Information Technology Reference
In-Depth Information
processing has involved the entire outline through step 5. You have
requested a transfer of money, the bank has performed the relevant
processing, and the ATM has notified you that the work is done. At
this stage, you want assurance that the balances in each of your ac
counts are correct; the risk of a computer crash or malfunction
would be that the record of your transfer was lost. To resolve these
types of problems, financial activities are considered as a sequence
of events, called
transactions
. Accounting standards require banks
to maintain logs, called
audit trails
, of each transaction. An audit
trail is simply a record of each event: an ongoing record of activity
for your accounts and all other accounts as well. In maintaining
these audit trails, editing is never allowed. Once a transaction oc
curs, it can never be erased. If a mistake is made, then a new trans
action is performed to correct the error, so both the error and its
correction appear in the audit trail.
Because of this system, the bank will have a record of your
transfer on its audit trail—even if the machine subsequently crashes
or malfunctions. After the ATM or bank's computer system is re
paired, these audit trails can be used with backup files to restore all
files, regardless of what information may have been damaged in the
malfunction itself.
Crashes or Malfunctions While You Are in the Process of Making
Your ATM Transfer: So what happens when a transaction is interrupted
in the middle? In this case, processing starts, but does not finish prop
erly. To handle these circumstances, many modern systems process
transactions in multiple phases. At the start of a transaction, the com
puter records the current state of data. Then, as the transaction pro
ceeds, the information is updated tentatively, and final updating of
records is deferred until all parts of the transaction are completed.
With this approach, any errors cause the transaction to be canceled,
with the original state of data restored. In the terminology of data
bases, the computer
rolls back
transactions, discarding updates.
In the case of your ATM transfer, the bank may have with
drawn money from one account, but not made the corresponding
deposit to your account at the time of a computer crash. No ac
count's records, however, are finalized until all relevant processing
has been done correctly; there is no chance that one account record
will be updated without the other having the corresponding revi
sion. Altogether, the roll back assures us that our records will stay
consistent for any banking transaction.
