Database systems also support the concept of a transaction. A transaction is a group of related changes to
the data, where all changes must occur, or else none must occur. The familiar example is removing funds from
a savings account and depositing those funds in a checking account. We want both the withdrawal and the
deposit to succeed, but if the withdrawal succeeds and the deposit fails, we want the withdrawal to be ???rolled
back??? and the money put back into the savings account. The two changes constitute a single transaction which
must either succeed in its entirety, or be rolled back to have no effects whatsoever. Database systems allow
changes to the data to be grouped into transactions that are either ???committed??? upon full success, or entirely
???rolled back??? upon any failure.
DBMSs also promote data security by organizing use by multiple users. Imagine an enterprise like
Amazon.com where many users from all over the world interrogate the database of available titles, and place
orders, simultaneously. The DBMS coordinates multiuser access so as to preserve data integrity. Changes made
by one user will not interfere with the use of the database by another. The DBMS manages potential conflicts
by providing temporary locks on the data when necessary.
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