Reachability%20analysis

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Reachability analysis

• A reachability analysis shows the product space
  of the two processes and the signal queues of
  their input ports.
• Say we have an SDL system. The diagrams on the
  following slides show the interface between a
  pair of processes A and B in the system.

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The SDL Block containing two processes
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Two communicating SDL Processes
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Reachability analysis (How to do a State Space
exploration)

• To perform the reachability analysis, the first
  step is to transform the process graph into
  transition charts.
• In a transition chart, each input is prefixed
  with a ? and each output by a !.
• Each transition consists of either an input or an
  output.

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The Transition chart for the SDL Processes
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State space exploration

• Assuming that there is a maximum queue size of 1,
  this is the global graph for the transition
  charts.

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State Space exploration graph
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Possible errors

• Unspecified receptions
• An unspecified reception occurs when a state in
  the global behavior is scheduled to receive a
  message that it does not know how to handle
• Queue overflow
• This occurs when there is an attempt to put a
  signal in a queue that is already full

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Possible errors(continued)

• Deadlocks
• deadlocks can also occur when 2 or more processes
  are unable to proceed because they wait endlessly
  for signals from each other.
• Livelocks
• livelocks occur when processes send signals
  endlessly without making any progress.

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Deriving role behaviours

• Now the global behaviour graph grows very
  rapidly.
• For large systems, we need to simplify the job.
• One method is deriving role behaviours.

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Deriving role behaviours

• Steps to follow
• Mark the transitions that involve signals to and
  from the environment with the special symbol ?.
  These transitions are called invisible
  transitions.
• Find the set of nodes reachable from a given node
  by following one or more ?-transitions (called
  ?-ambiguities) and group them together.
• We have a reduced transition chart. Then, proceed
  as usual.

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The Modified Transition chart with ?-transitions
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The Modified Transition chart with
?-transitions (without the ?-transitions)
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The new State-Space exploration graph
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Tau tool

• We can explore the state space by using the
  validator in Telelogic Tau tool.
• When a random walk is done, the tool will
  randomly select a branch and move down the graph
  until it reaches an unspecified reception,a
  deadlock or the end of the graph (maximum depth
  specified). It then selects randomly another
  branch.
• The following is the output given when the random
  walk was done on the egg timer system.

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Click here to Start the Bit-State or Random
Walk state space exploration or to go step by
step through the specification use the navigator
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• A bit-state can also be done.
• The bit-state will attempt to find all the
  possible outcomes from the top node. From these,
  it will repeat the same operation and go through
  their possible outcomes.
• The following is the output of the bit-state
  search in the validator.

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