Real Time Systems

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Real Time Systems

• Miscellaneous
• Topics

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Review
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Outline

• Exception Services
• More on Timing Services
• Cardinality and Capsule Structure

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Exception Services

• as we will see in upcoming weeks, fault tolerance
  is a critical requirement of almost any real-time
  system exception handling therefore becomes an
  important aspect of system design
• the Exception Services of RoseRT provide us with
  the concept of Exception (system) ports through
  which capsules will raise and handle exception
  signals (RTExceptionSignal)

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Exception Services (2)

• exception ports are of type Exception
• exception signals are of type RTExceptionSignal
• RTExceptionSignal methods
• raise( ) exceptions must be raised by the
  application
• defined exception signals
• userError(some user-defined RTTyped value)
• arithmaticError( ) error()
• notFoundError( ) notUnderstoodError( )
• serviceAccessError( ) streamError( )
• subclassResponsibilityError( )
• timesliceError( )

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Exception Services (3)

• example
• // on entry to deliver state
• // raise error if SAP registration fails
• if ( robotPort.registerSAP(deliveryService)
  ! 1 )
• errorPort.serviceAccessError(failed to connect
  to delivery service).raise( )
• // within transition to Error state
• RTString eMessage (RTString )getMsg()-gtgetData
  ()
• char eMessage (char )getMsg()-gtgetData()
• eLog.log(eMessage)

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More on Timing Services

• another critical aspect of designing a real-time
  system is the ability to model temporal
  information
• the event-based, state machine centric design
  approach used within RT-UML nicely supports the
  modeling of temporally ordered events,
• however, in order to model timed or periodic
  events (commonly found within RTS), we need
  another set of services - Timing Services

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More on Timing Services (2)

• the time services provide both relative and
  absolute timer functions
• built-in type RTTimespec provides a means of
  specifying time variables in seconds (or nsecs)
• timing ports provide a mechanism for capsules to
  set / cancel timers and access system time
• methods informIn( ), informEvery( ), informAt(
  ), cancelTimer( RTTimerID), currentTime( )
• built-in type RTTimerID provides a means of
  identifying specific timing port actions

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More on Timing Services (3)

• example
• // set a periodic timer on entry to Operate
  state
• // note RTTimerID perTID is defined on the
  capsule
• RTTimespec updatePeriod(0, 750000000)
• perTID perTimerPort.informEvery(updatePeriod)
• // cancel timer on entry to Shutdown state
• if ( perTimerPort.cancelTimer(perTID) ! 1 )
• systemErrorPort.userError(cancel timer
• error).raise( )

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More on Timing Services (4)

• another example
• // note the following yields a common run-time
  error // (bad priority on a timer port)
• someTimerPort.informIn(3, 500000000)
• // informIn is an overloaded method
• //where in the signature below, prio is optional
• TiminginformIn( RTTimespec, int prio)
• // and what you likely meant was
• someTimerPort.informIn(RTTimespec(3,
  500000000))

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Cardinality and Capsule Structure

• cardinality on capsule roles
• fixed defines the number of replicated capsule
  roles to create at containing capsule creation
• optional / plug-in defines the maximum number
  of capsule roles which can be created dynamically
  by the containing capsule
• cardinality on ports
• determines the number of instances of a port
• required to allow connections with replicated
  capsule roles
• each replicated port shares a common protocol

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Cardinality and Capsule Structure - Common
Structural Patterns

• Star

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Cardinality and Capsule Structure - Common
Structural Patterns (2)

• Bus

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Cardinality and Capsule Structure - Common
Structural Patterns (3)

• Array

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Cardinality and Capsule Structure - Common
Structural Patterns (4)

• Combination

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Miscellaneous Exercise

• modify the Production Line structure diagrams to
  support the following new requirements
• two more production lines have been added
• each production line is equipped with 5 identical
  workstations capable of producing widgets
• a. initially 3 delivery robots are available, one
  for each production line
• b. later a 4th robot becomes available and the
  manager decides it is best that they be viewed as
  belonging to the entire plant and are thus shared
  by all production lines

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Questions