native barcode generator for crystal reports crack REDO AND UNDO in Objective-C

Encode Data Matrix ECC200 in Objective-C REDO AND UNDO

CHAPTER 9 REDO AND UNDO
Print ECC200 In Objective-C
Using Barcode maker for iPhone Control to generate, create Data Matrix 2d barcode image in iPhone applications.
www.OnBarcode.com
Painting Barcode In Objective-C
Using Barcode printer for iPhone Control to generate, create Barcode image in iPhone applications.
www.OnBarcode.com
Figure 9-1. State of the system after an INSERT There are some cached, modified undo blocks, index blocks, and table data blocks. Each of these blocks is protected by entries in the redo log buffer.
QR Code ISO/IEC18004 Drawer In Objective-C
Using Barcode maker for iPhone Control to generate, create QR-Code image in iPhone applications.
www.OnBarcode.com
Barcode Maker In Objective-C
Using Barcode creator for iPhone Control to generate, create Barcode image in iPhone applications.
www.OnBarcode.com
Hypothetical Scenario: The System Crashes Right Now
Printing UPCA In Objective-C
Using Barcode creator for iPhone Control to generate, create Universal Product Code version A image in iPhone applications.
www.OnBarcode.com
Code 3/9 Encoder In Objective-C
Using Barcode generation for iPhone Control to generate, create Code 3 of 9 image in iPhone applications.
www.OnBarcode.com
Everything is OK. The SGA is wiped out, but we don t need anything that was in the SGA. It will be as if this transaction never happened when we restart. None of the blocks with changes got flushed to disk, and none of the redo got flushed to disk. We have no need of any of this undo or redo to recover from an instance failure.
Painting UCC - 12 In Objective-C
Using Barcode generation for iPhone Control to generate, create EAN128 image in iPhone applications.
www.OnBarcode.com
Encode UPC - E1 In Objective-C
Using Barcode printer for iPhone Control to generate, create UPCE image in iPhone applications.
www.OnBarcode.com
Hypothetical Scenario: The Buffer Cache Fills Up Right Now
Scanning Data Matrix ECC200 In .NET
Using Barcode recognizer for Visual Studio .NET Control to read, scan read, scan image in VS .NET applications.
www.OnBarcode.com
Encode Data Matrix ECC200 In Java
Using Barcode creation for Android Control to generate, create Data Matrix ECC200 image in Android applications.
www.OnBarcode.com
The situation is such that DBWR must make room and our modified blocks are to be flushed from the cache. In this case, DBWR will start by asking LGWR to flush the redo entries that protect these database blocks. Before DBWR can write any of the blocks that are changed to disk, LGWR must flush the redo information related to these blocks. This makes sense: if we were to flush the modified blocks for table T (but not the undo blocks associated with the modifications) without flushing the redo entries associated with the undo blocks, and the system failed, we would have a modified table T block with no undo information associated with it. We need to flush the redo log buffers before writing these blocks out so that we can redo all of the changes necessary to get the SGA back into the state it is in right now, so that a rollback can take place. This second scenario shows some of the foresight that has gone into all of this. The set of conditions described by If we flushed table T blocks and did not flush the redo for the undo blocks and the system failed is starting to get complex. It only gets more complex as we add users, and more objects, and concurrent processing, and so on. At this point, we have the situation depicted in Figure 9-1. We have generated some modified table and index blocks. These have associated undo segment blocks, and all three types of blocks have generated redo to protect them. If you recall from our discussion of the redo log buffer in 4 Memory Structures, it is flushed at least every three seconds, when it is one-third full or contains 1MB of buffered data, or whenever a commit takes place. It is very possible that at some point during our processing, the redo log buffer will be flushed. In that case, the picture will look like Figure 9-2.
Making Code 3/9 In None
Using Barcode creation for Font Control to generate, create Code 3 of 9 image in Font applications.
www.OnBarcode.com
EAN128 Encoder In Objective-C
Using Barcode encoder for iPad Control to generate, create GS1 128 image in iPad applications.
www.OnBarcode.com
CHAPTER 9 REDO AND UNDO
EAN / UCC - 13 Creation In Visual Basic .NET
Using Barcode creator for .NET framework Control to generate, create GTIN - 13 image in .NET applications.
www.OnBarcode.com
Make European Article Number 13 In Java
Using Barcode generation for Java Control to generate, create UPC - 13 image in Java applications.
www.OnBarcode.com
Figure 9-2. State of the system after a redo log buffer flush That is, we ll have modified blocks representing uncommitted changes in the buffer cache and redo for those uncommitted changes on disk. This is a very normal scenario that happens frequently.
USS-128 Decoder In VB.NET
Using Barcode recognizer for .NET framework Control to read, scan read, scan image in .NET applications.
www.OnBarcode.com
Decoding Data Matrix In C#.NET
Using Barcode scanner for .NET framework Control to read, scan read, scan image in Visual Studio .NET applications.
www.OnBarcode.com
The UPDATE
Barcode Scanner In VS .NET
Using Barcode decoder for .NET Control to read, scan read, scan image in VS .NET applications.
www.OnBarcode.com
Barcode Creation In Java
Using Barcode generation for Java Control to generate, create Barcode image in Java applications.
www.OnBarcode.com
The UPDATE will cause much of the same work as the INSERT to take place. This time, the amount of undo will be larger; we have some before images to save as a result of the UPDATE. Now we have the picture shown in Figure 9-3.
Make Barcode In .NET
Using Barcode printer for ASP.NET Control to generate, create Barcode image in ASP.NET applications.
www.OnBarcode.com
1D Barcode Drawer In Visual Basic .NET
Using Barcode encoder for Visual Studio .NET Control to generate, create 1D image in .NET applications.
www.OnBarcode.com
Figure 9-3. State of the system after the UPDATE
CHAPTER 9 REDO AND UNDO
We have more new undo segment blocks in the block buffer cache. To undo the UPDATE, if necessary, we have modified database table and index blocks in the cache. We have also generated more redo log buffer entries. Let s assume that some of our generated redo log from the insert is on disk and some is in cache.
Hypothetical Scenario: The System Crashes Right Now
Upon startup, Oracle would read the redo logs and find some redo log entries for our transaction. Given the state in which we left the system, with the redo entries for the INSERT in the redo log files and the redo for the UPDATE still in the buffer, Oracle would roll forward the INSERT. We would end up with a picture much like Figure 9-1, with some undo blocks (to undo the INSERT), modified table blocks (right after the INSERT), and modified index blocks (right after the INSERT). Oracle will discover that our transaction never committed and will roll it back since the system is doing crash recovery and, of course, our session is no longer connected. It will take the undo it just rolled forward in the buffer cache and apply it to the data and index blocks, making them look as they did before the INSERT took place. Now everything is back the way it was. The blocks that are on disk may or may not reflect the INSERT (it depends on whether or not our blocks got flushed before the crash). If they do, then the INSERT has been, in effect, undone, and when the blocks are flushed from the buffer cache, the data file will reflect that. If they do not reflect the INSERT, so be it they will be overwritten later anyway. This scenario covers the rudimentary details of a crash recovery. The system performs this as a twostep process. First it rolls forward, bringing the system right to the point of failure, and then it proceeds to roll back everything that had not yet committed. This action will resynchronize the data files. It replays the work that was in progress and undoes anything that has not yet completed.
Copyright © OnBarcode.com . All rights reserved.