native barcode generator for crystal reports crack Hypothetical Scenario: The Application Rolls Back the Transaction in Objective-C

Creator DataMatrix in Objective-C Hypothetical Scenario: The Application Rolls Back the Transaction

At this point, Oracle will find the undo information for this transaction either in the cached undo segment blocks (most likely) or on disk if they have been flushed (more likely for very large transactions). It will apply the undo information to the data and index blocks in the buffer cache, or if they are no longer in the cache request, they are read from disk into the cache to have the undo applied to them. These blocks will later be flushed to the data files with their original row values restored. This scenario is much more common than the system crash. It is useful to note that during the rollback process, the redo logs are never involved. The only time redo logs are read for recovery purposes is during recovery and archival. This is a key tuning concept: redo logs are written to. Oracle does not read them during normal processing. As long as you have sufficient devices so that when ARCH is reading a file, LGWR is writing to a different device, there is no contention for redo logs. Many other databases treat the log files as transaction logs. They do not have this separation of redo and undo. For those systems, the act of rolling back can be disastrous the rollback process must read the logs their log writer is trying to write to. They introduce contention into the part of the system that can least stand it. Oracle s goal is to make it so that logs are written sequentially, and no one ever reads them while they are being written.

Again, undo is generated as a result of the DELETE, blocks are modified, and redo is sent over to the redo log buffer. This is not very different from before. In fact, it is so similar to the UPDATE that we are going to move right on to the COMMIT.

We ve looked at various failure scenarios and different paths, and now we ve finally made it to the COMMIT. Here, Oracle will flush the redo log buffer to disk, and the picture will look like Figure 9-4.

Figure 9-4. State of the system after a COMMIT The modified blocks are in the buffer cache; maybe some of them have been flushed to disk. All of the redo necessary to replay this transaction is safely on disk and the changes are now permanent. If we were to read the data directly from the data files, we probably would see the blocks as they existed before the transaction took place, as DBWR most likely has not yet written them. That s OK the redo log files can be used to bring those blocks up to date in the event of a failure. The undo information will hang around until the undo segment wraps around and reuses those blocks. Oracle will use that undo to provide for consistent reads of the affected objects for any session that needs them.

It is important to understand how redo log files might impact us as developers. We will look at how the different ways we can write our code affect redo log utilization. We ve already seen the mechanics of redo earlier in the chapter, and now we ll look at some specific issues. You might detect many of these scenarios, but they would be fixed by the DBA as they affect the database instance as a whole. We ll start with what happens during a COMMIT, and then get into commonly asked questions and issues surrounding the online redo logs.

As a developer, you should have a good understanding of exactly what goes on during a COMMIT. In this section, we ll investigate what happens during the processing of the COMMIT statement in Oracle. A COMMIT is generally a very fast operation, regardless of the transaction size. You might think that the bigger a transaction (in other words, the more data it affects), the longer a COMMIT would take. This is not true. The response time of a COMMIT is generally flat, regardless of the transaction size. This is because a COMMIT does not really have too much work to do, but what it does do is vital.