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+===== Exception Handling in Control Systems =====
+//D.A. van Beek//
+This thesis deals with the required concepts and
+mechanisms for exception handling in control systems.
+There is much confusion in the literature about
+the meaning of exceptions and the relationship of exceptions and
+errors. In this thesis, the most important terms relating to
+errors and exceptions are accurately defined, while retaining a
+high level of compatibility with the way these terms are used in
+the literature.
+A treatment is presented of the most important
+concepts relating to the three stages of error handling: error
+detection, error diagnosis and confinement, and error recovery
+and fault repair. Only forward error recovery is covered in this
+thesis.
+Several satisfactory and more or less similar
+exception handling mechanisms exist for the handling of internal
+exceptions. These mechanisms and a traditional mechanism are
+briefly evaluated. The resume response from an exception handler
+is rejected as being undesirable in both single and multi-process
+environments.
+A literature search has yielded several proposals
+and existing mechanisms for exception handling in controlling
+systems or, more generally, in a multi-process environment. No
+publications, however, have been found which describe the essence
+of the required functionality of such mechanisms. Therefore,
+'constraint of an operation' and 'constraint violation' have been
+introduced as new concepts. The constraint of an operation is
+that part of its precondition which is invariant over the
+operation: it has to be valid throughout the execution of the
+operation. A violation of an operation's constraint causes an
+exception occurrence in the process executing the operation and
+should result in the raising of an exception.
+The concepts constraint and constraint violation
+have been used to describe the required functionality of
+mechanisms for the handling of exceptions in controlling systems.
+Several existing and proposed mechanisms have been evaluated
+using this functionality. The mechanisms have been evaluated as
+either offering a functionality which is too restricted for
+controlling systems, as offering an incorrect or undesirable
+functionality, or as inadequate in other ways.
+A new mechanism for the handling of constraint
+violations has been introduced. The mechanism has been realized
+by means of constraint monitors which are used to specify and
+monitor constraints of operations independently of other
+operations, which is an important requirement for the creation of
+modular subprograms. A constraint monitor bound to a single
+operation can also be used to specify a constraint which is
+common to several operations.
+A constraint monitor is bound to an operation,
+and consists of a constraint and an exception. The violation of
+the constraint which is monitored by a constraint monitor results
+in the creation of a pending exception. The exception is not
+immediately raised, since this can result in time-dependent
+run-time errors due to violations of the internal invariants of a
+process. Pending exceptions are raised at interaction points,
+which are natural places for internal invariants to hold, but
+they are not raised in exception handlers.
+Several constraints can be violated at the same
+time by concurrently executing processes. This can result in more
+than one pending exception in a process. Some criteria for the
+selection of a pending exception have been evaluated. The pending
+exception that should be selected is the one belonging to the
+constraint monitor which was enabled first, and thus at the
+outermost level. The other pending exceptions can be discarded.
+The only systems considered are systems that can
+be modeled as discrete event systems.
+The new mechanism is independent of a particular
+programming language. The functionality of the mechanism
+therefore deals with the common requirements of languages for the
+control of industrial systems. Language-specific elements are not
+treated.
+Constraint monitors have been successfully
+implemented in Process Calculus, which is a language for the
+specification, simulation and control of industrial systems. The
+simplicity and power of the new mechanism is illustrated with a
+case concerning the control of a transport system.
+//PhD thesis//, Eindhoven University of Technology, The Netherlands, 1993.
+{{thesis.pdf|pdf}}

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