Exception Handling and Transaction management

Exception Handling:

The protocol for managing synchronous errors (errors which are generated by the process) is another issue defined by transaction management (extending the basic operating systems facilities). Generally the data management system wants to abort the transaction if the application program fails. This is in general handled by organizing the exceptions into a hierarchy. If a lower level of the hierarchy not succeeds to handle the error it is passed to a higher node of the hierarchy. The data manager typically has a few handlers very near the top of the hierarchy (the operating system gets the root of the hierarchy.)

a) Either the process or else the data management system (or both) may perhaps  ----establish an exception handler to field errors.

b) When an exception is noticed then the exception is signalled.

c) Exception handlers are invoked in some fixed order (typically order of establishment) until one productively corrects the error. This operation is described percolation.

PL/l ‘ON units’ or else the IBM Operating System set-task-abnormal-exit (STAE) are instances of this mechanism. Example of exception conditions are- arithmetic exception conditions (that is overflow), invalid program reference (that is to protected storage) wild branches, infinite loops, deadlock, .. as well as attempting to read beyond end of file.

There may be numerous exception handlers active for a process at a particular instant. The program's handler is typically given the first try at recovery if the program has established a handler. The handler will in general diagnose the failure as one that was expected (overflow) one that was unexpected however can be handled (invalid program reference), or one that is unexpected as well as cannot be dealt with by the handler (infinite loop). If the failure is able to be corrected, the handler makes the correction as well as continues processing the program (perhaps at a different point of execution.) If the failure can’t be corrected by this handler, then the exception will permeate to the next exception handler for that process.

The system in general aborts any process, which infiltrates to the system revival routine or doesn’t participate in recovery. This process involves terminating each processing being done on behalf of the process, restoring each non-consumable resources in use by the process to operating system control (that is storage) and removing to the greatest extent possible the effects of the transaction.

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