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Oracle also provides read consistency of data, meaning that all the data selected by your queries comes from a single point in time Oracle s undo segments are part of the undo tablespace mentioned earlier in this chapter Oracle uses locking mechanisms to ensure data concurrency By allowing one user to lock individual rows or entire tables, that user is guaranteed exclusive use of the table for updating purposes An important feature of the Oracle locking mechanisms is that they are, for the most part, automatic You don t need to concern yourself with the details of how to lock the objects you want to modify Oracle will take care of it for you behind the scenes Oracle uses two basic modes of locking The exclusive lock mode is used for updates, and the share lock mode is used for SELECT operations on tables.

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The share lock mode enables several users to simultaneously read the same rows in a table The exclusive lock mode, because it involves updates to the table, can only be used by one user at any given time Exclusive locks are almost always applied to the specific rows being updated, permitting simultaneous use of the database by several users Oracle releases the locks it holds on the tables and other internal resources automatically after the issue of a COMMIT, SAVEPOINT, or ROLLBACK command Oracle locking is complex, and you ll learn about it in detail in 6, along with how Oracle ensures data consistency and concurrency..

The database writer, as you saw earlier, is responsible for writing all modified buffers in the database buffer cache to the data files. Further, it has the responsibility of ensuring there is free space in the buffer cache so the server process can read in new data from the data files when necessary. The

set -A DATE `date +"%S %M %k %d %m %Y"

(log) write ahead protocol also requires that the redo records in the redo log buffer associated with the changed data in the data buffer cache are written to the redo log data files before the changes are recorded in the data files themselves. The importance of the redo log contents makes it imperative that Oracle write the contents of the redo log file to permanent storage before it writes the changes to the data files on disk. When users commit their transactions, the log writer process immediately writes only a single commit record to the redo log files. The entire set of records affected by the committed transaction may not be written simultaneously to the data files. This fast commit mechanism, along with the write ahead protocol, ensures that the database is not kept waiting for all the physical writes to be completed after each transaction. As you can well imagine, a huge OLTP database with numerous changes throughout the day cannot function optimally if it has to write to disk after every committed data change.

If there are a large number of transactions and, therefore, a large number of commit requests, the log writer process may not write each committed transaction s redo entries to the redo log immediately. It may batch multiple commit requests if it is busy writing previously issued commit records. This batched writing of redo entries from multiple committed transactions is known as group commits.

The system change number (SCN) is an important quantifier that the Oracle database uses to keep track of its state at any given point in time When you read (SELECT) the data in the tables, you don t affect the state of the database, but when you modify, insert, or delete a row, the state of the database is different from what it was before Oracle uses the SCN to keep track of all the changes made to the database over time The SCN is a logical time stamp that is used by Oracle to order events that occur within the database The SCN is very important for several reasons, not the least of which is the recovery of the database after a crash SCNs are like increasing sequence numbers, and Oracle increments them in the SGA.

Table 2-1 shows commonly used primitives provided by the runtime. Table 2-1. CTS Primitives and C++/CLI Type Names

When a transaction modifies or inserts data, Oracle first writes a new SCN to the rollback segment The log writer process then writes the commit record of the transaction immediately to the redo log, and this commit record will have the unique SCN of the new transaction In fact, the writing of this SCN to the redo log file denotes a committed transaction in an Oracle database The SCN helps Oracle determine whether crash recovery is needed after a sudden termination of the database instance or after a SHUTDOWN ABORT command is issued Every time the database checkpoints, Oracle writes a START SCN command to the data file headers.

declare -a DATE=( `date +"%S %M %k %d %m %Y"` )