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The control file is a file that the Oracle DBMS maintains to manage the state of the database, and it is probably the single most important file in the Oracle database. Every database has one control file, but due to the file s importance, multiple identical copies (usually three) are maintained when the database writes to the control file, all copies of the file get written to. The control file is critical to the functioning of the database, and recovery is difficult without access to an up-to-date control file. Oracle creates the control file (and the copies) during the initial database creation process. The control file contains the names and locations of the datafiles, redo log files, current log sequence numbers, backup set details, and the all-important system change number (SCN), which indicates the most recent version of committed changes in the database information that is not accessible by users even for reading purposes. Only Oracle can write information to the control file, and the Oracle server process continually updates the control file during the operation of the database. Control files are vital when the Oracle instance is operating. When you turn the instance on, Oracle reads the control file for the location of the data and log files. During the normal operation of the database, the database consults the control file periodically for necessary information regarding virtually every structure of the database.

ClientConnected ClientConnected RequestStarted RequestStarted RequestProcessed RequestProcessed OnRequestProcessed OnRequestStarted OnClientConnect

The control file is also important in verifying the integrity of the database and when recovering the database. The checkpoint process instructs the database writer to write data to the disk when some specific conditions are met, and the control file notes all checkpoint information from the online redo log files. This information comes in handy during a recovery the checkpoint information in the control file enables Oracle to decide how far back it needs to go in recovering data from the online redo log files. The checkpoint indicates the SCN up to which the datafiles are already written to the datafiles, so the recovery process will disregard all the information in the online redo log files before the checkpoint noted in the control file. When you start an Oracle instance, it consults the control file first, to identify all the datafiles and the redo log files that must be opened for database operations.

The checkpoint process is discussed in more detail in the Checkpoint section later in this chapter.

Due to its obvious importance, Oracle recommends that you keep multiple copies of the control file. The V$CONTROLFILE dynamic view gives you the names of all the control files. The STATUS column will be NULL if the name can be determined, which is the case always. If the name can t be determined (which shouldn t happen), you ll see the value INVALID in the STATUS column. The IS_RECOVER_DEST_FILE column shows YES if the control file was created in the flash recovery area and a value of NO otherwise. Here s the output of a query on the V$CONTROLFILE view: SQL> SELECT status, name, is_recovery_dest_file FROM V$CONTROLFILE; STATUS ----------NAME ------------------------------------C:\ORACLE\ORADATA\MARK1\CONTROL01.CTL C:\ORACLE\ORADATA\MARK1\CONTROL02.CTL C:\ORACLE\ORADATA\MARK1\CONTROL03.CTL IS_RECOVERY_DEST ---------------NO NO NO

Figure 12-17. The signal diagram of HostEmulator The Coordinator is shown making statically bound calls to the three Worker objects on the right, entailing type coupling between the Coordinator and the workers. To eliminate the coupling, you could have made the Coordinator talk to the workers using event notifications. This approach would have been a bit more complicated, because you would have needed a JIT BuilderBinder, since ConnectionProcessor and RequestProcessor are created on the fly when a client connects and sends a request. Listing 12-25 and Listing 12-26 show the salient portions of FormMain s implementation in C# and VB .NET. Listing 12-25. The Salient Part of the FormMain Implementation (C#) public class FormMain : System.Windows.Forms.Form { int numberOfActiveConnections; int numberOfRequestsReceived; int numberOfRequestsInProgress; CoordinatorIncomingTraffic incomingTraffic;

The redo log files record all the changes made to the database, and they are vital during the recovery of a database. If you need to restore your database from a backup, you can recover the latest changes made to the database from the redo log files. The set of redo log files that are currently being used to record the changes to the database are called online redo log files. These logs can be archived (copied) to a different location before being reused, and the saved logs are called archived redo logs. Oracle writes all final changes made to data (committed data) first to the redo log files, before applying those changes to the actual datafiles themselves. Thus, if a system failure prevents these data changes from being written to the permanent datafiles, Oracle will use the redo logs to recover all transactions that committed but couldn t be applied to the datafiles. Thus, redo log files guarantee that no committed data is ever lost. If you have all the archived redo logs since the last database backup, and a set of the current redo logs as well, you can always bring a database up to date.