Computer World. Матросова Т.А. - 57 стр.

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56
given time. The DBMS must allow such concurrent accesses, prevent problems such
as two agents assigning the same seat simultaneously, and protect against loss of
records if the system suddenly fails.
Banking Systems
Data items include names and addresses of customers, accounts, loans, and their
balances, and the connection between customers and their accounts and loans, e . g .
who has signature authority over which accounts. Queries for account balances are
common, but far more common are modifications representing a single payment
from, or deposit to, an account.
As with the airline reservation system, we expect that many tellers and
customers (through ATM machines or the Web) will be querying and modifying the
bank's data at once. It is vital that simultaneous accesses to an account not cause the
effect of a transaction to be lost. Failures cannot be tolerated. For example, once the
money has been ejected from an ATM machine, the bank must record the debit,
even if the power immediately fails. On the other hand, it is not permissible for the
bank to record the debit and then not deliver the money if the power fails. The
proper way to handle this operation is far from obvious and can be regarded as one of
the significant achievements in DBMS architecture.
Corporate Records
Many early applications concerned corporate records, such as a record of each
sale, information about accounts payable and receivable, or information about
employees – their names, addresses, salary, benefit options, tax status, and so on.
Queries include the printing of reports such as accounts receivable or employees'
w eekly payc hec ks. Each sale, purchase, bill, rec eipt, employee hired, fired, or
promoted, and so on, results in a modification to the database.
TEXT 3
Read the text. The paragraphs are mixed, put them in their correct order.
For example, since the rate at which data can be read from a given disk is
fairly low, a few megabytes per second, we can speed processing if we use many
disks and read them in parallel (even if the data originates on tertiary storage, it is
“cached” on disks before being accessed by the DBMS). These disks may be part
of an organized parallel machine, or they may be components of a distributed
system, in which many machines, each responsible for a part of the database,
communicate over a high-speed network when needed.
Of course, the ability to move data quickly, like the ability to store large
amounts of data, does not by itself guarantee that queries can be answered quickly.
We still need to use algorithms that break queries up in ways that allow parallel
computers or networks of distributed computers to make effective use of all the
resources. Thus, parallel and distributed management of very large databases remains
an active area of research and development.

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