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Oracle

Posted Dec 29, 2008

Oracle Database 11g: Flashback Transaction Backout

By Jim Czuprynski

Synopsis. Oracle Database 10g offered two new Flashback features- Flashback Versions Query and Flashback Transaction Query – that allowed an authorized user to see all versions of any changes made to one or more rows in a table, as well as the capability to view the entire transaction associated with any row change, based on the transaction ID of the transaction. Database 11gR1 builds upon these 10g flashback features to their logical destination: the ability to back out one or more independent or dependent transactions with Flashback Transaction Backout.

An IT project manager for a large multinational insurance firm and one of my closest colleagues recently told me of her frustration with a production fix that their team’s offshore resources were attempting to implement. “I got a request today from one of our developers to approve the execution of some ‘DML that he’d built against several production database tables,” she said. When I asked why she allowed her developers to submit what I like to call “naked” DML against their production database, she replied, “We really had no choice. It was actually the third time this week he’d requested this type of approval, because on Monday he submitted some DML to fix a problem in some transactions. On Tuesday, he submitted a fix to his original fix, and Wednesday he found out he’d broken several other transactions with his Tuesday ‘fix.’”

I grimaced in sympathy, as this had happened to me before as well. “Let me guess,” I said. “He forgot to look at what triggers would fire against the tables in his SQL statement as a result of his original Monday ‘repairs,’ right?” “Exactly!” she replied. “And when he made the second fix, he forgot to look at what other triggers would fire as a result of that fix. I made him go back and take a look at all the objects affected by his third attempt to repair things, and we found that even more triggers would have fired had he not checked first. What a nightmare.”

Unfortunately for my friend, her client was not using an Oracle database on the back end of their application, so I couldn’t explain to her how Oracle 10g offered several features that would have made her job much simpler:

  • Flashback Versions Query allows an authorized session to query against all versions of one or more rows. Any DML statement issued against a table is therefore viewable, and the developer can use that information to decide how to best reverse errant transactions.
  • Flashback Transaction Query, on the other hand, works at “right angles” to Flashback Versions Query because it can utilize the transaction ID that’s stored alongside each row version in a Flashback Versions Query to display every “atom” of the transaction that changed the row – including any that may have fired via a table-level DML trigger.

However, both of these tools are limited by the amount of undo that’s currently retained within the database’s UNDO tablespace, because that’s from where their transaction information is retrieved. What if I need to delve into a transaction whose reciprocal change vectors have been aged out of the UNDO tablespace?

Delving Deeply Into Transaction Detail with LogMiner

Oracle’s LogMiner tool set finally gets the recognition it’s due as a “power player” in Oracle 11g – it’s the perfect addendum to the Flashback Versions Query and Flashback Transaction Query capabilities that were introduced in Oracle 10g. For starters, Oracle 11g’s Enterprise Manager Database Control finally offers an intuitive, wizard-based interface for this powerful tool, a major improvement over prior releases’ “thick” Java Console GUI. The new LogMiner wizard interface is activated by selecting the Availability tab and then choosing the View and Manage Transactions link as shown below:

LogMiner wizard interface

The wizard interface is not the only new LogMiner feature in Oracle 11g, however:

  • LogMiner query parameters are easily specified at either the time stamp or SCN granularity
  • A long-running LogMiner query can now be halted so that any corresponding partial results can be displayed; equally impressive, it’s now possible to show the remaining estimated time to complete the query if execution was resumed.
  • The original LogMiner query can be used as a target of a “re-mining” operation based on the initial results retrieved, and the original LogMiner query results can now be saved for future evaluations
  • Finally, the detailed results of a LogMiner query – including all transaction details, dependencies, and the compensating undo SQL scripts – can be easily obtained, and this feature set facilitates a brand-new feature set in Oracle 11g: the ability to quickly and accurately reverse individual committed transactions via Flashback Transaction Backout.

I’ll illustrate these extended LogMiner query capabilities a bit later in this article.

Reversing Transactions Accurately Via Flashback Transaction Backout

Oracle Database 11g’s new Flashback Transaction Backout (FTB) feature allows an authorized user session to back out a single transaction -- as well as any corresponding dependent transactions -- by applying appropriate compensating statements for the affected transaction(s). FTB uses undo data, redo data generated for the UNDO blocks, and supplemental logging to accomplish this.

Flashback Transaction Backout: Prerequisites

Before a Flashback Transaction Backout operation can be activated, the DBA must make sure the following prerequisites have been addressed:

  • The database must be in ARCHIVELOG mode.
  • Supplemental logging must have been enabled at the database level; in addition, at a minimum, primary key level supplemental logging must be enabled on the tables for which FTB is desired.
  • Any user accounts other than SYS that will be participating in an FTB operation must have been granted the EXECUTE object privilege on package DBMS_FLASHBACK, and the SELECT ANY TRANSACTION system privilege.
  • Finally, any user account other than SYS must be granted FLASHBACK privileges on the table(s) on which FTB will be performed. Likewise, any user account that will be performing FTB must also either own the tables on which FTB will be performed, or have complete (SELECT, INSERT, UPDATE, and DELETE) DML privileges on those tables

Performing FTB Using Enterprise Manager Database Control

To illustrate the flexibility and power of Flashback Transaction Backout, I’ve set up the following simple scenario:

  • First, I’ve recreated my Accounts Payable (AP) schema and added just one table (AP.VENDORS) as shown in the code in Listing 1. I’ve also activated supplemental logging and appropriate permissions for the AP schema so that it can perform FTB.
  • Via the code in Listing 2, I’ve populated that table with just a few rows of sample data and then updated two of the five rows in the table, capturing the SCNs and timestamps of these transactions. The updated transactions will be the target of my FTB operation.
  • Listing 3 shows the queries I executed against V$LOGMNR_CONTENTS to view corresponding change vectors that LogMiner will provide to FTB for making intelligent decisions about which transactions may be backed out, as well as the corresponding output from those queries.

Now I’m ready to use the power of FTB to roll back just the last two update transactions. As shown in Figure 1, I’ll select the Perform Recovery link from the Availability tab of Oracle 11g Enterprise Manager Database Control, and then I’ll start FTB by selecting the Transactions option for the scope of my recovery operation:

Invoking Database Recovery Wizard
Figure 1. Invoking Database Recovery Wizard.

Now that the FTB recovery operation is underway, Oracle 11g allows me to supply either a range of SCNs or a range of timestamps to limit the amount of archived or online redo logs that’ll need to be read to find the corresponding change vectors. As Figure 2 shows below, I’ve also restricted the search for possible transactions to only those that affect table AP.VENDORS:

Specifying SCN / Timestamp Ranges
Figure 2. Specifying SCN / Timestamp Ranges.

Oracle 11g next searches for any matching transaction information, and it provides a concise summary of how the search is progressing, as Figure 3 below shows:

Tracking FTB Transaction Retrieval Processing
Figure 3. Tracking FTB Transaction Retrieval Processing.

After a few minutes, the Recovery Wizard then presents me with a list of possible transactions against AP.VENDORS that I’d be most interested in reversing, as shown in Figure 4.

Reviewing Candidate FTB Transactions
Figure 4. Reviewing Candidate FTB Transactions.

If I’m interested in reviewing the individual atomic transactions before making a decision on which one(s) to back out, I can simply select one of them by clicking on the Transaction ID “breadcrumb.” Figure 5 shows the results of selecting the link to my proposed target transaction, the two UPDATE statements in Transaction ID x’05000F00D050000’:

Reviewing FTB Transaction Details
Figure 5. Reviewing FTB Transaction Details

After selecting the desired transaction(s) for FTB, Oracle provides a tracking screen to notify me that it’s begun to process them for reversal, as Figure 6 shows.

Monitoring FTB Progress
Figure 6. Monitoring FTB Progress.

Almost done! Once Oracle verifies that the transactions can be rolled back without causing any disruption of the atomicity of the transaction – also known as NOCASCADE mode – it acknowledges that the FTB reversal transactions are ready for execution, as Figure 7 below shows.

Final Review before FTB Application
Figure 7. Final Review before FTB Application.

I have several options at this point:

  • I can simply execute the FTB transactions.
  • I can formulate and execute a custom query against the target table to view its contents immediately after the FTB compensating transactions have been applied.
  • Finally, I can view the actual compensating statements themselves by clicking the Show Undo SQL Script button. Figure 8 shows the results of doing just that:

Viewing Undo SQL Statements
Figure 8. Viewing Undo SQL Statements.

Once I’ve chosen to continue, FTB will apply the compensating transactions it has already prepared to the table, and returns an acknowledgment that the operation has completed successfully, as shown in Figure 9.

Acknowledgement of Successful FTB Processing
Figure 9. Acknowledgement of Successful FTB Processing.

Handling Special FTB Situations

This was an extremely straightforward example of using FTB to reverse a simple set of UPDATE statements; but what if the transactions that I want to reverse are much more complex? For example, what if the transaction that I want to back out has changed data that another transaction depends upon? Or what if I want to reverse a transaction – say, delete an Order from OE.ORDERS – that one or more rows in another table depend upon in a parent-child relationship, or otherwise use that Order’s information to enforce referential integrity via a foreign key constraint?

These two examples represent the different types of dependent transaction conditions that (thankfully!) FTB can detect and act upon:

  • Write-After-Write (WAW). In this case, another transaction needs to change the same data that was originally changed by the target transaction.
  • Primary/Unique Constraint Relationship. In this situation, an FTB transaction might even need to reinsert the same primary key value that was deleted by the original target transaction.

The good news is that FTB can detect these situations, and if one is detected, it provides three different additional options that can be specified to allow a more complex FTB transaction to continue:

Table 1. Flashback Transaction Backout: Backout Options

Option

Description

NOCASCADE

The default value; it indicates that the executing session expects no dependent transactions, but if any are found, an exception will be raised, and the first dependent transaction will be listed.

CASCADE

Tells Oracle to simply remove all transactions and their dependents in a posted-order fashion, i.e. in the reverse order in which they were committed.

NOCONFLICT_ONLY

Tells Oracle to back out only the non-conflicting rows for the transaction. This directly implies that database consistency will be maintained, even though the atomicity of the transaction has been broken while completing its repair.

NOCASCADE_FORCE

Tells Oracle to back out all transactions regardless of any of the dependent transactions. In other words, all compensating transactions will be executed in reverse order of the time that each transaction was originally COMMITted.

Flashback Transaction Backout: Metadata

Two new data dictionary views allow the DBA to monitor the progress of ongoing FTB operations:

Table 2. Flashback Transaction Backout: Metadata Views

View

Description

DBA_FLASHBACK_TXN_STATE

Shows a transaction’s current state, i.e. whether it has been backed out successfully from the target database, or if it is still a “live” transaction. The set of compensating transaction(s) may contain multiple rows because of the dependency between the transactions that have been “compensated” by the compensating transaction(s).

DBA_FLASHBACK_TXN_REPORT

Contains detailed metadata in XML format for the actual transactions that have been recently backed out of the database, including every compensating transaction, that has been committed in the database. One row is presented for each compensating transaction.

I’ve presented some SQL*Plus queries against these views in Listing 4, and I’ve included the results of running these queries against the FTB operation I performed via EM earlier.

Flashback Transaction Backout: Caveats and Best Practices

Of course, you may have guessed that all this power does require some restraint. There are a few additional restrictions on FTB at the table level:

  • Just as with Flashback Versions Query and Flashback Transaction Query, a FTB operation cannot be performed on transactions that have been punctuated by DDL changes to the table(s) being flashed back.
  • Also, if a table’s columns include data types other than those supported by LogMiner in Oracle 11g (e.g. BFILEs, VARRAYs), FTB may not be performed against that table.

Finally, Oracle recommends the following best practices regarding FTB:

  • Once the need for FTB is discovered, it’s generally best to start the FTB operation as soon as possible. Since FTB needs to traverse transactional change vectors that may be stored in both archived and current online redo logs, it performs best when huge redo logs don’t have to be interrogated to find the change vectors necessary to create the compensating transactions.
  • To make later FTB auditing simpler, the DBA should assign a transaction name to the FTB operation. (If no transaction name is provided, FTB will automatically generate one.)
  • Don’t forget that function DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER provides an excellent means to capture and return an SCN from within PL/SQL code blocks.

Conclusion

Oracle Database 11g’s new LogMiner and Flashback Transaction Backout features offer excellent capabilities for perusing as well as correcting erroneous data on a transaction-by-transaction basis. The fine-grained control over which transactions should be reversed, the ability to preview the compensating transactions before deciding to reverse them, and the capability to break the atomicity of transactions (only if desired, of course!) are welcome additions to any DBA’s tool belt.

References and Additional Reading

While I’m hopeful that I’ve given you a thorough grounding in the technical aspects of the features I’ve discussed in this article, I’m also sure that there may be better documentation available since it’s been published. I therefore strongly suggest that you take a close look at the corresponding Oracle documentation on these features to obtain crystal-clear understanding before attempting to implement them in a production environment. Please note that I’ve drawn upon the following Oracle Database 11g documentation for the deeper technical details of this article:

B28279-02 Oracle Database 11g New Features Guide

B28320-01 Oracle Database 11g Reference Guide

B28419-02 Oracle Database 11g PL/SQL Packages and Types Reference

B28424-03 Oracle Database 11g Advanced Application Developer’s Guide

In addition, the following MetaLink documentation helps clarify this feature set:

737332.1 Flashback Transactions Using DBMS_FLASHBACK.TRANSACTION_BACKOUT procedure

» See All Articles by Columnist Jim Czuprynski



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