Page 1429 - (ISC)² CISSP Certified Information Systems Security Professional Official Study Guide

account 1001 and then subtract $250 from account 2002:

BEGIN TRANSACTION
UPDATE accounts
SET balance = balance + 250
WHERE account_number = 1001;

UPDATE accounts
SET balance = balance – 250
WHERE account_number = 2002

END TRANSACTION

Imagine a case where these two statements were not executed as part
of a transaction but were instead executed separately. If the database
failed during the moment between completion of the first transaction
and completion of the second transaction, $250 would have been
added to account 1001, but there would be no corresponding

deduction from account 2002. The $250 would have appeared out of
thin air! Flipping the order of the two statements wouldn’t help—this
would cause $250 to disappear into thin air if interrupted! This simple
example underscores the importance of transaction-oriented
processing.

When a transaction successfully finishes, it is said to be committed to
the database and cannot be undone. Transaction committing may be

explicit, using SQL’s COMMIT command, or it can be implicit if the end
of the transaction is successfully reached. If a transaction must be
aborted, it can be rolled back explicitly using the ROLLBACK command
or implicitly if there is a hardware or software failure. When a
transaction is rolled back, the database restores itself to the condition
it was in before the transaction began.

All database transactions have four required characteristics: atomicity,

consistency, isolation, and durability. Together, these attributes are
known as the ACID model, which is a critical concept in the
development of database management systems. Let’s take a brief look
at each of these requirements:

Atomicity Database transactions must be atomic—that is, they must