April 15, 2021 - Alon Moss, Chief Innovation Officer

Microsoft has announced on April 13, 2021, that MSIX app attach is now generally available. MSIX app attach is Microsoft’s entry into the field of dynamic application deployment - sometimes referred to as application layering and used for distributing applications to virtual desktops. MSIX app attach is a key part of Microsoft’s new VDI offering Windows Virtual Desktop (WVD). It may be the most dramatic change to application deployment since the launch of the personal computer in 1981.

To understand why MSIX app attach is so revolutionary, we need to dive into a bit of IT technology history. Desktop applications have been the mainstay of the enterprise work environment since the personal computer entered the enterprise space in the 1980s. However, fundamentally, the way applications are deployed has not changed since: Apps must be installed on every machine where they are to be used. This is their biggest weakness! If a thousand users need access to an application, then that application must be installed and configured on a thousand devices. Over the years we have gotten a lot better at the distribution and installation of apps. By introducing tools such as Microsoft SCCM and Intune, file formats such as MSI and App-V, it became a lot easier to install applications for users (a lot easier than walking around offices with CD-ROM’s and clicking “next”, anyway). However, the apps must still be installed on user devices, leading to all sorts of localized issues such as compatibility, “DLL hell”, conflicts between apps, increasing deployment and maintenance costs, headaches to IT managers, and a diminished end user experience.

Two technological concepts bear discussion to understand why MSIX app attach is such a breakthrough: The first is Application Layering. The rise of VDI required a new way of delivering apps to virtual devices and the virtual hard drive that supported it. Originally introduced by Citrix (see an excellent explanation of the technology here), application layering basically creates a “flattened” image made by an application installer to a separate file in VHD or similar format. It then becomes possible to layer such files on top of each other and on top of the base VHD image that contains the OS, allowing the Virtual machine to use a single “disk” that is made up of several transparent layers. This is very similar to layering transparencies over each other, where each layer adds more details creating a composite image. This makes it easy to add an application to an image – simply by adding another layer. Administrators can also “uninstall” an application if there is an issue by simply removing that specific app layer (see diagram below).


The second technology that makes MSIX app attach possible is Microsoft’s MSIX package format. Launched in 2018, it has many unique and important features such as application containerization. Initially introduced, albeit only conceptually, with App-V, application containerization allows applications to be decoupled from the OS by running in a “sandboxed” environment. This also allows applications to be easily added and removed from a system – although they are still installed. Crucially, though, everything an application needs is provided within the MSIX container.

So how does MSIX app attach put all this together? MSIX app attach provides a functionality that is similar to App Layering, but better in many ways. Some readers may be familiar with the concept of shared network drives, MSIX app attach works in a similar way. The application image is not “layered” on top of the device, it is “attached” to the device through the creation of a virtual disk drive, which basically contains a flattened version of the application in MSIX format, hence MSIX is required for MSIX app attach. This concept is, frankly, amazing in its simplicity. After attaching applications look and feel as if they were locally installed to the user as well as the operating system and can be made available instantly and removed instantly.

The possibilities that this technology provides are astounding: To list just a few:

  • Physical devices supported: Since the application is delivered as a drive, rather than a disk “layer”, MSIX app attach can and does work for physical devices (it has, in fact, been generally available for physical devices for some time). Therefore, even enterprises that work on-premises can take advantage of this unique technology.
  • Application on demand: Long the “holy grail” of application deployment, this is now possible. The number of applications that can potentially be made available to users rises dramatically. Just imagine a user needing an app for a one-off task. Or, imagine that an application can be selected and added to a desktop as easily and quickly as we currently browse websites, and you’ll see the possibilities.
  • Reduced reliance on gold OS Images: The applications are entirely independent of the OS image greatly reducing the work required to maintain gold images.
  • Truly decoupled applications: Clearly, devices will continue to require an OS since device hardware resources must be controlled and managed. However, this pattern will in the long term lead to the operating system being irrelevant to the application. Microsoft has been moving in this direction for some time through improvements to the Windows subsystem for Linux (WSL) and through Project Latte - a declared intent to allow android apps to run on Windows.
  • The democratization of applications: With truly decoupled applications that are always up-to-date and available on-demand operating systems will become simpler and more stable, focusing on basic I/O services and just enough software to run a container leaving the architecture less prone to compatibility and update issues. Critically, any application will be able to run on any operating system and on any device.

As WVD rollouts continue across the enterprise IT space it will be interesting to see the creative and innovative ways customers find to take advantage of this breakthrough technology to enable their business at a faster pace with the best user experience. We will continue to report on this.

In my next blog, I will discuss the uniqueness of WVD.

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