PowerPivot for SharePoint 2016 Gallery Snapshots Fail for SSL Host Header Sites

If you have been following the multiple threads  on issues and changes to the PowerPivot for SharePoint 2016 Gallery Snapshots, you may be aware of an issue where gallery snapshots do not function properly (infinite "hourglass") for SSL sites when there are multiple sites bound to the same port. While not limited to port 443 we will use a generic port 443 site as an example here as it is the most common usage.

I found this issue back in December 2016 during testing and again in a really well researched blog post by Brian Laws (http://info.summit7systems.com/blog/bug-report-powerpivot-for-sharepoint-2016-thumbnails-not-generating) in February of this year. After that, we started to see some cases roll in and I had a basis to start engaging our product team on the issue.

I will not go into extensive detail on the snapshot process or the log entries (you can see Brian's blog above for most of it or follow along in the ULS logs), but here is the 10,000 foot view:

  1. A user uploads a new PowerPivot workbook to the Gallery.
  2. Event receiver detects the file and whether or not it is a file valid for a snapshot.
  3. SharePoint calls the ExcelRest.aspx for each relevant item in the workbook via a URL similar to the following:
    •  https://localhost/_vti_bin/ExcelRest.aspx/PowerPivot%20Gallery/GenericWorkbook.xlsx/Model/Sheets('Sheet1')
  4. As part of the above call the full URL of the document is sent with the request to OOS on behalf of the user that uploaded the document. (Obviously "localhost" is not a valid URL for OOS, so we also send the full URL as part of the request so OOS knows where to go to retrieve the snapshots)
  5. Office Online Server loads the workbook, retrieves the snapshots, and returns them to the SharePoint WFE server.
  6. The snapshots are applied to the document.

The Issue:

The new snapshot process causes an issue for SSL sites that share a port and use unique certificates. When you combine multiple unique URLs each with their own unique SSL certificates and localhost calls, you wind up with IIS getting very confused on what certificate to pin to the request (since it is localhost, it takes the first one in line generally). In an effort to avoid an authentication double hop in a scenario where more than one SharePoint web front end (WFE) is involved, the product team opted to utilize localhost. If this was not in place, Kerberos Constrained Delegation (KCD) would need to be configured between the SharePoint WFE servers to avoid an overcomplicated configuration just for gallery snapshots to function properly.

In the SharePoint ULS logs, you will see a set of entries similar to the following detailing the failure:

00/00/2017 00:00:00:00 w3wp.exe (0xXXXX) 0x18A8 PowerPivot Service Unknown 46 Medium Capturing Report Gallery snapshot information from the following URL: https://localhost/_vti_bin/ExcelRest.aspx/PowerPivot Gallery/GenericWorkbook.xlsx/model/Sheets 37abef9d-92a8-20b9-17bb-d369b513965b

00/00/2017 00:00:00:00 w3wp.exe (0xXXXX) 0x18A8 PowerPivot Service Unknown 46 High Snapshot Exception: Unable to take snapshots or get details of the file: https://ssl.contoso.com/PowerPivot Gallery/GenericWorkbook.xlsx from the uri: https://localhost/_vti_bin/ExcelRest.aspx/PowerPivot Gallery/GenericWorkbook.xlsx/model/Sheets. 37abef9d-92a8-20b9-17bb-d369b513965b

00/00/2017 00:00:00:00 w3wp.exe (0xXXXX) 0x18A8 PowerPivot Service Unknown 46 High Snapshot Exception: Ensure localhost uris are allowed. 37abef9d-92a8-20b9-17bb-d369b513965b

00/00/2017 00:00:00:00 w3wp.exe (0xXXXX) 0x18A8 PowerPivot Service Report Gallery 99 High EXCEPTION: System.Net.WebException: The underlying connection was closed: An unexpected error occurred on a send. <<Truncated stack>> 37abef9d-92a8-20b9-17bb-d369b513965b

During my multiple attempts to narrow this down, I could see the snapshot process attempting to load the first site bound certificate in my list (for my "secondary" SSL site) instead of the certificate for ssl.contoso.com. Because of the localhost call, IIS is confused (obviously) about what certificate to hand out for the request. This issue is specific to farms where everything else is configured properly and functional, but it is not limited to just this scenario. This just happens to be the most common (and basic) configuration.

The product team has determined this is by design and will not be changed due to product limitations with SharePoint and Office Online Server.

"The Workaround"

  • You can use a wildcard certificate. This is the only possible workaround we have found so far. With a wildcard certificate for your sites, you can utilize multiple SSL host header sites and your gallery snapshots will function (this has been tested and confirmed working). Again, this is assuming that everything else is configured properly for snapshots to function. We realize this is not viable for everyone, but at this point it is only option to keep sites SSL and have functional snapshots.
    • Note: Be sure that you research all aspects of using Wildcard certificates before implementing this solution so you are fully informed of the risks and rewards of using them.

Other Options

  • You can restrict your usage to one SSL site  with host header and certificate and no IP specific bindings (site specific SSL certs and IP specific bindings usually go hand-in-hand).
  • Stop using the PowerPivot Gallery all together. You can schedule data refreshes for PowerPivot workbooks in any document library as long as the PowerPivot solution is deployed to the web app and the features are activated on the site.
  • Convert your SSL sites to HTTP. (Maybe even just the sites that host workbooks?)
  • Set "All Documents" to the default view in the PowerPivot Gallery.
    1. Navigate to the list a site owner.
    2. Click "Library" > "Library Settings" from the ribbon menu.
    3. Scroll down to the "Views" section and click "All Documents"
    4. Check the "Make this the default view" box and click "OK"
  • You can take the above a step further by deleting the Silverlight views in the PowerPivot Gallery.
    • (Do the "Set "All Documents" to the default view" section above first)
      1. Navigate to the list a site owner.
      2. Click "Library" > "Library Settings" from the ribbon menu.
      3. Scroll down to the "Views" section and click on any of the 3 PowerPivot Silverlight views "Gallery", "Theatre" and "Carousel".
      4. Click "Delete".
  • You can also Delete/Hide the extra views with PowerShell (sample scripts attached below):
    • Hide Script
    • Delete Script
      NOTE: For your current PowerPivot Galleries, they will continue to attempt to take snapshots even if you disable/delete the views. The only way to stop them from attempting to create snapshots is to replace the gallery with a normal document library. As long as you keep the PowerPivot features deployed to the site, you will still be able to schedule data refreshes as usual. We STRONGLY recommend that you test these scripts extensively in a development environment before using them on a farm that matters.

Something else to consider here is that PowerBI Report Server is being released soonish (Q2 this year is the current timeline). You can check out more info and try out the preview version already here: https://powerbi.microsoft.com/en-us/report-server/

For more information about changes to Gallery Snapshots in SharePoint 2016, see our post here: https://blogs.technet.microsoft.com/excel_services__powerpivot_for_sharepoint_support_blog/2017/01/20/changes-to-powerpivot-gallery-and-snapshots-in-sharepoint-2016/

This article covers an issue specific to PowerPivot Gallery Snapshots in SharePoint 2016 and does not refer to or reference any other products. If you see similar errors in other versions of SharePoint, the solutions/recommendations list here will not help you and are not relevant to your issue. The PowerShell scripts provided in this article are provided as samples for testing with no guarantee or warrantee by Microsoft.

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