MSIX For App Attach On WVD

April 2021

Microsoft publically released App Attach on the 13th of April 2021, but we'd seen an explosion of interest in the technology for about a year leading up to that date. App Attach promises to revolutionise how applications are delivered on Windows Virtual Desktop, so first let's look at why.

Before the release of App Attach three options were available for delivering applications to your pooled Windows Virtual Desktops. Option 1 was to maintain multiple images, option 2 to have a single large image and hide some applications from some users with FsLogix, or a combination of these approaches. Option 3 was to stream applications with App-V, however this has some impact on the run time performance the user sees, even if fully locally cached or using Shared Content Store.

App Attach separates applications from the operating system, similarly to the way FsLogix separates user profiles. Each application or application suite is attached as its own virtual disk on a per user basis, providing complete flexibility. Unlike application layering technologies, because App Attach runs applications in isolated containers there is no merging of them with each other or an OS layer, with the significant delay that causes to Windows startup.

So from an app delivery architecture point of view, Microsoft have done a lot right here and the enthusiasm is understandable.

The containers supported by App Attach are MSIX packages. MSIX is based on the AppX package format that's been around for many years now for delivery of apps from the Microsoft Store. Unfortunately, MSIX has significant limitations supporting applications that weren't written for the store, which of course is the majority of software today.

One statement on Microsoft's What is MSIX? page has caused some confusion. "MSIX provides a reliable install boasting a 99.96% success rate over millions of installs with a guaranteed uninstall." This can be mistaken for meaning pretty much all applications will be fully functional as a MSIX. This is a bit like me saying I can make 99.96% or pool shots. I pretty much always complete a shot, but that doesn't mean the ball goes in the pocket! Likewise, the MSIX installing doesn't mean the application runs properly.

Testing of a wide sample of common applications for MSIX compatibility showed that less than 60% are fully functional right now. The applications tested didn't include any with drivers or otherwise clearly not a candidate for virtualisation, or that are very large such as AutoCAD. You can expect that to account for a further 20%, meaning for a real world portfolio the fully functional number will be around 40%.

It is early days for the technology. Microsoft has provided compatibility shimming for MSIX, known as the Package Support Framework, similarly to how they approached application compatibility with the Windows operating system as that changed. For now this is quite limited but can be expected to expand. They also continue to encourage software vendors to rewrite their applications to fit the Microsoft Store model, but developers aren't likely to rush to embrace this.

So where does that leave MSIX? Having 40% or your applications individually attachable to your pooled WVDs, quickly and smoothly, is still a great result. Yes, you will need to put some apps in the image and mask them with FsLogix for users that shouldn't get them. In fact even if MSIX compatibility with applications improves that will always be the case. App-V is a much more mature "app virtualisation" format and gives something like 75% fully functional. Running applications in a container, especially one they weren't written for, is always going to cause some issues.

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