DAT 390 Wk 2 – Practice: Reading

Accurately complete the 2.1 to 2.5 Reading, Participation, and Challenge Activities to receive full points for this practice. You have unlimited attempts. The points you see in zyBooks do not reflect the points you receive in the gradebook.

A transaction is a sequence of database operations that must be either completed or rejected as a whole. Partial execution of a transaction results in inconsistent or incorrect data.

Ex: The debit-credit transaction transfers funds from one bank account to another. The first operation removes funds, say $100, from one account, and the second operation deposits $100 in another account. If the first operation succeeds but the second fails, $100 is mysteriously lost. The transaction must complete and save either both operations or neither operation.

Saving complete transaction results in the database is called a commit. Rejecting an incomplete transaction is called a rollback. A rollback reverses the transaction and resets data to initial values. A variety of circumstances cause a rollback:

• The operating system detects a device failure. Ex: Magnetic disk fails during execution of a transaction, and transaction results cannot be written to the database.
• The database detects a conflict between concurrent transactions. Ex: Two airline customers attempt to reserve the same seat on a flight.
• The application program detects an unsuccessful database operation. Ex: In the debit-credit transaction, funds are removed from the debit account, but the credit account is deleted prior to deposit.

When a failure occurs, the database is notified and rolls back the transaction. If the failure is temporary, such as intermittent network problems, the database attempts to restart the transaction. If the failure is persistent, such as a deleted bank account, the databases ‘kills’ the transaction permanently.