Frequently Asked Questions for snapshot debugging in Visual Studio

When the Snapshot Debugger captures a snapshot of your app, it forks the app's process and suspends the forked copy. When you debug a snapshot, you're debugging against the forked copy of the process. This process takes only 10-20 milliseconds but doesn't copy the full heap of the app. Instead, it copies only the page table and sets pages to copy on write. If some of your app's objects on the heap change, their respective pages are then copied. That's why each snapshot has a small in-memory cost (on the order of hundreds of kilobytes for most apps).

When you have multiple instances of your app, snappoints get applied to every single instance. Only the first snappoint to hit with the conditions specified creates a snapshot. If you have multiple snappoints, later snapshots come from the same instance that created the first snapshot. Logpoints sent to the output window will only show messages from one instance, while logpoints sent to application logs send messages from every instance.

The Snapshot Debugger requires that you have the matching symbols for your application either local or deployed to your Azure App Service. (Embedded PDBs are currently not supported.) The Snapshot Debugger automatically downloads symbols from your Azure App Service. Starting with Visual Studio 2017 version 15.2, deploying to Azure App Service also deploys your app's symbols.

Yes - the Snapshot Debugger is intended to work against release builds. When a snappoint is placed in a function, the function is recompiled back to a debug version, making it debuggable. Stopping the Snapshot Debugger returns functions to the version of the release build.

No - any log messages you add to your app are evaluated virtually. They can't cause any side effects in your application. However, some native properties may not be accessible with logpoints.

Yes, snapshot debugging can work for servers under load. The Snapshot Debugger throttles and doesn't capture snapshots in situations where there's a low amount of free memory on your server.

You can uninstall the Snapshot Debugger site extension on your App Service with the following steps:

1. Turn off your App Service either through the Cloud Explorer in Visual Studio or the Azure portal.
2. Navigate to your App Service's Kudu site (that is, yourappservice.scm.azurewebsites.net) and navigate to Site extensions.
3. Click the X on the Snapshot Debugger site extension to remove it.

Snapshot Debugger needs to open a set of ports in order to debug the snapshots taken in Azure, these are the same ports required for remote debugging. You can find the list of ports here.

For App Services:

1. Disable Remote Debugger extension via the Azure portal for your App Service.
2. Azure portal > your Application Service resource blade > Application Settings
3. Navigate to the Debugging section and click the Off button for Remote debugging.

For AKS:

1. Update your Dockerfile to remove the sections corresponding to the Visual Studio Snapshot Debugger on Docker images.
2. Rebuild and redeploy the modified Docker image.

For virtual machine/virtual machine scale sets remove the Remote Debugger extension, Certificates, KeyVaults and InBound NAT pools as follows:

1. Remove Remote Debugger extension

   There are several ways to disable the Remote Debugger for virtual machines and virtual machine scale sets:

   • Disable the Remote Debugger through Cloud Explorer

     • Cloud Explorer > your virtual machine resource > Disable Debugging (Disabling Debugging does not exist for virtual machine scale set on Cloud Explorer).

   • Disable the Remote Debugger with PowerShell Scripts/Cmdlets

     For virtual machine:

     Remove-AzVMExtension -ResourceGroupName $rgName -VMName $vmName -Name Microsoft.VisualStudio.Azure.RemoteDebug.VSRemoteDebugger
     

     For virtual machine scale sets:

     $vmss = Get-AzVmss -ResourceGroupName $rgName -VMScaleSetName $vmssName
     $extension = $vmss.VirtualMachineProfile.ExtensionProfile.Extensions | Where {$_.Name.StartsWith('VsDebuggerService')} | Select -ExpandProperty Name
     Remove-AzVmssExtension -VirtualMachineScaleSet $vmss -Name $extension
     

   • Disable the Remote Debugger through the Azure portal

     • Azure portal > your virtual machine/virtual machine scale sets resource blade > Extensions
     • Uninstall Microsoft.VisualStudio.Azure.RemoteDebug.VSRemoteDebugger extension

     Note

     Virtual machine scale sets - The portal does not allow removing the DebuggerListener ports. You will need to use Azure PowerShell. See below for details.

2. Remove Certificates and Azure KeyVault

   When installing the Remote Debugger extension for virtual machine or virtual machine scale sets, both client and server certificates are created to authenticate the Visual Studio client with the Azure Virtual Machine/virtual machine scale sets resources.

   • The Client Cert

     This cert is a self-signed certificate located in Cert:/CurrentUser/My/

     Thumbprint                                Subject
     ----------                                -------
     
     1234123412341234123412341234123412341234  CN=ResourceName
     

     One way to remove this certificate from your machine is via PowerShell

     $ResourceName = 'ResourceName' # from above
     Get-ChildItem -Path Cert:\CurrentUser\My | Where-Object {$_.Subject -match $ResourceName} | Remove-Item
     

   • The Server Certificate

     • The corresponding server certificate thumbprint is deployed as a secret to Azure KeyVault. Visual Studio will attempt to find or create a KeyVault with prefix MSVSAZ* in the region corresponding to the virtual machine or virtual machine scale sets resource. All virtual machine or virtual machine scale sets resources deployed to that region therefore will share the same KeyVault.
     • To delete the server certificate thumbprint secret, go to the Azure portal and find the MSVSAZ* KeyVault in the same region that's hosting your resource. Delete the secret which should be labeled remotedebugcert<<ResourceName>>
     • You will also need to delete the server secret from your resource via PowerShell.

     For virtual machines:

     $vm.OSProfile.Secrets[0].VaultCertificates.Clear()
     Update-AzVM -ResourceGroupName $rgName -VM $vm
     

     For virtual machine scale sets:

     $vmss.VirtualMachineProfile.OsProfile.Secrets[0].VaultCertificates.Clear()
     Update-AzVmss -ResourceGroupName $rgName -VMScaleSetName $vmssName -VirtualMachineScaleSet $vmss
     

3. Remove all DebuggerListener InBound NAT pools (virtual machine scale set only)

   The Remote Debugger introduces DebuggerListener in-bound NAT pools that are applied to your scaleset's load balancer.

   $inboundNatPools = $vmss.VirtualMachineProfile.NetworkProfile.NetworkInterfaceConfigurations.IpConfigurations.LoadBalancerInboundNatPools
   $inboundNatPools.RemoveAll({ param($pool) $pool.Id.Contains('inboundNatPools/DebuggerListenerNatPool-') }) | Out-Null
   
   if ($LoadBalancerName)
   {
      $lb = Get-AzLoadBalancer -ResourceGroupName $ResourceGroup -name $LoadBalancerName
      $lb.FrontendIpConfigurations[0].InboundNatPools.RemoveAll({ param($pool) $pool.Id.Contains('inboundNatPools/DebuggerListenerNatPool-') }) | Out-Null
      Set-AzLoadBalancer -LoadBalancer $lb
   }
   

For App Service:

1. Disable Snapshot Debugger via the Azure portal for your App Service.

2. Azure portal > your Application Service resource blade > Application Settings

3. Delete the following App settings in the Azure portal and save your changes.

   • INSTRUMENTATIONENGINE_EXTENSION_VERSION
   • SNAPSHOTDEBUGGER_EXTENSION_VERSION

   Warning

   Any changes to Application Settings will initiate an app restart. For more information about Application Settings, see Configure an App Service app in the Azure portal.

For AKS:

1. Update your Dockerfile to remove the sections corresponding to the Visual Studio Snapshot Debugger on Docker images.
2. Rebuild and redeploy the modified Docker image.

For virtual machine/virtual machine scale sets:

There are several ways to disable the Snapshot Debugger:

• Cloud Explorer > your virtual machine/virtual machine scale set resource > Disable Diagnostics

• Azure portal > your virtual machine/virtual machine scale set resource blade > Extensions > Uninstall Microsoft.Insights.VMDiagnosticsSettings extension

• PowerShell Cmdlets from Az PowerShell

  Virtual machine:

      Remove-AzVMExtension -ResourceGroupName $rgName -VMName $vmName -Name Microsoft.Insights.VMDiagnosticsSettings
  

  Virtual machine scale sets:

      $vmss = Get-AzVmss -ResourceGroupName $rgName -VMScaleSetName $vmssName
      Remove-AzVmssExtension -VirtualMachineScaleSet $vmss -Name Microsoft.Insights.VMDiagnosticsSettings
  

See also

• Debugging in Visual Studio
• Debug live ASP.NET apps using the Snapshot Debugger
• Debug live ASP.NET Azure Virtual Machines\Virtual Machines scale sets using the Snapshot Debugger
• Debug live ASP.NET Azure Kubernetes using the Snapshot Debugger
• Troubleshooting and known issues for snapshot debugging