Processing Transactions
This part describes how to do the transaction processing. You learn the fundamental techniques that safeguard the consistency of your database, involving how to control whether changes to the Oracle data are made permanent or undo.
The jobs or tasks that Oracle manages are termed as the sessions. The user session is started whenever you run an application program or an Oracle tool and connect to the Oracle. To permit the user sessions to work "concurrently" and share the computer resources, the Oracle should control the concurrency, the accessing of similar data by various users. Without sufficient concurrency controls, there might be a loss of the data integrity. That is, the changes to data might be prepared in the wrong order.
Oracle uses the locks to control the concurrent access to data. The lock gives you the temporary ownership of a database resource like a table or a row of data. And hence, the data cannot be changed by other users unless you finish with it. You never require explicitly lock a resource as default locking mechanisms protect the Oracle data and structures. Though, you can request data locks on the tables or rows when it is to your benefit to override the default locking. You can prefer from various modes of locking like the exclusive and row share.
The deadlock can occur when two or many users try to access the similar schema object. For illustration, the two users updating the similar table might wait if each tries to update a row presently locked by the other. As each user is waiting for the resources held by the other user, neither can continue till the Oracle breaks the deadlock by signaling an error to the last participating transaction.
If a table is being queried by one user and updated by the other at similar time, the Oracle generates a read-consistent view of the data for the query. That is, the data read by the query does not change, once a query begins and as it proceeds. The Oracle take snapshot of the table's data and records changes in the rollback segment. The Oracle uses rollback segments to build read-consistent query results and to undo the changes when necessary.