Chapter%205,%20Analysis:%20Dynamic%20Modeling

1
Chapter 5, AnalysisDynamic Modeling
2
Outline

• Dynamic modeling
• Sequence diagrams
• State diagrams
• Using dynamic modeling for the design of user
  interfaces
• Analysis example
• Requirements analysis document template

3
Example of use case format

• Use case name
• ReportEmergency
• Entry condition
• 1. The FieldOfficer activates the Report
  Emergency function of her terminal.
• Flow of events
• 2. FRIEND responds by presenting a form to the
  officer...
• 3. The FieldOfficer fills the form....
• 4. The Dispatcher reviews the information
  submitted by the FieldOfficer ...
• Exit condition
• 5. The FieldOfficer receives the acknowledgment
  and the selected response.

4
How do you find classes?

• From previous lectures
• Application domain analysis Talk to client to
  identify abstractions
• Apply general world knowledge and intuition
• Scenarios
• Natural language formulation of a concrete usage
  of the system
• Use Cases
• Natural language formulation of the functions of
  the system
• Textual analysis of problem statement (Abbot)
• From this lecture
• Dynamic model
• Events Candiates for operations to be offered
  by classes
• Sequence diagrams as sources for objects
• From future lectures
• Design Patterns

5
Dynamic Modeling with UML

• Diagrams for dynamic modeling
• Interaction diagrams describe the dynamic
  behavior between objects
• Statecharts describe the dynamic behavior of a
  single object
• Interaction diagrams
• Sequence Diagram
• Dynamic behavior of a set of objects arranged in
  time sequence.
• Good for real-time specifications and complex
  scenarios
• Collaboration Diagram
• Shows the relationship among objects. Does not
  show time
• State Charts
• A state machine that describes the response of an
  object of a given class to the receipt of outside
  stimuli (Events).
• Activity Diagram
• Special type of statechart where all states are
  action states

6
Dynamic Modeling

• Definition of dynamic model
• A collection of multiple state chart diagrams,
  one state chart diagram for each class with
  important dynamic behavior.
• Purpose
• Detect and supply methods for the object model
• How do we do this?
• Start with use case or scenario
• Model interaction between objects gt sequence
  diagram
• Model dynamic behavior of single objects gt
  statechart diagram

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Start with Flow of Events from Use Case

• Flow of events from Dial a Number Use case
• Caller lifts receiver
• Dail tone begins
• Caller dials
• Phone rings
• Callee answers phone
• Ringing stops
• ....

8
What is an Event?

• Something that happens at a point in time
• Relation of events to each other
• Causally related Before, after,
• Causally unrelated concurrent
• An event sends information from one object to
  another
• Events can be grouped in event classes with a
  hierarchical structure. Event is often used in
  two ways
• Instance of an event class New IETM issued on
  Thursday September 14 at 930 AM.
• Event class New IETM, Subclass Figure
  Change
• Attribute of an event class
• IETM Update (930 AM, 9/14/99)
• Car starts at ( 445pm, Monroeville Mall,
  Parking Lot 23a)
• Mouse button down(button, tablet-location)

9
Sequence Diagram

• From the flow of events in the use case or
  scenario proceed to the sequence diagram
• A sequence diagram is a graphical description of
  objects participating in a use case or scenario
  using a DAG notation
• Relation to object identification
• Objects/classes have already been identified
  during object modeling
• Objects are identified as a result of dynamic
  modeling
• Heuristic
• An event always has a sender and a receiver. Find
  them for each event gt These are the objects
  participating in the use case

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An Example

• Flow of events in a Get SeatPosition use case
  
• 1. Establish connection between smart card and
  onboard computer
• 2. Establish connection between onboard computer
  and sensor for seat
• 3. Get current seat position and store on smart
  card
• Which are the objects?

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Sequence Diagram for Get SeatPosition
Seat
Onboard Computer
Smart Card
1. Establish connection between smart card and
onboard computer 2. Establish connection between
onboard computer and sensor for seat 3. Get
current seat position and store on smart card
Establish Connection
Establish Connection
Accept Connection
Accept Connection
Get SeatPosition
500,575,300
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Heuristics for Sequence Diagrams

• Layout
• 1st column Should correspond to the actor who
  initiated the use case
• 2nd column Should be a boundary object
• 3rd column Should be the control object that
  manages the rest of the use case
• Creation
• Control objects are created at the initiation of
  a use case
• Boundary objects are created by control objects
• Access
• Entity objects are accessed by control and
  boundary objects,
• Entity objects should never call boundary or
  control objects This makes it easier to share
  entity objects across use cases and makes entity
  objects resilient against technology-induced
  changes in boundary objects.

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Is this a good Sequence Diagram?
Seat
Onboard Computer
Smart Card

• Did the modeler follow the heuristics?

Establish Connection
Establish Connection
Accept Connection
Accept Connection
Get SeatPosition
500,575,300
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UML Statechart Diagram Notation
Event trigger With parameters
State1
State2
Event1(attr) condition/action
do/Activity
Guard condition
entry /action
exit/action
Also internal transition and deferred events

• Notation based on work by Harel
• Added are a few object-oriented modifications
• A UML statechart diagram can be mapped into a
  finite state machine

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Statechart Diagrams

• Graph whose nodes are states and whose directed
  arcs are transitions labeled by event names.
• Distinguish between two types of operations
• Activity Operation that takes time to complete
• associated with states
• Action Instantaneous operation
• associated with events
• associated with states (reduces drawing
  complexity) Entry, Exit, Internal Action
• A statechart diagram relates events and states
  for one class
• An object model with a set of objects has a
  set of state diagrams

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State

• An abstraction of the attribute of a class
• State is the aggregation of several attributes a
  class
• Basically an equivalence class of all those
  attribute values and links that do no need to be
  distinguished as far as the control structure of
  the system is concerned
• Example State of a bank
• A bank is either solvent or insolvent
• State has duration

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Example of a StateChart Diagram
coins_in(amount) / set balance
Collect Money
Idle
coins_in(amount) / add to balance
cancel / refund coins
item empty
select(item)
changelt0
do test item and compute change
changegt0
change0
do dispense item
do make change
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Nested State Diagram

• Activities in states are composite items denoting
  other lower-level state diagrams
• A lower-level state diagram corresponds to a
  sequency of lower-level states and events that
  are invisible in the higher-level diagram.
• Sets of substates in a nested state diagram
  denoting a superstate are enclosed by a large
  rounded box, also called contour.

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Example of a Nested Statechart Diagram
coins_in(amount) / set balance
Collect Money
Idle
coins_in(amount) / add to balance
cancel / refund coins
item empty
select(item)
changelt0
Superstate
do test item and compute change
changegt0
change0
do dispense item
do make change
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Expanding activity dodispense item
Dispense item as an atomic activity
change0
do dispense item
Dispense item as a composite activity
do push item off shelf
do move arm to row
do move arm to column
Arm ready
Arm ready
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Superstates

• Goal
• Avoid spaghetti models
• Reduce the number of lines in a state diagram
• Transitions from other states to the superstate
  enter the first substate of the superstate.
• Transitions to other states from a superstate are
  inherited by all the substates (state inheritance)

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Modeling Concurrency

• Two types of concurrency
• 1. System concurrency
• State of overall system as the aggregation of
  state diagrams, one for each object. Each state
  diagram is executing concurrently with the
  others.
• 2. Object concurrency
• An object can be partitioned into subsets of
  states (attributes and links) such that each of
  them has its own subdiagram.
• The state of the object consists of a set of
  states one state from each subdiagram.
• State diagrams are divided into subdiagrams by
  dotted lines.

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Example of Concurrency within an Object
Splitting control
Synchronization
Emitting
Do Dispense
Cash taken
Cash
Ready
Setting
to r
eset
Up
Ready
Do Eject
Card
Card taken
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State Chart Diagram vs Sequence Diagram

• State chart diagrams help to identify
• Changes to objects over time
• Sequence diagrams help to identify
• The temporal relationship of between objects over
  time
• Sequence of operations as a response to one ore
  more events

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Dynamic Modeling of User Interfaces

• Statechart diagrams can be used for the design of
  user interfaces
• Also called Navigation Path
• States Name of screens
• Graphical layout of the screens associated with
  the states helps when presenting the dynamic
  model of a user interface
• Activities/actions are shown as bullets under
  screen name
• Often only the exit action is shown
• State transitions Result of exit action
• Button click
• Menu selection
• Cursor movements
• Good for web-based user interface design

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Navigation Path Example (15-499 Spring 96)

• Diagnostics
• User can move cursor to Control Panel or Graph

• Graph
• User can select data group and type of graph

• Control panel
• User can select functionality of sensors

• Selection
• User selects data group
• Field site
• Car
• Sensor group
• Time range
• User selects type of graph
• time line
• histogram
• pie chart

• Define
• User defines a sensor event
• from a list of events

• Disable
• User can disable a sensor event from a list of
  sensor events

• Enable
• User can enable a sensor event from a list of
  sensor events

• List of events
• User selects event(s)

• Visualize
• User views graph
• User can add data groups for being viewed

• List of sensor events
• User selects sensor event(s)

• Link
• User makes a link (doclink)

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Practical Tips for Dynamic Modeling

• Construct dynamic models only for classes with
  significant dynamic behavior
• Avoid analysis paralysis
• Consider only relevant attributes
• Use abstraction if necessary
• Look at the granularity of the application when
  deciding on actions and activities
• Reduce notational clutter
• Try to put actions into state boxes (look for
  identical actions on events leading to the same
  state)

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Summary Requirements Analysis
Functional Modeling

• 1. What are the transformations?
• Create scenarios and use case diagrams
• Talk to client, observe, get historical records,
  do thought experiments
• 2. What is the structure of the system?
• Create class diagrams
• Identify objects. What are the associations
  between them? What is their multiplicity?
• What are the attributes of the objects?
• What operations are defined on the objects?
• 3. What is its control structure?
• Create sequence diagrams
• Identify senders and receivers
• Show sequence of events exchanged between
  objects. Identify event dependencies and event
  concurrency.
• Create state diagrams
• Only for the dynamically interesting objects.

Object Modeling
Dynamic Modeling
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Lets Do Analysis

• 1. Analyze the problem statement
• Identify functional requirements
• Identify nonfunctional requirements
• Identify constraints (pseudo requirements)
• 2. Build the functional model
• Develop use cases to illustrate functionality
  requirements
• 3. Build the dynamic model
• Develop sequence diagrams to illustrate the
  interaction between objects
• Develop state diagrams for objects with
  interesting behavior
• 4. Build the object model
• Develop class diagrams showing the structure of
  the system

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Problem Statement Direction Control for a Toy
Car

• Power is turned on
• Car moves forward and car headlight shines
• Power is turned off
• Car stops and headlight goes out.
• Power is turned on
• Headlight shines
• Power is turned off
• Headlight goes out.
• Power is turned on
• Car runs backward with its headlight shining.

• Power is turned off
• Car stops and headlight goes out.
• Power is turned on
• Headlight shines
• Power is turned off
• Headlight goes out.
• Power is turned on
• Car runs forward with its headlight shining.

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Find the Functional Model Do Use Case Modeling

• Use case 1 System Initialization
• Entry condition Power is off, car is not moving
• Flow of events
• Driver turns power on
• Exit condition Car moves forward, headlight is
  on
• Use case 2 Turn headlight off
• Entry condition Car moves forward with
  headlights on
• Flow of events
• Driver turns power off, car stops and headlight
  goes out.
• Driver turns power on, headlight shines and car
  does not move.
• Driver turns power off, headlight goes out
• Exit condition Car does not move, headlight is
  out

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Use Cases continued

• Use case 3 Move car backward
• Entry condition Car is stationary, headlights
  off
• Flow of events
• Driver turns power on
• Exit condition Car moves backward, headlight on
• Use case 4 Stop backward moving car
• Entry condition Car moves backward, headlights
  on
• Flow of events
• Driver turns power off, car stops, headlight
  goes out.
• Power is turned on, headlight shines and car
  does not move.
• Power is turned off, headlight goes out.
• Exit condition Car does not move, headlight is
  out.
• Use case 5 Move car forward
• Entry condition Car does not move, headlight
  is out
• Flow of events
• Driver turns power on
• Exit condition
• Car runs forward with its headlight shining.

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Use Case Pruning

• Do we need use case 5?
• Use case 1 System Initialization
• Entry condition Power is off, car is not moving
• Flow of events
• Driver turns power on
• Exit condition Car moves forward, headlight is
  on
• Use case 5 Move car forward
• Entry condition Car does not move, headlight
  is out
• Flow of events
• Driver turns power on
• Exit condition
• Car runs forward with its headlight shining.

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Find the Dynamic Model Create sequence diagram

• Name Drive Car
• Sequence of events
• Billy turns power on
• Headlight goes on
• Wheels starts moving forward
• Wheels keeps moving forward
• Billy turns power off
• Headlight goes off
• Wheels stops moving
• . . .

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Sequence Diagram for Drive Car Scenario
Wheel
Headlight
BillyDriver
Power(on)
Power(on)
Power(off)
Power(off)
Power(on)
Power(on)
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Toy Car Dynamic Model
Wheel
Forward
power

power

off
on
Stationary
Stationary
power

power

on
off
Backward
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Toy Car Object Model
Car
Headlight
Power
Status (On, Off)
Status (On, Off)
Switch_On()
TurnOn()
Switch_Off()
TurnOff()
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When is a model dominant?

• Object model The system has non-trivial data
  structures.
• Dynamic model The model has many different types
  of events Input, output, exceptions, errors,
  etc.
• Functional model The model performs complicated
  transformations such as difficult computations
  consisting of many steps.
• Examples
• Compiler Functional model most important.
  Dynamic model is trivial because there is only
  one type input and only a few outputs.
• Database systems Object model most important.
  Functional model is trivial, because their
  purpose is usually only to store, organize and
  retrieve data.
• Spreadsheet program Functional model most
  important. Object model is trivial, because the
  spreadsheet values are trivial and cannot be
  structured further. The only interesting object
  is the cell.

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Collaborative Analysis

• A system is a collection of subsystems providing
  services
• Analysis of services is provided by a set of the
  teams who provide the models for their subsystems
• Integration of subsystem models into the full
  system model by the architecture team
• Analysis integration checklist
• Are all the classes mentioned in the data
  dictionary?
• Are the names of the methods consistent with the
  names of actions, activities, events or
  processes?
• Check for assumptions made by each of the
  services
• Missing methods, classes
• Unmatched associations

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Analysis UML Activity Diagram
41
Object Model Integration in JAMES (Fall 1997)
All Teams
Model Changes
Team 1
Integrated
System
Model

User Interface
Module 1
Module
Integration
Architecture Team
Module 4
Module 3
Module 2
Module 5
Analysis
Team 5
Team 3
Team 4
Team 2
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Consistency, Completeness, Ambiguities

• Consistency
• Identification of crossed wires between classes
• Naming of classes, attributes, methods
• Completeness
• Identification of dangling associations
  (associations pointing to nowhere)
• Identification of double- defined classes
• Identification of missing classes (referred to by
  one subsystem but not defined anywhere)
• Ambiguities
• Misspelling of names
• Classes with the same name but different meanings

43
Requirements Analysis Document Template

• 1. Introduction
• 2. Current system
• 3. Proposed system
• 3.1 Overview
• 3.2 Functional requirements
• 3.3 Nonfunctional requirements
• 3.4 Constraints (Pseudo requirements)
• 3.5 System models
• 3.5.1 Scenarios
• 3.5.2 Use case model
• 3.5.3 Object model
• 3.5.3.1 Data dictionary
• 3.5.3.2 Class diagrams
• 3.5.4 Dynamic models
• 3.5.5 User interfae
• 4. Glossary

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Section 3.5 System Model

• 3.5.1 Scenarios
• - As-is scenarios, visionary scenarios
• 3.5.2 Use case model
• - Actors and use cases
• 3.5.3 Object model
• - Data dictionary
• - Class diagrams (classes, associations,
  attributes and operations)
• 3.5.4 Dynamic model
• - State diagrams for classes with significant
  dynamic behavior
• - Sequence diagrams for collaborating objects
  (protocol)
• 3.5.5 User Interface
• - Navigational Paths, Screen mockups

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Section 3.3 Nonfunctional Requirements

• 3.3.1 User interface and human factors
• 3.3.2 Documentation
• 3.3.3 Hardware considerations
• 3.3.4 Performance characteristics
• 3.3.5 Error handling and extreme conditions
• 3.3.6 System interfacing
• 3.3.7 Quality issues
• 3.3.8 System modifications
• 3.3.9 Physical environment
• 3.3.10 Security issues
• 3.3.11 Resources and management issues

46
Nonfunctional Requirements Trigger Questions

• 3.3.1 User interface and human factors
• What type of user will be using the system?
• Will more than one type of user be using the
  system?
• What sort of training will be required for each
  type of user?
• Is it particularly important that the system be
  easy to learn?
• Is it particularly important that users be
  protected from making errors?
• What sort of input/output devices for the human
  interface are available, and what are their
  characteristics?
• 3.3.2 Documentation
• What kind of documentation is required?
• What audience is to be addressed by each
  document?
• 3.3.3 Hardware considerations
• What hardware is the proposed system to be used
  on?
• What are the characteristics of the target
  hardware, including memory size and auxiliary
  storage space?

47
Nonfunctional Requirements (continued)

• 3.3.4 Performance characteristics
• Are there any speed, throughput, or response time
  constraints on the system?
• Are there size or capacity constraints on the
  data to be processed by the system?
• 3.3.5 Error handling and extreme conditions
• How should the system respond to input errors?
• How should the system respond to extreme
  conditions?
• 3.3.6 System interfacing
• Is input coming from systems outside the proposed
  system?
• Is output going to systems outside the proposed
  system?
• Are there restrictions on the format or medium
  that must be used for input or output?

48
Nonfunctional Requirements, ctd

• 3.3.7 Quality issues
• What are the requirements for reliability?
• Must the system trap faults?
• Is there a maximum acceptable time for restarting
  the system after a failure?
• What is the acceptable system downtime per
  24-hour period?
• Is it important that the system be portable (able
  to move to different hardware or operating system
  environments)?
• 3.3.8 System Modifications
• What parts of the system are likely candidates
  for later modification?
• What sorts of modifications are expected?
• 3.3.9 Physical Environment
• Where will the target equipment operate?
• Will the target equipment be in one or several
  locations?
• Will the environmental conditions in any way be
  out of the ordinary (for example, unusual
  temperatures, vibrations, magnetic fields, ...)?

49
Nonfunctional Requirements, ctd

• 3.3.10 Security Issues
• Must access to any data or the system itself be
  controlled?
• Is physical security an issue?
• 3.3.11 Resources and Management Issues
• How often will the system be backed up?
• Who will be responsible for the back up?
• Who is responsible for system installation?
• Who will be responsible for system maintenance?

50
Pseudo Requirements (Constraints)

• Pseudo requirement
• Any client restriction on the solution domain
• Examples
• The target platform must be an IBM/360
• The implementation language must be COBOL
• The documentation standard X must be used
• A dataglove must be used
• ActiveX must be used
• The system must interface to a papertape reader

51
Project Agreement

• The project agreement represents the acceptance
  of the analysis model (as documented by the
  requirements analysis document) by the client.
• The client and the developers converge on a
  single idea and agree about the functions and
  features that the system will have. In addition,
  they agree on
• a list of priorities
• a revision process
• a list of criteria that will be used to accept or
  reject the system
• a schedule, and a budget

52
Prioritizing requirements

• High priority (Core requirements)
• Must be addressed during analysis, design, and
  implementation.
• A high-priority feature must be demonstrated
  successfully during client acceptance.
• Medium priority (Optional requirements)
• Must be addressed during analysis and design.
• Usually implemented and demonstrated in the
  second iteration of the system development.
• Low priority (Fancy requirements)
• Must be addressed during analysis (very
  visionary scenarios).
• Illustrates how the system is going to be used in
  the future if not yet available technology
  enablers are

53
Summary

• In this lecture, we reviewed the construction of
  the dynamic model from use case and object
  models. In particular, we described In
  particular, we described
• Sequence diagrams for identifying missing objects
  and operations.
• Statechart diagrams for identifying missing
  attributes.
• Definition of an event hierarchy.
• In addition, we described the requirements
  analysis document and its use when interacting
  with the client.

54
Chapter 6, System DesignLecture 1
55
Design

• There are two ways of constructing a software
  design One way is to make it so simple that
  there are obviously no deficiencies, and the
  other way is to make it so complicated that there
  are no obvious deficiencies.
• - C.A.R. Hoare

56
Why is Design so Difficult?

• Analysis Focuses on the application domain
• Design Focuses on the implementation domain
• Design knowledge is a moving target
• The reasons for design decisions are changing
  very rapidly
• Halftime knowledge in software engineering About
  3-5 years
• What I teach today will be out of date in 3 years
• Cost of hardware rapidly sinking
• Design window
• Time in which design decisions have to be made

57
The Purpose of System Design
Problem

• Bridging the gap between desired and existing
  system in a manageable way
• Use Divide and Conquer
• We model the new system to be developed as a set
  of subsystems

New System
Existing System
58
System Design
System Design
Failure
2. System
Decomposition
Layers/Partitions Coherence/Coupling
7. Software Control
Monolithic Event-Driven Threads Conc. Processes
3. Concurrency
6. Global

4. Hardware/
Identification of Threads
5. Data
Resource Handling
Softwar
e

Management
Mapping
Access control Security
Persistent Objects
Special purpose
Files
Buy or Build Trade-off
Databases
Allocation
Data structure
Connectivity
59
Overview

• System Design I
• 0. Overview of System Design
• 1. Design Goals
• 2. Subsystem Decomposition
• System Design II (next lecture)
• 3. Concurrency
• 4. Hardware/Software Mapping
• 5. Persistent Data Management
• 6. Global Resource Handling and Access Control
• 7. Software Control
• 8. Boundary Conditions

60
How to use the results from the Requirements
Analysis for System Design

• Nonfunctional requirements gt
• Activity 1 Design Goals Definition
• Use Case model gt
• Activity 2 System decomposition (Selection of
  subsystems based on functional requirements,
  coherence, and coupling)
• Object model gt
• Activity 4 Hardware/software mapping
• Activity 5 Persistent data management
• Dynamic model gt
• Activity 3 Concurrency
• Activity 6 Global resource handling
• Activity 7 Software control
• Activity 8 Boundary conditions

61
Section 1. Design Goals

• Reliability
• Modifiability
• Maintainability
• Understandability
• Adaptability
• Reusability
• Efficiency
• Portability
• Traceability of requirements
• Fault tolerance
• Backward-compatibility
• Cost-effectiveness
• Robustness
• High-performance

• Good documentation
• Well-defined interfaces
• User-friendliness
• Reuse of components
• Rapid development
• Minimum of errors
• Readability
• Ease of learning
• Ease of remembering
• Ease of use
• Increased productivity
• Low-cost
• Flexibility

62
Relationship Between Design Goals
End User
Functionality User-friendliness Ease of Use Ease
of learning Fault tolerant Robustness
Low cost Increased Productivity Backward-Compatib
ility Traceability of requirements Rapid
development Flexibility
Runtime Efficiency
Reliability
Portability Good Documentation
Client
(Customer,
Sponsor)
Minimum of errors Modifiability,
Readability Reusability, Adaptability Well-defined
interfaces
63
Typical Design Trade-offs

• Functionality vs. Usability
• Cost vs. Robustness
• Efficiency vs. Portability
• Rapid development vs. Functionality
• Cost vs. Reusability
• Backward Compatibility vs. Readability

64
Nonfunctional Requirements give a clue for the
use of Design Patterns

• Read the problem statement again
• Use textual clues (similar to Abbots technique
  in Analysis) to identify design patterns
• Text manufacturer independent, device
  independent, must support a family of products
• Abstract Factory Pattern
• Text must interface with an existing object
• Adapter Pattern
• Text must deal with the interface to several
  systems, some of them to be developed in the
  future, an early prototype must be
  demonstrated
• Bridge Pattern

65
Textual Clues in Nonfunctional Requirements

• Text complex structure, must have variable
  depth and width
• Composite Pattern
• Text must interface to an set of existing
  objects
• Façade Pattern
• Text must be location transparent
• Proxy Pattern
• Text must be extensible, must be scalable
• Observer Pattern
• Text must provide a policy independent from the
  mechanism
• Strategy Pattern

66
Section 2. System Decomposition

• Subsystem (UML Package)
• Collection of classes, associations, operations,
  events and constraints that are interrelated
• Seed for subsystems UML Objects and Classes.
• Service
• Group of operations provided by the subsystem
• Seed for services Subsystem use cases
• Service is specified by Subsystem interface
• Specifies interaction and information flow
  from/to subsystem boundaries, but not inside the
  subsystem.
• Should be well-defined and small.
• Often called API Application programmers
  interface, but this term should used during
  implementation, not during System Design

67
Services and Subsystem Interfaces

• Service A set of related operations that share a
  common purpose
• Notification subsystem service
• LookupChannel()
• SubscribeToChannel()
• SendNotice()
• UnscubscribeFromChannel()
• Services are defined in System Design
• Subsystem Interface Set of fully typed related
  operations. Also called application programmer
  interface (API)
• Subsystem Interfaces are defined in Object Design

68
Choosing Subsystems

• Criteria for subsystem selection Most of the
  interaction should be within subsystems, rather
  than across subsystem boundaries (High
  coherence).
• Does one subsystem always call the other for the
  service?
• Which of the subsystems call each other for
  service?
• Primary Question
• What kind of service is provided by the
  subsystems (subsystem interface)?
• Secondary Question
• Can the subsystems be hierarchically ordered
  (layers)?
• What kind of model is good for describing layers
  and partitions?

69
Example STARS Subsystem Decomposition
Is this the right decomposition or is this too
much ravioli?
70
Definition Subsystem Interface Object

• A Subsystem Interface Object provides a service
  
• This is the set of public methods provided by the
  subsystem
• The Subsystem interface describes all the methods
  of the subsystem interface object
• Use a Facade pattern for the subsystem interface
  object

71
STARS as a set of subsystems communicating via a
software bus
Authoring
Modeling
Workflow
Augmented Reality
Inspection
Repair
Workorder
A Subsystem Interface Object publishes the
service ( Set of public methods) provided by
the subsystem
72
STARS as a 3-layered Architecture
What is the relationship between Modeling and
Authoring? Are other subsystems needed?
73
Coupling and Coherence

• Goal Reduction of complexity
• Coherence measures the dependence among classes
• High coherence The classes in the subsystem
  perform similar tasks and are related to each
  other (via associations)
• Low coherence Lots of misc and aux objects, no
  associations
• Coupling measures dependencies between subsystems
• High coupling Modifications to one subsystem
  will have high impact on the other subsystem
  (change of model, massive recompilation, etc.)
• Subsystems should have as maximum coherence and
  minimum coupling as possible
• How can we achieve loose coupling?
• Which subsystems are highly coupled?

74
Partitions and Layers

• A large system is usually decomposed into
  subsystems using both, layers and partitions.
• Partitions vertically divide a system into
  several independent (or weakly-coupled)
  subsystems that provide services on the same
  level of abstraction.
• A layer is a subsystem that provides services to
  a higher level of abstraction
• A layer can only depend on lower layers
• A layer has no knowledge of higher layers

75
Subsystem Decomposition into Layers

• Subsystem Decomposition Heuristics
• No more than 7/-2 subsystems
• More subsystems increase coherence but also
  complexity (more services)
• No more than 5/-2 layers

76
Layer and Partition Relationships between
Subsystems

• Layer relationship
• Layer A Calls Layer B (runtime)
• Layer A Depends on Layer B (make dependency,
  compile time)
• Partition relationship
• The subsystem have mutual but not deep knowledge
  about each other
• Partition A Calls partition B and partition B
  Calls partition A

77
Virtual Machine (Dijkstra, 1965)

• A system should be developed by an ordered set of
  virtual machines, each built in terms of the ones
  below it.

Problem
VM1
C1
C1
C1
attr
attr
attr
opr
opr
opr
C1
C1
VM2
attr
attr
opr
opr
C1
VM3
C1
attr
attr
opr
opr
C1
VM4
attr
opr
Existing System
78
Virtual Machine

• A virtual machine is an abstraction that provides
  a set of attributes and operations.
• A virtual machine is a subsystem connected to
  higher and lower level virtual machines by
  "provides services for" associations.
• Virtual machines can implement two types of
  software architecture closed and open
  architectures.

79
Closed Architecture (Opaque Layering)

• A virtual machine can only call operations from
  the layer below
• Design goal High maintainability

80
Open Architecture (Transparent Layering)

• A virtual machine can call operations from any
  layers below
• Design goal Runtime efficiency

VM1
VM2
VM3
VM4
81
Properties of Layered Systems

• Layered systems are hierarchical. They are
  desirable because hierarchy reduces complexity.
• Closed architectures are more portable.
• Open architectures are more efficient.
• If a subsystem is a layer, it is often called a
  virtual machine.
• Layered systems often have a chicken-and egg
  problem
• Example Debugger opening the symbol table when
  the file system needs to be debugged

82
Software Architectures

• Subsystem decomposition
• Identification of subsystems, services, and their
  relationship to each other.
• Specification of the system decomposition is
  critical.
• Patterns for software architecture
• Client/Server Architecture
• Peer-To-Peer Architecture
• Repository Architecture
• Model/View/Controller
• Pipes and Filters Architecture

83
Client/Server Architecture

• One or many servers provides services to
  instances of subsystems, called clients.
• Client calls on the server, which performs some
  service and returns the result
• Client knows the interface of the server (its
  service)
• Server does not need to know the interface of the
  client
• Response in general immediately
• Users interact only with the client

84
Client/Server Architecture

• Often used in database systems
• Front-end User application (client)
• Back end Database access and manipulation
  (server)
• Functions performed by client
• Customized user interface
• Front-end processing of data
• Initiation of server remote procedure calls
• Access to database server across the network
• Functions performed by the database server
• Centralized data management
• Data integrity and database consistency
• Database security
• Concurrent operations (multiple user access)
• Centralized processing (for example archiving)

85
Design Goals for Client/Server Systems

• Portability
• Server can be installed on a variety of machines
  and operating systems and functions in a variety
  of networking environments
• Transparency
• The server might itself be distributed (why?),
  but should provide a single "logical" service to
  the user
• Performance
• Client should be customized for interactive
  display-intensive tasks
• Server should provide CPU-intensive operations
• Scalability
• Server has spare capacity to handle larger number
  of clients
• Flexibility
• Should be usable for a variety of user interfaces
• Reliability
• System should survive individual node and/or
  communication link problems

86
Problems with Client/Server Architectures

• Layered systems do not provide peer-to-peer
  communication
• Peer-to-peer communication is often needed
• Example Database receives queries from
  application but also sends notifications to
  application when data have changed

87
Peer-to-Peer Architecture

• Generalization of Client/Server Architecture
• Clients can be servers and servers can be clients
• More difficult because of possibility of deadlocks

88
Example of a Peer-to-Peer Architecture

• ISOs OSI Reference Model
• ISO International Standard Organization
• OSI Open System Interconnection
• Reference model defines 7 layers of network
  protocols and strict methods of communication
  between the layers.

89
Middleware Allows You To Focus On The Application
Layer
90
Model/View/Controller

• Subsystems are classified into 3 different types
• Model subsystem Responsible for application
  domain knowledge
• View subsystem Responsible for displaying
  application domain objects to the user
• Controller subsystem Responsible for sequence
  of interactions with the user and notifying views
  of changes in the model.
• MVC is a special case of a repository
  architecture
• Model subsystem implements the central
  datastructure, the Controller subsystem
  explicitly dictate the control flow

91
Example of a File System based on MVC
Architecture
92
Sequence of Events
93
Repository Architecture

• Subsystems access and modify data from a single
  data structure
• Subsystems are loosely coupled (interact only
  through the repository)
• Control flow is dictated by central repository
  (triggers) or by the subsystems (locks,
  synchronization primitives)

94
Examples of Repository Architecture
Compiler
SyntacticAnalyzer
Optimizer
CodeGenerator
LexicalAnalyzer

• Hearsay II speech understanding system
  (Blackboard architecture)
• Database Management Systems
• Modern Compilers

SyntacticEditor
95
Summary

• System Design
• Reduces the gap between requirements and the
  machine
• Decomposes the overall system into manageable
  parts
• Design Goals Definition
• Describes and prioritizes the qualities that are
  important for the system
• Defines the value system against which options
  are evaluated
• Subsystem Decomposition
• Results into a set of loosely dependent parts
  which make up the system