Failing over AlwaysOn Availability Groups

Merry Christmas Eve!

A few weeks ago I posted on Availability Group fail-over tests with PowerShell. That post covered the basics of how to check the Availability Group’s state and perform the commands for conducting a zero data loss fail over. As a Christmas Present to all of you, I wanted to get a little bit more advanced and provide a completed script which handles the entire fail-over process.

Features of the script

• Fails over a single Availability Group (AG) with zero data loss.
• Does not require the current or target configurations to use synchronous commit in order to achieve zero data loss.
• Accepts JSON files or strings as input for the target AG state. This is useful for saving configurations for fail-back and for performing scripted tests regularly.
• It is aware of the AG state, enabling it to move from configuration A to B without stumbling on limitations such as maximum number of synchronous commit replicas.
• Before and after print outs of the AG state for logging and troubleshooting purposes.
• Available on GitHub here.

Prerequisites

• SQLPS module – this is installed with Microsoft SQL Server 2012 and above, by default. If you don’t have SQL Server installed you can also install the, smaller footprint, feature pack here.
• PowerShell v3.0 – to be honest, this might work in PowerShell v2.0 but I have only tested it in v3.0. v3.0 is packaged with Windows Server 2012 and Windows 8 or above but, if you aren’t using one of those operating systems, you can install it yourself by following these instructions.

The script

[sql][CmdletBinding(SupportsShouldProcess=$true)]
Param
(
[Parameter(Mandatory=$false,
ValueFromPipeline=$true,
ValueFromPipelineByPropertyName=$true,
HelpMessage=”Path to a file with a json string representing the desired fail-over configuration.”)]
[string]$jsonFilePath,
[Parameter(Mandatory=$false,
ValueFromPipeline=$true,
ValueFromPipelineByPropertyName=$true,
HelpMessage=”A json string representing the desired fail-over configuration.”)]
[string]$jsonString
)

function Get-AGState ()
{
return Invoke-Sqlcmd -Query “SELECT replica_server_name
, AG.name AS [AG_Name], HAGS.primary_replica
, availability_mode_desc, failover_mode_desc
FROM sys.availability_replicas AR
INNER JOIN sys.dm_hadr_availability_group_states HAGS
INNER JOIN sys.availability_groups AG ON AG.group_id = HAGS.group_id
ON HAGS.group_id = AR.group_id;” -ServerInstance “$selectedNode”;
}

function Get-DatabaseSyncStates([string]$AG, [string]$Server)
{
return Invoke-Sqlcmd -Query “select DB_NAME(database_id) [Database]
, ST.synchronization_state_desc [SyncState]
FROM sys.dm_hadr_database_replica_states ST
INNER JOIN sys.availability_groups AG ON AG.group_id = ST.group_id
INNER JOIN sys.availability_replicas AR ON AR.replica_id = ST.replica_id
WHERE AG.name = `’$AG`’
AND AR.replica_server_name = `’$Server`’;” -ServerInstance “$Server”;
}

function ValidateTargetReplicaConfiguration()
{
[string[]]$primaries = $targetConfig.Replicas | Where-Object {$_.isPrimary -like “true”}
if($primaries.Count -ne 1)
{
throw “There must be one, and only one, primary in the configuration.”;
}

[string[]]$syncReplicas = $targetConfig.Replicas | Where-Object {$_.AvailabilityMode -like “SynchronousCommit”}
if($syncReplicas.Count -gt 3)
{
throw “The requested configuration exceeds the maximum of three synchronous replicas which can be in an Availability Group configuration.”;
}

if(($targetConfig.Replicas.Count -gt 5) -and ($majorSQLVersion -lt 12))
{
throw “The requested configuration exceeds the maximum of five replicas which can be in an Availability Group configuration for a SQL Server version below 12.”;
}

if(($targetConfig.Replicas.Count -gt 9) -and ($majorSQLVersion -ge 12))
{
throw “The requested configuration exceeds the maximum of nine replicas which can be in an Availability Group configuration for a SQL Server version of 12 or greater.”;
}
}

#Error handling
$ErrorActionPreference = “Stop”;
Trap
{
$err = $_.Exception
while ( $err.InnerException )
{
$err = $err.InnerException
write-output $err.Message
};
}

$totalScriptSteps = 13;
$currentStep = 0;

#Load dependencies
$currentStep++;
Write-Progress -Activity “Loading dependencies.” -Status “Importing SQLPS module.” -PercentComplete ($currentStep/$totalScriptSteps*100);
Import-Module SQLPS -DisableNameChecking;

#Validate input / load jsonString
$currentStep++;
Write-Progress -Activity “Validating input.” -Status “Checking parameter requirements.” -PercentComplete ($currentStep/$totalScriptSteps*100);

if ((!$jsonString) -and ($jsonFilePath))
{
if(!(Test-Path $jsonFilePath))
{
throw “Invalid file path. ($jsonFilePath)”;
}
$jsonString = Get-Content $jsonFilePath
}
elseif (!$jsonString)
{
throw “You must input a JSON file path or a JSON string.”;
}

#Load JSON
$currentStep++;
Write-Progress -Activity “Validating input.” -Status “Loading JSON.” -PercentComplete ($currentStep/$totalScriptSteps*100);
$targetConfig = $jsonString | ConvertFrom-Json;
$currentStep++;
Write-Progress -Activity “Validating input.” -Status “Validating target replica configuration.” -PercentComplete ($currentStep/$totalScriptSteps*100);
ValidateTargetReplicaConfiguration;

#Build server object
$currentStep++;
Write-Progress -Activity “Validating input.” -Status “Testing server connection.” -PercentComplete ($currentStep/$totalScriptSteps*100);
$selectedNode = ($targetConfig.Replicas | Where-Object {$_.isPrimary -like “true”})[0].Name;
$newPrimaryReplicaName = $selectedNode;
if($newPrimaryReplicaName -notcontains “\”)
{
$newPrimaryReplicaName += “\DEFAULT”;
}

# Test connection
# We check the version number as a light weight method of checking the connection.
# Error handling will abort script if this command fails.
Invoke-Sqlcmd -ServerInstance $selectedNode -Query “SELECT @@Version;” | Out-Null;

$currentStep++;
Write-Progress -Activity “Validating input.” -Status “Retrieving current Availability Group state.” -PercentComplete ($currentStep/$totalScriptSteps*100);
$AGState = Get-AGState;
Write-Host -Object “Current Availability Group state:” -ForegroundColor Green -BackgroundColor Black;
$AGState | Format-Table;
Write-Host -Object “———————————” -ForegroundColor Green -BackgroundColor Black;

if($AGState -eq $null)
{
throw “Failed to retrieve Availability Group state from $newPrimaryReplicaName”;
}

[string]$currentPrimaryReplica = $AGState[0].primary_replica;
if($currentPrimaryReplica -notcontains “\”)
{
$currentPrimaryReplica += “\DEFAULT”;
}

if($currentPrimaryReplica -eq “$newPrimaryReplicaName”)
{
throw “$newPrimaryReplicaName is already the primary replica.”;
}

# Failover prep
# You cannot have more than 3 synchronous commit replicas at one time.
# But we want to maintain synchronous commit replicas during the fail-over, if possible.
# So, we set just enough to async so that we can set our fail-over replica to synchronous.
$currentStep++;
Write-Progress -Activity “Failover preparation.” -Status “Pre-configuring async replicas to avoid hitting synchronous commit limit during fail-over.” -PercentComplete ($currentStep/$totalScriptSteps*100);
[string]$AGname = $AGState[0].AG_Name;
[string[]]$syncReplicas = $AGState | Where-Object {$_.availability_mode_desc -like “SYNCHRONOUS_COMMIT”};
$count = 0;
while($syncReplicas.Count -ge 3)
{
[string]$asyncReplicaName = ($targetConfig.Replicas | Where-Object {$_.Name -notlike ($currentPrimaryReplica.Replace(‘\DEFAULT’,”))} `
| Sort-Object AvailabilityMode, Name)[$count].Name;
Set-SqlAvailabilityReplica -AvailabilityMode “AsynchronousCommit” -FailoverMode “Manual” `
-Path “SQLSERVER:\Sql\$currentPrimaryReplica\AvailabilityGroups\$AGname\AvailabilityReplicas\$asyncReplicaName”;

$count++;
$AGState = Get-AGState;
[string[]]$syncReplicas = $AGState | Where-Object {$_.availability_mode_desc -like “SYNCHRONOUS_COMMIT”};
}

$currentStep++;
Write-Progress -Activity “Failover preparation.” -Status “Setting new primary target replica to synchronous commit to achieve zero data loss during fail-over.” -PercentComplete ($currentStep/$totalScriptSteps*100);
$tempName = $newPrimaryReplicaName.Replace(‘\DEFAULT’,”);
Set-SqlAvailabilityReplica -AvailabilityMode “SynchronousCommit” -FailoverMode “Manual” `
-Path “SQLSERVER:\Sql\$currentPrimaryReplica\AvailabilityGroups\$AGname\AvailabilityReplicas\$tempName”;

#wait for new primary to sync up
$currentStep++;
Write-Progress -Activity “Failover preparation.” -Status “Verifying that data synchronization is complete.” -PercentComplete ($currentStep/$totalScriptSteps*100);
[System.Data.DataRow[]]$synchronizingDatabases = (Get-DatabaseSyncStates -AG $AGname -Server $tempName) | Where-Object {$_.SyncState -like “SYNCHRONIZING”};
$stopWatch = [System.Diagnostics.Stopwatch]::StartNew()
$currentStep++;
while($synchronizingDatabases.Count -gt 0)
{
$currentTime = $stopWatch.Elapsed;
$elapsedTime = [string]::Format(“Elapsed time: {0:d2}:{1:d2}:{2:d2}”, $CurrentTime.hours, $CurrentTime.minutes, $CurrentTime.seconds)
Write-Progress -Activity “Waiting for databases to synchronize.” -Status $elapsedTime -PercentComplete ($currentStep/$totalScriptSteps*100);
Start-Sleep -s 2;
$synchronizingDatabases = (Get-DatabaseSyncStates -AG $AGname -Server $tempName) | Where-Object {$_.SyncState -like “SYNCHRONIZING”};
}
$stopWatch.Stop();

# Failover
$to = $newPrimaryReplicaName.Replace(‘\DEFAULT’,”);
$from = $currentPrimaryReplica.Replace(‘\DEFAULT’,”);
$currentStep++;
Write-Progress -Activity “Failover.” -Status “Failing over, without data loss, from $from to $to.” -PercentComplete ($currentStep/$totalScriptSteps*100);
Switch-SqlAvailabilityGroup -Path “SQLSERVER:\Sql\$newPrimaryReplicaName\AvailabilityGroups\$AGname”;

#Final Configuration

# Set availability and failover modes
# Sort ASC by AvailabilityMode so that Async is before Sync.
# This prevents us from adding a 4th sync replica before we establish the asyncs
$currentStep++;
Write-Progress -Activity “Final configuration.” -Status “Set all replicas availability mode and failover mode to match target configuration.” -PercentComplete ($currentStep/$totalScriptSteps*100);
foreach($replica in ($targetConfig.Replicas | Sort-Object AvailabilityMode))
{
[string]$newAvailabilityMode = $replica.AvailabilityMode;
[string]$newFailoverMode = $replica.FailoverMode;
[string]$replicaName = $replica.Name;
Set-SqlAvailabilityReplica -AvailabilityMode $newAvailabilityMode -FailoverMode $newFailoverMode `
-Path “SQLSERVER:\Sql\$newPrimaryReplicaName\AvailabilityGroups\$AGname\AvailabilityReplicas\$replicaName”;
}

$currentStep++;
Write-Progress -Activity “State review.” -Status “Getting Availability Group state for review.” -PercentComplete ($currentStep/$totalScriptSteps*100);
Write-Host -Object “Final Availability Group state:” -ForegroundColor Green -BackgroundColor Black;
Get-AGState | Format-Table;
Write-Host -Object “———————————” -ForegroundColor Green -BackgroundColor Black;[/sql]

Example JSON object

{
	"Replicas" : 
	[
		{
			"Name" : "V-HAMMER-01",
			"AvailabilityMode" : "AsynchronousCommit",
			"FailoverMode" : "Manual",
			"isPrimary" : "false"
		},
		{
			"Name" : "V-HAMMER-02",
			"AvailabilityMode" : "AsynchronousCommit",
			"FailoverMode" : "Manual",
			"isPrimary" : "false"
		},
		{
			"Name" : "V-HAMMER-03",
			"AvailabilityMode" : "SynchronousCommit",
			"FailoverMode" : "Automatic",
			"isPrimary" : "true"
		},
		{
			"Name" : "V-HAMMER-04",
			"AvailabilityMode" : "SynchronousCommit",
			"FailoverMode" : "Automatic",
			"isPrimary" : "false"
		}
	]
}