COBOL Manual for TNS and TNS/R Programs

HP COBOL Manual for TNS and TNS/R Programs—522555-006
32-1
32 Fault-Tolerant Processes
A process is running in a fault-tolerant manner when no single point of failure can stop
the process or corrupt its data or the files it is manipulating. Processes are not
automatically fault tolerant—they must be designed and implemented to be fault
tolerant.
How might a single point of failure affect a process? Suppose a process is operating
an automated teller machine (ATM) for a financial institution. If you, as a customer,
come to the ATM and request $20 from your account:
•
You want the ATM to service your request, not terminate before completing the
transaction.
•
You want the ATM to record at most one debit of $20 from your account; the
institution wants the ATM to record at least one debit of $20 from your account.
•
You want the ATM to dispense at least $20 to you; the institution wants the ATM to
dispense at most $20 to you.
If the process is not fully fault tolerant, a number of possible failures can interfere with
the preceding desires:
•
A process might be running on a processor that fails (blows a fuse, is accidentally
unplugged, is stopped by an operator, or whatever), so the transaction does not
complete.
•
A process might get part way through your withdrawal transaction, deducting the
$20 from your balance but not yet reaching the point of dispensing the cash to you.
If the process is automatically retried, it might deduct the $20 from your account
again and dispense you the cash; but the balance in your account would then be
down by $40.
•
A process might fail during the transaction after disbursing the $20 to you, but
before recording the fact, and resume at the point of asking what you want. If you
again asked for $20, the process could disburse another $20 (total = $40) and
record only a $20 withdrawal.
•
A process might disburse the $20 cash to you and fail before making a permanent
record of the transaction.
Topics:
•
NonStop Operating System
•
Fault-Tolerant Facility
•
TMF