Transaction Concept

Transaction Concept

A transaction is a set of operations that collectively perform a single logical function.

E.g., transfer of funds from a checking account to a savings account

a single logical function from the customer's standpoint

to achieve this function, two operations are involved: (1) decrease the checking account by an X amount; and (2) increases the savings account by the same X amount

A transaction must see a consistent database.

In the funds transfer example, suppose the checking account balance before the execution of the transaction is $1000.00 and the amount to be transferred is $300.00. Then, the checking account balance should remain $1000.00 before it is changed to $700.00. It should not be that before the transaction reads the balance, the balance is $1000.00 and after reading the balance but before changing it to $700.00, it becomes $1200.00 (probably updated by another transaction).

When the transaction is committed, the database must also be consistent. Note that the database may not be consistent during the execution of the transaction.

· Two main issues to deal with:

-- failures of various kinds, such as hardware failures and system crashes

-- concurrent execution of multiple transactions