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Locks ensure data consistency while allowing data concurrency by letting multiple users simultaneously access the database Oracle does its locking implicitly; you don t have to worry about which table to lock or how to lock it, as Oracle will automatically place locks on your transaction s behalf when necessary By default, Oracle uses row-level locking, which involves the least restrictive amount of locking, thus guaranteeing the highest amount of concurrency By default, Oracle stores the locked row information in the data blocks Also, Oracle never uses lock escalation that is, it doesn t go from a lowerlevel granularity like row-level locking to a higher level of granularity like table-level locking.

Oracle s multiversion concurrency control system is a variation of the time-stamp approach to concurrency control; it maintains older versions of table data to ensure that any transaction can read the original data even after it has been changed by other transactions Unlike locking, no waits are involved here; transactions use different versions of the same table instead of waiting for other transactions to complete When transactions want to update a row, Oracle first writes the original beforeimage to an undo record in the undo tablespace Queries then have a consistent view of the data, which provides read consistency they only see data from a single point in time Using the same mechanism, Oracle is also capable of providing transaction-level read consistency, meaning that all the separate statements in a transaction see data from a single point in time.

Figure 11-1. A screenshot of SystemBrowser with the Folders navigator selected The navigator and content panes must be mutually synchronized, in the sense that selecting an item in one updates the other. When you select a folder in the Folders navigator, the content pane must show the folders and files in the selected folder. The status bar must display the complete path of the folder. When you select a file in the content pane, the status bar must show the file s size and its complete file path. Double-clicking a folder in the content pane should make two things happen: The Folders navigator should select the folder, and the content pane should show the contents of the folder. The other navigator is a Search control that allows you to recursively search all subdirectories of a given directory for a file. SystemBrowser supports regular expressions for filenames, so you can do a search with names like m .exe or *.doc. You must be able to interrupt the search at any time by clicking a button. While the search runs, the status bar must show the number of files found. The content pane must display a list of the files found. Selecting a file in the content pane must make the file s size and path appear in the status bar. Figure 11-2 shows the system after running a search.

The multiversion concurrency control system used by Oracle enables you to get by with the less-stringent readcommitted mode of isolation instead of having to use the slower but safer serializable isolation level Here are some important features of Oracle locking: Oracle implements locks by setting a bit in the data item being locked The locking information is stored in the data block where the row lives Locks are held for the entire length of a transaction and are released when a COMMIT or a ROLLBACK statement is issued..

Oracle doesn t use lock escalation. Oracle doesn t need to escalate locks, as it stores the locking information in the individual data blocks. Lock escalation for example, an escalation from the row level to the table level reduces concurrency. Oracle does use lock conversion, which involves changing the restrictiveness of a lock while keeping the granularity of the lock the same. For example, a row share table lock is converted into a more restrictive row exclusive table lock when a SELECT FOR UPDATE statement starts updating the previously locked rows in the table. I explain locking granularity and Oracle locking types in more detail in the following sections. In the next few sections, you ll learn more about the locking methods and lock types used by Oracle s concurrency control mechanism.

Figure 11-2. A screenshot of SystemBrowser with the Search pane selected The program must have a menu bar. The File menu has only an Exit command. The View menu has two groups of menu items, as shown in Figure 11-3.