Why transactions?

Perhaps the first question to answer is "Why transactions?" There are a number of reasons to include transactional support in your applications. The most common ones are the following:

Applications often need to ensure that no matter how the system or application fails, previously saved data is available the next time the application runs. This is often called Durability.
Applications may need to make multiple changes to one or more databases, but ensure that either all of the changes happen, or none of them happens. Transactions guarantee that a group of changes are atomic; that is, if the application or system fails, either all of the changes to the databases will appear when the application next runs, or none of them.
Applications may need to make changes in isolation, that is, ensure that only a single thread of control is modifying a key/data pair at a time. Transactions ensure each thread of control sees all records as if all other transactions either completed before or after its transaction.