Mirror, Mirror, In the Farm …

This is a post I’ve been meaning to write for some time, but I’m only now getting around to it. It’s a quick one, and it’s intended to share a couple of observations and a script that may be of use to those of you who are SharePoint 2010 administrators.

Mirroring and SharePoint

The use of SQL Server mirroring isn’t something that’s unique to SharePoint, and it was possible to leverage mirroring with SharePoint 2007 … though I tended to steer people away from trying it unless they had a very specific reason for doing so and no other approach would work. There were simply too many hoops you needed to jump through in order to get mirroring to work with SharePoint 2007, primarily because SharePoint 2007 wasn’t mirroring-aware. Even if you got it working, it was … finicky.

SharePoint 2010, on the other hand, is fully mirroring-aware through the use of the Failover Partner keyword in connection strings used by SharePoint to connect to its databases.

(Side note: if you aren’t familiar with the Failover Partner keyword, here’s an excellent breakdown by Michael Aspengren on how the SQL Server Native Provider leverages it in mirroring configurations.)

There are plenty of blog posts, articles (like this one from TechNet), and books (like the SharePoint 2010 Disaster Recovery Guide that John Ferringer and I wrote) that talk about how to configure mirroring. It’s not particularly tough to do, and it can really help you in situations where you need a SQL Server-based high availability and/or remote redundancy solution for SharePoint databases.

This isn’t a blog post about setting up mirroring; rather, it’s a post to share some of what I’ve learned (or think I’ve learned) and related “ah-ha” moments when it comes to mirroring.

What Are You Pointing At?

This all started when Jay Strickland (one of the Quality Assurance (QA) folks on my team at Idera) ran into some problems with one of our SharePoint 2010 farms that was used for QA purposes. The farm contained two SQL Server instances, and the database instances were setup such that the databases on the second instance mirrored the databases on the first (principal) instance. Jay had configured SharePoint’s service applications and Web applications for mirroring, so all was good.

But not really. The farm had been running properly for quite some time, but something had gone wrong with the farm’s mirroring configuration – or so it seemed. That’s when Jay pinged me on Skype one day with a question (which I’m paraphrasing here):

Is there any way to tell (from within SharePoint) which SQL Server instance is in-use by SharePoint at any given time for a database that is being mirrored?

It seemed like a simple question that should have a simple answer, but I was at a loss to give Jay anything usable off the top of my head. I told Jay that I’d get back to him and started doing some digging.

The SPDatabase Type

Putting on my developer hat for a second, I recalled that all SharePoint databases are represented by an instance of the SPDatabase type (Microsoft.SharePoint.Administration.Database specifically) or one of the other classes that derive from it, such as SPContentDatabase. Running down the available members for the SPDatabase type, I came up with the following properties and methods that were tied to mirroring in some way:

• FailoverServer
• FailoverServiceInstance
• AddFailoverServiceInstance()

What I thought I would find (but didn’t) was one or more properties and/or methods that would allow me to determine which SQL Server instance was serving as the active connection point for SharePoint requests.

In fact, the more digging that I did, the more that it appeared that SharePoint had no real knowledge of where it was actually connecting to for data in mirrored setups. It was easy enough to specify which database instances should be used for mirroring configurations, but there didn’t appear to be any way to determine (from within SharePoint) if the principal was in-use or if failover to the mirrored instance had taken place.

The Key Takeaway

If you’re familiar with SQL Server mirroring and how it’s implemented, then the following diagram (which I put together for discussion) probably looks familiar:

This diagram illustrates a couple of key points:

1. SharePoint connects to SQL Server databases using the SQL Server Native Client
2. SharePoint supplies a connection string that tells the native client which SQL Server instances (as Data Source and Failover Partner) should be used as part of a mirroring configuration.
3. It’s the SQL Server Native Client that actually determines where connections are made, and the results of the Client’s decisions don’t directly surface through SharePoint.

Number 3 was the point that I kept getting stuck on. I knew that it was possible to go into SQL Server Management Studio or use SQL Server’s Management Objects (SMO) directly to gain more insight around a mirroring configuration and what was happening in real-time, but I thought that SharePoint must surely surface that information in some form.

Apparently not.

Checking with the Experts

I hate when I can’t nail down a definitive answer. Despite all my reading, I wanted to bounce the conclusions I was drawing off of a few people to make sure I wasn’t missing something obvious (or hidden) with my interpretation.

• I shot Bill Baer (Senior Technical Product Manager for SharePoint and an MCM) a note with my question about information surfacing through SharePoint. If anyone could have given me a definitive answer, it would have been him. Unfortunately, I didn’t hear back from him. In his defense, he’s pretty doggone busy.
• I put a shout out on Twitter, and I did hear back from my good friend Todd Klindt. While he couldn’t claim with absolute certainty that my understanding was on the mark, he did indicate that my understanding was in-line with everything he’d read and conclusions he had drawn.
• I turned to Enrique Lima, another good friend and SQL Server MCM, with my question. Enrique confirmed that SQL SMO would provide some answers, but he didn’t have additional thoughts on how that information might surface through SharePoint.

Long and short: I didn’t receive rock-solid confirmation on my conclusions, but my understanding appeared to be on-the-mark. If anyone knows otherwise, though, I’d love to hear about it (and share the information here – with proper recognition for the source, of course!)

Back to the Farm

In the end, I wasn’t really able to give Jay much help with the QA farm that he was trying to diagnose. Since I couldn’t determine where SharePoint was pointing from within SharePoint itself, I did the next best thing: I threw together a PowerShell script that would dump the (mirroring) configuration for each database in the SharePoint farm.

<#
.SYNOPSIS
   SPDBMirrorInfo.ps1
.DESCRIPTION
   Examines each of the databases in the SharePoint environment to identify which have failover partners and which don't.
.NOTES
   Author: Sean McDonough
   Last Revision: 19-August-2011
#>
function DumpMirroringInfo ()
{
	# Make sure we have the required SharePoint snap-in loaded.
	$spCmdlets = Get-PSSnapin Microsoft.SharePoint.PowerShell -ErrorAction silentlycontinue
	if ($spCmdlets -eq $Null)
	{ Add-PSSnapin Microsoft.SharePoint.PowerShell }

	# Grab databases and determine which have failover support (and which don't)
	$allDatabases = Get-SPDatabase
	$dbsWithoutFailover = $allDatabases | Where-Object {$_.FailoverServer -eq $null} | Sort-Object -Property Name
	$dbsWithFailover = $allDatabases | Where-Object {$_.FailoverServer -ne $null} | Sort-Object -Property Name
	
	# Write out unmirrored databases
	if ($dbsWithoutFailover -eq $null)
	{ Write-Host "`n`nNo databases are configured without a mirroring partner." }
	else
	{ 
		Write-Host ("`n`nDatabases without a mirroring partner: {0}" -f $dbsWithoutFailover.Count) 
		$dbsWithoutFailover | Format-Table -Property Name, Server -AutoSize 
	}

	# Dump results for mirrored databases
	if ($dbsWithFailover -eq $null)
	{ Write-Host "`nNo databases are configured with a mirroring partner." }
	else
	{ 
		Write-Host ("`nDatabases with a mirroring partner: {0}" -f $dbsWithFailover.Count) 
		$dbsWithFailover | Format-Table -Property Name, Server, FailoverServer -AutoSize
	}
	
	# For ease of reading
	Write-Host ("`n`n")
}
DumpMirroringInfo

The script itself isn’t rocket science, but it did actually prove helpful in identifying some databases that had apparently “lost” their failover partners.

Additional Reading and Resources

1. MSDN: Using Database Mirroring
2. Whitepaper: Using database mirroring (Office SharePoint Server)
3. Blog Post: Clarification on the Failover Partner in the connectionstring in Database Mirror setup
4. TechNet: Configure availability by using SQL Server database mirroring (SharePoint Server 2010)
5. Book: The SharePoint 2010 Disaster Recovery Guide
6. Blog: John Ferringer’s “My Central Admin”
7. Blog: Jay Strickland’s “Slinger’s Thoughts
8. Company: Idera
9. MSDN: SPDatabase members
10. MSDN: SQL Server Management Objects (SMO)
11. Blog: Bill Baer
12. Blog: Todd Klindt’s SharePoint Admin Blog
13. Blog: Enrique Lima’s Intentional Thinking