An Integrated Framework for Scenarios and State Machines

1
An Integrated Framework forScenarios and State
Machines
Rance
Cleaveland
Department of Computer Science
University of Maryland, College
Park

• Bikram Sengupta
• IBM Research
• India

2
Outline

• Background
• Motivation
• Triggered Message Sequence Charts (TMSCs)
• Communicating State Machines (CSMs)
• Acceptance Trees
• Combining TMSCs and CSMs
• An Integrated Framework
• Semantics
• Case Study
• Automated Resuscitation and Stabilization System
  (ARSS)
• Conclusions and Future Work

3
Motivation

• Heterogeneous specifications, featuring a mixture
  of high-level requirements and lower-level design
  artifacts, are motivated by several development
  methodologies
• In spiral system development processes,
  requirements elicitation and system design often
  proceed hand-in-hand
• Intermediate stages of refinement-based
  strategies contain a mixture of design elements
  and requirements
• UML supports notations for both requirements
  modeling and operational design
• However, research in the area of heterogeneous
  specifications has remained confined to the more
  theoretical domains of process algebra, temporal
  logic and mu-calculus
• We need to explore how these ideas may be adapted
  to more accessible notations used in practice
• We propose a framework for heterogeneous system
  specifications consisting of a mix of
• Higher-level scenario-based requirements,
  expressed as Triggered Message Sequence Charts
  (TMSCs)
• State-machine-based subsystem designs, given as
  Communicating State Machines (CSMs)

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Triggered Message Sequence Charts (TMSCs)
Q
P
a
trigger
Conditional Scenario
b
action
terminates
extensible
Partial Scenario
5
Communicating State Machines
A2
A1
6
Acceptance Trees the must pre-order

• P ltmust Q, if Q is more deterministic than P

acceptance set

P1
P2
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Combining TMSCs and CSMs

• TMSCs and CSMs both specify system behavior in
  terms of sequences of events that may occur
• A scenario shows only one possible interaction
  between instances, and is inherently incomplete
• Conditional and partial TMSC scenarios make the
  behavior even more incomplete
• In CSMs, the individual behavior of each instance
  is generally given over all interactions, hence
  they are more complete
• Any common account of TMSCs and CSMs should
• Be able to express both underspecified as well as
  fully specified behavior in a uniform manner
• Provide operators to weave together multiple
  scenarios and to allow networks of CSMs to be
  formed
• Prescribe when a CSM correctly implements a
  TMSC specification, or, more generally, when one
  heterogeneous specification refines another
• An acceptance tree-based framework will have the
  right ingredients
• Execution-based
• Behavior may be expressed at various levels of
  detail through acceptance sets
• Must-preorder may be used to check the
  relationship between scenarios, state-machines,
  and a mixture of these notations, once they are
  expressed as acceptance trees

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Example From TMSCs to Acceptance Trees
abc
X
Y
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A Common Framework

• H M (single TMSC)
• S (single ISM)
• X (variable)
• H H (communicating
  parallel comp)
• H H (interleaving
  parallel comp)
• H H (delayed choice)
• H H (internal
  choice)
• H /\ H (logical AND)
• H H (sequential
  composition)
• recX . H (recursive
  operation)

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Semantics of Heterogeneous Expressions

• Combine acceptance trees of sub-expressions

• Semantics is compositional
• If P ltmust Q then P op R ltmust Q op R

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Case Study Automated Resuscitation and
Stabilization System
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Initial System Requirements
RB M1 M2 RP M3 M4 RD M5 M6
BS RB RP RD
RS BS /\ T1
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Initial Design
C1
RB M1 M2 ltmust C1
C2
RB ltmust C1, RP ltmust C2, RD ltmust C3
BS RBRPRD ltmust C1C2C3
C3
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Intermediate Heterogeneous Design
Initial Design ID
C1C2C3 ltmust ID ltmust ID /\ T1
15
Refined Design New Requirement
Refined Design RD
ID ltmust RD HS2 RD /\ T2
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Final Design
RD /\ T2 ltmust FD
RS BS /\ T1 ltmust C1C2C3 /\ T1
ltmust RD ltmust RD /\ T2 ltmust FD
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Conclusions

• Requirements and design phases often overlap in
  practice
• Need for a common framework that would allow
  requirements and design notations to
  inter-operate
• We presented a framework for heterogeneous
  specifications involving
• Requirements expressed as TMSCs
• Design elements represented as CSMs
• Semantics is based on acceptance trees
• Precise notion of refinement in terms of the
  must pre-order
• Supports principled evolution of higher-level
  requirements to lower-level operational
  specifications
• Future Work
• Extend framework to cater to other notations
• Synthesize state-machines from TMSC expressions