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Many people think of EBSs as large, distributed, asynchronous systems, with middleware components handling messages and notifications, running independently of event publishers. While EBSs certainly can be asynchronous, they don t have to be. For one thing, not all EBSs use middleware, and even those that do might have reasons to prefer synchronous delivery. Moreover, not all EBSs are distributed, or even large. In small systems, synchronous delivery is often preferred. Synchronous delivery has two important advantages: The delivery order of notifications is predictable, and the systems are cheaper and easier to build. Another consideration is that not all the notifications in a system need be delivered using the same synchrony. For example, in a distributed system, notifications between systems might use asynchronous delivery, while those between objects in the same system might use synchronous delivery. In the following sections, I ll describe both types of timing, showing the pros and cons of each.

You can monitor RMAN s backups using several important data dictionary views. The V$BACKUP_ CORRUPTION and V$COPY_CORRUPTION views, for example, provide important information about corrupt blocks. (I ll discuss data block corruption in the Database Corruption Detection section later in this chapter.) You can use the V$RMAN_OUTPUT view to monitor a running RMAN job. The V$RMAN_STATUS view shows the status of all completed jobs as well as commands, as shown here: SQL> SELECT operation, status, start_time, end_time FROM v$rman_status; OPERATION --------LIST VALIDATE BACKUP BACKUP . . . SQL> STATUS --------COMPLETED COMPLETED FAILED COMPLETED START_TIME ---------28-APR-08 28-APR-08 28-APR-08 29-APR-08 END_TIME --------28-APR-08 28-APR-08 28-APR-08 29-APR-08

You can estimate the backup s progress using the following query on the V$SESSION_LONGOPS view: SQL> SELECT TO_CHAR(start_time,'DD-MON-YY HH24:MI') "Start of backup",Sofar, totalwork, elapsed_seconds/60 "ELAPSED TIME IN MINUTES", ROUND(sofar/totalwork*100,2) "Percentage Completed so far" FROM v$session_longops WHERE opname='prod1_dbbackup'; To ensure the backups made using RMAN are useful during a recovery, you can use the CROSSCHECK and VALIDATE commands, as described in the following sections.

RMAN provides the useful CROSSCHECK command to enable you to check that the backup sets and image copies listed in the recovery catalog actually exist in their specified locations and haven t been accidentally deleted or written over. In addition, the command verifies the headers and ensures that RMAN can read the files. The CROSSCHECK command can thus test both the existence and the readability of the backups. Here s an example of the use of the CROSSCHECK command in RMAN: RMAN> CROSSCHECK BACKUPSET 326; allocated channel: ORA_DISK_1 . . . channel ORA_DISK_4: sid=21 devtype=DISK crosschecked backup piece: found to be 'AVAILABLE' Crosschecked 1 objects RMAN> As you can see, RMAN has cross-checked the backup piece and found it to be available, which confirms that the backup files exist and are usable.

When a notification is delivered synchronously, the sender blocks while the receiver is busy processing the notification. Synchronous delivery is used most often with notifications sent via procedure calls. The reason is simplicity, because the mechanics of blocking are built into the procedure call substrate. The simplest way to achieve synchronous delivery is with an LPC. Since a call transfers execution control to the callee, the caller is forced to block until the callee returns. LPCs are natively synchronous, because it takes extra work to make them asynchronous. RPCs are similar to LPCs from the viewpoint of the programs using them, because all the interprocess plumbing details are basically transparent to the caller and callee. Under the covers, there are all sorts of networking protocols at work, with proxies, stubs, marshaling, and other hideous gunk that application-level programmers prefer to know little about. When delivering notifications using shared resources, true synchronous delivery is not possible, since the sender and receiver are generally in separate processes running concurrently. By resorting to a simple stratagem, it s possible to make a notification appear to have been delivered synchronously, from the perspective of the sender. To produce blocking, a secondary shared resource is required to notify the sender when the receiver has finished processing the notification. This design pattern uses a sequence of reads and writes to emulate a single synchronous operation. Figure 3-29 shows the details.