(Look in the Misconceptions blog category for the rest of the month’s posts and check out the 60-page PDF with all the myths and misconceptions blog posts collected together when you join our Insider list, plus my online Myths and Misconceptions training course on Pluralsight.)
I've got a session coming up at SQL Connections in a couple of weeks called DBA Mythbusters. And I've got a whole stack of DBA myths and misconceptions around SQL Server to choose from (I came up with 69 just in an hour on the plane last weekend) and I won't be able to cover them all in the session, so I thought it would be cool to do a Debunk-A-Myth-A-Day blog post series in April. And so here we are.
Every day (hopefully!) I'll pick a myth and lay it to rest. They won't be long posts and they may be links to previous posts, but hopefully they'll be useful.
Myth #1: After a failover, any in-flight transactions are continued.
Any time a failover occurs, some form of crash recovery has to occur. If the transaction hadn't committed on the server that the connection was to when the server/database went down, there's no way for SQL Server to automatically recreate all the transaction context and resurrect the in-flight transaction on the failover server – whether the failover used clustering, mirroring, log-shipping, or SAN replication.
For failover clustering, when a failover occurs, the SQL instance is restarted on another cluster node. All the database go through crash recovery – which will roll-back all uncommitted transactions. (See my TechNet Magazine article from February 2009 for an explanation of crash recovery and how it works: Understanding Logging and Recovery in SQL Server.)
For database mirroring, the transaction log on the mirror database is constantly being used to perform redo operations (of the log records coming from the principal). When the mirror switches to being the principal, the transaction log switches into crash-recovery mode and recovers the database as if a crash had occured – and then lets connections into the database.
For log shipping, transaction log backups are being replayed on the log shipping secondary database(s) periodically. When a failure on the primary occurs, the DBA brings the log shipping secondary online by completing the restore sequence (either bringing the database immediately online, or replaying all possible transaction log backups to bring the database as up-to-date as possible). The final part of any restore sequence is to run the undo portion of crash recovery – rolling back any uncommitted transactions.
With SAN replication, I/Os to the local SAN are shipped to the remote SAN to be replayed. When a failure occurs, the system connected to the remote SAN comes online and the databases go through crash recovery, in just the same way as for failover clustering (this is an overly simplistic explanation – but you get the idea).
The *only* technology that allows unbroken connections to a database when a failure occurs, is using virtualization with a live migration feature, where the VM comes up and the connections don't know they're talking to a different physical host server of the VM.
But whatever mechanism you're using – if a *connection* is broken, the in-flight transaction is lost. So your application needs to be coded to gracefully cope with broken connections and some form of transaction retry.
1 down – 29 to go!