digitized waveformsTo-XY - displayClip – clip images to displayTrigger Subsystem - MeasureMain Problem: How should user interact with the system?
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Instrum. Soft: Modified Pipe and Filter Model
Notes1. Performance problems – waveforms are large; copying is
expensive2. Different speed of the different filters3. Solution several types “colors” of pipes; one copies, one doesn’t4. Speed handled by pipelining
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Traditional CompilerLexical analysisLexical analysis
Syntax AnalysisSyntax Analysis
Semantic AnalysisSemantic Analysis
OptimizationOptimization
Code GenerationCode Generation
Modified with globally accessible symbol tableModified with globally accessible symbol table
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Modern Compiler
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Canonical Compiler Revisited
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Layered Design with Different StylesPROVOX system designed by Fisher ControlsPROVOX system designed by Fisher Controls
Level 1 – Process measurementLevel 1 – Process measurement
Level 2 – Process supervision – monitoring and Level 2 – Process supervision – monitoring and controlling level 1controlling level 1
Level 3 – Process management – plant automation, Level 3 – Process management – plant automation, management reports, optimization strategiesmanagement reports, optimization strategies
Level 4 – Plant Management – interactions; cost Level 4 – Plant Management – interactions; cost accounting, inventoryaccounting, inventory
Level 5 – Corporation Management – Order Level 5 – Corporation Management – Order proecessing/billingproecessing/billing
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Layered Design with Different Styles
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Layered Design with Different StylesLevels 1-3 were Object-orientedLevels 1-3 were Object-oriented
Levels 4 and 5 were primarily respository (database) Levels 4 and 5 were primarily respository (database) modelsmodels
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Rule Based Systems
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Blackboard model: Hearsay II (speech processing)
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Evolution of Software EngineeringWhat is engineering?What is engineering?
Phrases in answers:Phrases in answers:
1.1. Creating cost-effective solutionsCreating cost-effective solutions
2.2. … … to practical problems …to practical problems …
3.3. … … by applying scientific knowledge …by applying scientific knowledge …
4.4. … … building things …building things …
5.5. … … in the service of mankind …in the service of mankind …
Applied science for practical problemsApplied science for practical problems
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Traditional EngineeringDesign experience built up over the yearsDesign experience built up over the years
Key design parameters abstracted from problemsKey design parameters abstracted from problems
Design problem formalizedDesign problem formalized
Knowledge codifiedKnowledge codified
Handbooks of DesignHandbooks of Design
Well there are no handbooks of design for software.Well there are no handbooks of design for software.
There are algorithms and libraries and now class There are algorithms and libraries and now class libraries, but these are components.libraries, but these are components.
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Evolution of an Engineering Discipline