App Attach, App Layering & App Volumes
Three methods with the same goal: delivering applications without installing them permanently into the image. They come from three vendors, work differently under the hood, and are regularly confused with one another.
The difference in one sentence
App Attach is Microsoft’s method and mounts MSIX containers through native Windows interfaces. Citrix App Layering is Citrix’s own solution, attaching applications as layers at sign-in. Omnissa App Volumes attaches virtual disks through a filter driver so that applications appear to the operating system as if locally installed.
Naming one of these as the answer to a question about another compares apples with pears — which is exactly what happens in product comparisons.
Side by side
| App Attach | Citrix App Layering | Omnissa App Volumes | |
|---|---|---|---|
| Vendor | Microsoft | Citrix | Omnissa |
| Format | MSIX, Appx or App-V package, stored as CIM, VHDX or VHD | proprietary layer format | VHD, VMDK or MSI |
| Mechanism | native Windows interfaces | layers merged into the image | filter driver merges file system and registry in real time |
| Timing | at session start | at sign-in, by AD group | at sign-in or when the app is launched |
| Additional infrastructure | file share | layering appliance | App Volumes Manager plus SQL database; in Horizon Cloud neither is needed |
| Operating system | client and server; Windows Server 2022 and 2025 since April 2026 | client and server | client and server |
| User persistence | no | yes, via the User Layer | yes, via writable volumes |
App Attach
Microsoft’s method for Azure Virtual Desktop and Windows 365. The application is packaged as an MSIX container with its own virtual registry and file system, then mounted on the session host at runtime rather than installed. Besides MSIX, Appx images and App-V packages can also be attached — a route without repackaging for organisations with existing App-V packages, particularly since App-V server infrastructure fell out of support in April 2026.
Microsoft recommends CIM over VHDX as the storage format because it mounts faster and consumes less CPU and memory; that recommendation explicitly applies to session hosts running Windows 11. Microsoft advises against VHD. A valid code signing certificate is a prerequisite, and its whole chain must be trusted on the session hosts.
The former restriction to Windows client operating systems has gone: as of 14 April 2026 Windows Server 2022 and 2025 are supported. Server-based multi-session is therefore no longer a reason to rule App Attach out.
Citrix supports App Attach as well, but only on desktop VDAs — not on server VDAs.
Citrix App Layering
Citrix’s own approach to image management. Operating system, platform and applications sit in separate layers that are merged into a published image. Elastic Layers are attached later, at sign-in, based on the user’s AD group membership — so a single base image serves many application combinations.
The User Layer is a writable elastic layer. It gives users a persistent experience on a non-persistent image: they can install applications and save settings that live outside the user profile.
App Layering supports both Citrix provisioning models, MCS and PVS, and through platform layers delivers the same application set across different hypervisors as well as Azure and Google Cloud.
Omnissa App Volumes
Omnissa’s method, originating in the VMware world. Applications are captured on a dedicated packaging VM and stored as virtual disks in VHD or VMDK format. At sign-in a filter driver merges that disk’s file system and registry with the base operating system in real time — to Windows, the application looks locally installed.
Newer versions offer Apps on Demand: the disk is attached only when the user launches the application, noticeably shortening sign-in times.
In your own data centre App Volumes requires its own management layer: the App Volumes Manager with an attached SQL database, and for production environments Omnissa recommends an external SQL instance rather than the Express edition installed by default. In the Horizon Cloud service both fall away — there App Volumes is part of the control plane and runs as a managed service.
In product comparisons the question “does the platform support App Attach?” is often answered with “yes, App Layering” or “yes, App Volumes”. That is misleading: all three solve the same problem, but they are different products with different formats and different infrastructure. When assessing a platform, ask instead: “how does it deliver applications without rebuilding the image?” — then all three are valid answers.
Which method when
App Attach fits when the environment runs on Azure Virtual Desktop or Windows 365, client operating systems are in use, and no additional management layer is wanted.
App Layering fits Citrix environments with many application combinations, particularly where users should be able to install their own software on non-persistent desktops.
App Volumes fits Omnissa environments and cases where applications with deep system hooks must be delivered that cannot be packaged as MSIX.