All those TLA’s (three letter acronyms)!

March 16, 2011 at 11:15 am | Posted in Oracle | Leave a comment
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If you’re working on your OCA, and possibly even your OCP, you have already run into quite a few product names and acronyms used by Oracle for “add on”  products that enhance the fundamental capabilities of the Oracle database.  But who has the time to investigate all these products while preparing for an exam? Exactly. So I thought I would offer our Oracle exam candidates the Cliff’s notes version of one of Oracle’s most popular products.

RAC, or Real Application Clusters, is probably the hottest “add on” product that Oracle has released in the last 5 years.   It’s important to recall what you already know regarding the Oracle architecture as well as Backup and Recovery if you’re going to properly understand the benefits of RAC.  If you have your OCA in Database Administration, you know that if the Oracle instance fails, the DBA just starts the database back up and it goes through a process called automatic instance recovery. During this process the database is reading the redo log files (and the undo log files) to determine which transactions logically completed but  haven’t been written to the datafiles.  This process also checks the redo and undo log files to determine if a transaction wasn’t completed (either because it was rolled back or just never committed) but the datafiles were updated.  This situation needs to be “fixed” when the database is restarted, so that the logical state of all transactions is synchronized with the physical reality of what’s written to the datafiles.   During this automatic instance recovery, the database  “rolls backward” and “rolls forward” to ensure that data that has been committed but hasn’t been written to the datafiles does get physically written to the datafiles, and that data that hasn’t been committed but has been written to the datafiles is physically scraped off the datafiles.  Once complete, the Oracle database is available for use.

This is all very cool, no? How the only action on the part of the DBA was to start the database. The database recognizes when there was a need to perform automatic instance recovery and begins that process as soon as the database was restarted. Nothing different the DBA does other than start the database regardless of whether it previously suffered instance failure or not.  You just have to wait a little longer for it to become available to users if the software determines it needs to go through this automatic instance recovery process.

STOP! Take note – the time required for the database to complete automatic instance recovery following a crash, even though it happens automatically, may take longer than permitted based upon your business situation.  In some mission critical applications, a corporation may lose thousands of dollars for each minute the database in unavailable.  The solution to this problem is RAC.  RAC provides a second instance to be running mapped to the same set of database files.  Now, if instance failure takes place on one node, the user sessions are swapped over to the second instance which is already running.   There is no loss of data and each user continues to work in their session.  It’s amazing to see this work, if you have a chance to catch a demo of this be sure to take advantage of the opportunity.  RAC is considered to be one of the “high availability” tools that Oracle provides since it eliminates downtime when instance failure takes place.

I realize this doesn’t take care of that growing list of acronyms, but I hope it lets you check at least one thing off that list!

–Bob Bungenstock

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