is an umbrella term that describes software technologies that improve portability, manageability and compatibility of applications
by encapsulating them from the underlying operating system
on which they are executed. A fully virtualized application is not installed in the traditional sense, although it is still executed as if it is. The application is fooled at runtime into believing that it is directly interfacing with the original operating system and all the resources managed by it, when in reality it is not. Application virtualization differs from operating system virtualization
in that in the latter case, the whole operating system
is virtualized rather than only specific applications.
Limited application virtualization is used in modern operating systems such as Microsoft Windows
. For example, IniFileMappings were introduced with Windows NT
to virtualize (into the Registry
) the legacy INI files
of applications originally written for Windows 3.1
. Similarly, Windows Vista
implements limited file and Registry
virtualization so that legacy applications that try to save user data in a read-only system location can still work on the platform without elevating users' access.
Full application virtualization requires a virtualization layer. This layer must be installed on a machine to intercept all file and Registry operations of virtualized applications and transparently redirect these operations into a virtualised location. The application performing the file operations never knows that it's not accessing the physical resource it believes it is. In this way, applications with many dependent files and settings can be made portable by redirecting all their input/output to a single physical file, and traditionally incompatible applications can be executed side-by-side. Microsoft Application Virtualization, Software Virtualization Solution, and VMware ThinApp are examples of this technology for the Windows platform.
A common misconception is that a runtime environment is application virtualization. However a runtime layer is required for an application to be able to execute, while a virtualization layer is not.
A similar, but different principle is Operating system virtualizations which encapsulate the entire operating system from the hardware, as opposed to only the applications from the operating system.
Technology categories that fall under application virtualization include:
- Application Streaming. The application is delivered in a package, that may include a subset of OS files and configuration settings. Running the package requires the installation of a lightweight client application. Packages are usually delivered over a protocol such as HTTP or RTSP. Application virtualization is commonly paired with application streaming to deliver applications on demand.
- Desktop Virtualization/Virtual Desktop Infrastructure (VDI). The application is hosted in a VM or blade PC that also includes the operating system (OS). These solutions include a management infrastructure for automating the creation of virtual desktops, and providing for access control to target virtual desktop. VDI solutions can usually fill the gaps where application streaming falls short.
Benefits of application virtualization
- Allows applications to run in environments that do not suit the native application (e.g. Wine allows Microsoft Windows applications to run on Linux).
- May protect the operating system and other applications from poorly written or buggy code.
- Uses fewer resources than a separate virtual machine.
- Run applications that are not written correctly, for example applications that try to store user data in a read-only system-owned location.
- Run incompatible applications side-by-side, at the same time and with minimal regression testing against one another.
- Maintain a standard configuration in the underlying operating system across multiple computers in an organization, regardless of the applications being used, thereby keeping costs down.
- Implement the security principle of least privilege by removing the requirement for end-users to have Administrator privileges in order to run poorly written applications.
- Simplified operating system migrations.
- Accelerated application deployment, through on-demand application streaming.
- Improved security, by isolating applications from the operating system.
Disadvanges of application virtualization
- Applications have to be "packaged" or "sequenced" before they will run in a virtualized way.
- Minimal increased resource requirements (memory and disk storage).
- Not all software can be virtualized. Some examples include applications that require a device driver and 16-bit applications that need to run in shared memory space.
- Some compatibility issues between legacy applications and newer operating systems cannot be addressed by application virtualization (although they can still be run on an older operating system under a virtual machine). For example, some applications that don't manage the heap correctly will not execute on Windows Vista, regardless of whether they are virtualized or not, as they still allocate memory in the same way. For this reason, specialist application compatibility fixes ("SHIMs") may still be needed, even if the application is virtualized.
Similar application schemes