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Transaction Recovery overcomes the shortcomings of traditional recoveries by eliminating downtime and avoiding the loss of good data. Transaction Recovery is “the process of removing the undesired effects of specific transactions from the database”. Traditional recovery is at the database object level: for example, at the data space, table space or index level. When performing a traditional recovery, a specific database object is chosen. Then, a backup copy of that object is applied, followed by reapplying log entries for changes that occurred after the image copy was taken. Remember that all changes made to a relational database are captured in the database log. So, if the change details can be read from the log, recovery can be achieved by reversing the impact of the logged changes.

One of the forms of Transaction Recovery is Log based transaction recovery takes two form: UNDO recovery or REDO recovery. For UNDO recovery, the database log is read to find the data modifications that were applied during a given timeframe and: INSERT is turned into DELETE. UPDATE are turned around to UPDATE to the old value In effect, an UNDO recovery reverses database modifications using SQL. UNDO Transaction Recovery is online database recovery. Of course, whether or not it is desirable to keep the database online during a Transaction Recovery will depend on the nature and severity of the database problem.

The second type of Transaction Recovery is REDO Transaction Recovery. Since the REDO process does not generate the problem transactions, performing a recovery and then executing the REDO SQL can restore the data to a current state that does not include the problem transactions. A REDO Transaction Recovery requires an outage for the PIT recovery. When redoing transactions in an environment where availability is crucial, the database can be brought down during the PIT recovery and when done, the database can brought back online. The subsequent redoing of the valid transactions to complete the recovery can be done with the data online, thereby reducing application downtime.

Transaction Recovery allows a user to recover a specific portion of the data based on user-defined criteria. A traditional recovery is performed object by object through the database. A transaction is a set of related operations that, when grouped together, define a logical unit of work within an application. Transactions are defined by the user's view of the process. While Transaction Recovery may seem like the answer to all your database recovery problems, there are times when it is not possible or not advisable. The cost needed to balance against the cost of long scans of log data sets to isolate data to undo and the cost of applying that data using SQL.

The ultimate database recovery solution should analyze your overall environment and the transactions needing to be recovered, and recommend which type of data recovery to perform. Furthermore, it should automatically generate the appropriate jobs to perform the recovery to avoid the errors that are sure to be introduced with manually developed scripts and jobs.

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