[Note from Pinal]: This is a new episode of Notes from the Fields series. AlwaysOn is a very complex subject and not everyone knows many things about this. The matter of the fact is there is very little information available on this subject online and not everyone knows everything about this. This is why when a very common question related to AlwaysOn comes, people get confused.
In this episode of the Notes from the Field series database expert John Sterrett (Group Principal at Linchpin People) explains a very common issue DBAs and Developer faces in their career and is related to Planned and Unplanned Availablity Group Failovers. Linchpin People are database coaches and wellness experts for a data driven world. Read the experience of John in his own words.
Whenever a disaster occurs it will be a stressful scenario regardless of how small or big the disaster is. This gets multiplied when it is your first time working with newer technology or the first time you are going through a disaster without a proper run book. Today, were going to help you establish a run book for creating a planned failover with availability groups.
To make today’s session simple were going to have two instances of SQL Server 2012 included in an availability group and walk through the steps of doing an unplanned failover. We will focus on using the user interface and T-SQL to complete the failovers. We are going to use a two replica Availability Group where each replica is in another location. Therefore, we will be covering Asynchronous (non automatic failover) the following is a breakdown of our availability group utilized today.
Seeing the following screen might be scary the first time you come across an unplanned failover. It looks like our test database used in this Availability Group is not functional and it currently isn’t. The database status is not synchronizing which makes sense because the primary replica went down so it couldn’t synchronize. With that said, we can still failover and make it functional while we troubleshoot why we lost our primary replica.
To start we are going to right click on the availability group that needs to be restarted and select failover.
This will bring up the following wizard, which will walk you through several steps needed to complete the failover using the graphical user interface provided with SQL Server Management Studio (SSMS).
You are going to see warning messages simply because we are in Asynchronous commit mode and can not guarantee ‘no data loss’ when we do failover.
Just incase you missed it; you get another screen warning you about potential data loss because we are in Asynchronous mode.
Next we get to connect to the specific replica we want to become the primary replica after the failover occurs. In our case, we only have two replicas so this is trivial.
In order to failover, it’s required to connect to the replica that will become primary. The following screen shows that the connection has been made successfully.
Next, you will see the final summary screen. Once again, this reminds you that the failover action will cause data loss as were using Asynchronous commit mode due to the distance between instances used for disaster recovery.
Finally, once the failover is completed you will see the following screen.
If you followed along this long you might be wondering what T-SQL scripts are generated for clicking through all the sections of the wizard. If you have used Database Mirroring in the past you might be surprised. It’s not too different, which makes sense because the data is being replicated via SQL Server endpoints just like the good old database mirroring.
Now were going to take a look at how to do a failover with just T-SQL. First, were going to need to open a new query window and run our query in SQLCMD mode. Just incase you haven’t used SQLCMD mode before we will show you how to enable it below.
Now you can run the following statement. Notice, we connect to the replica we want to become primary after failover and specify to force failover to allow data loss. We can use the following script to failback over when our primary instance comes back online.
-- YOU MUST EXECUTE THE FOLLOWING SCRIPT IN SQLCMD MODE.
ALTER AVAILABILITY GROUP [AGSQL2] FORCE_FAILOVER_ALLOW_DATA_LOSS;
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Reference: Pinal Dave (http://blog.sqlauthority.com)