Modeling and Analysis of Architectural Exceptions

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Modeling and Analysis ofArchitectural
Exceptions

• Fernando Castor Filho Patrick Henrique da
  S. Brito
• fernando_at_ic.unicamp.br
  patrick.silva_at_ic.unicamp.br
• Cecília Mary F. Rubira
• cmrubira_at_ic.unicamp.br
• FM2005 Workshop on Rigorous Engineering of
  Fault-Tolerant Systems
• REFT2005, Newcastle upon Tyne, July 19th 2005

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Exception Handling

• Popular mechanism for structuring forward error
  recovery in software systems
• Exceptions can be derived incrementally at
  different phases of development
• Requirements
• Architecture
• Detailed Design
• Implementation

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Exception Handling

• Popular mechanism for structuring forward error
  recovery in software systems
• Exceptions can be derived incrementally at
  different phases of development
• Requirements
• Architecture
• Detailed Design
• Implementation

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Exceptions at the Architectural Level

• A systems exceptional activity should be
  addressed since the early phases of development
• In recent years, many approaches combining
  software architecture and exception handling have
  been proposed
• There hasnt been much focus on the description
  of exceptions at the architectural level
• This may be required for systems with strict
  dependability requirements such as commercial
  applications, control systems, and so on.

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An Air-Traffic Control System Example
Exceptions
Exceptions
Exceptions
Exceptions
Exceptions
Exceptions
Source Bass, Clements, and Kazman,
Software Architecture in Practice, 2nd Edition,
2003.
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... Some Interesting questions...

• What does a double-headed arrow mean?
• What are the exceptions that each component
  signals and handles?
• Are there any relevant cause-effect
  relationships?
• Is this analyzable?

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Problem

• To describe software architectures so that it is
  possible to reason about the flow of exceptions
  at the architectural level

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Requirements of the Solution

1. Easy to use (pictorial representation)
2. Integrated with the concept of architectural
   style
3. Precise (unambiguous)
4. Analyzable
5. Capable of expressing rules of existing exception
   handling models

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Alloy Design Language

• Lightweight formal method
• Similar to Z (less expressive but supports
  automated analysis)
• Support for complex data structures
• Declarative
• Alloy constraint analyzer
• Easy to use
• Requirements 3-5

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Proposed Framework Aereal
Normal Architectural Styles
Exceptional Architectural Styles
Architecture Description Extended with Exceptions
Translation
Arch. Description Exception Flow View
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Proposed Framework Aereal
Normal Architectural Styles
Exceptional Architectural Styles
Architecture Description Extended with Exceptions
Translation
Arch. Description Exception Flow View

• Documentation
• Analysis of stylistic constraints

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Proposed Framework Aereal
Normal Architectural Styles
Exceptional Architectural Styles
Architecture Description Extended with Exceptions
Translation
Arch. Description Exception Flow View

• Exception flow analysis

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Proposed Framework Aereal
Normal Architectural Styles
ACME
Exceptional Architectural Styles
Architecture Description Extended with Exceptions
Translation
Arch. Description Exception Flow View
Alloy
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Elements of the Model
Components Signals Raises Encounters Handles SignalsTo CatchesFrom PortMap Ducts Signals Raises Encounters CatchesFrom SignalsTo Exceptions
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An Example
CoalFeederController
Duct1
AirFlowController
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An Example
GENERIC MODEL sig Component Signals Exception-gtDuct, SignalsTo set Duct, sig Duct Encounters set Exception, CatchesFrom one Component INSTANTIATION sig AirFlowCtr extends Component sig Duct1 extends Duct sig AirFlowActuatorTimeout extends Exception fact SystemStructure AirFlowCtr.SignalsTo Duct1 Duct1.CatchesFrom AirFlowCtr fact ExceptionFlow AirFlowCtr.Signals AirFlowActuatorTimeout-gtDuct1 Duct1.Encounters AirFlowActuatorTimeout
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Properties of Interest

• Basic EH mechanism properties
• Desirable EH properties
• Application-specific properties
• Verified using the Alloy Analyzer
• Violations of properties generate graphical
  counter-examples

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Examples of Properties

• Exceptions encountered by a component and not
  handled or propagated are signaled
• If a component raises an exception, it must also
  signal the exception
• The exceptions encountered by a component are all
  the exceptions signaled by ducts in the
  components CatchesFrom set
• No useless handlers

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Example No useless handlers

• pred no_useless_handlers()
• all C Component all D C.CatchesFrom
• D.(C.Handles) in D.(C.Encounters)
• D.(C.Encounters)lt(D.(C.Propagates))D.(C.Prop
  agates)

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Future Directions

• Model coordinated exception handling
• Technical report describing the whole model
• Extend the implementation of Aereal in order to
  automatically compute the sets of exceptions that
  are caught and signaled

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Thank You!
Contact information Fernando Castor
Filho fernando_at_ic.unicamp.br fernando.castor_at_newca
stle.ac.uk