WIS (Windows Installer Service) performs application installs transactionally. That is, it keeps track of files and registry settings that are added, replaced or removed. If an unrecoverable error occurs at any time during installation, MSI lets you roll back the entire process to avoid disabling your system with half-installed components.
When used with Active Directory, an MSI can be associated with a group policy to let end users install the application on their systems even if they don't have rights to modify the file system or registry. The request is passed on to WIS for the actual install. Because the package was approved by the administrator using association with the AD group policy, for example, the install will continue.
The MSI format lets you easily modify or customize the software install by creating a transform. An MSI transform is a file (.mst) that describes how WIS should install an MSI package. The transform may indicate which package features to install, provide a custom installation location or designate responses to the user-information screens commonly presented by software installers. Rather than try to modify the original MSI package, it's better to create an MSI transform so that different transforms can be created, all using the same base MSI.
Software developers choosing the MSI format can use advanced MSI capabilities, like self-healing or specifying on-first-access feature installs. These capabilities, however, require the use of MSI programming interfaces, so don't expect this level of functionality from legacy applications that are repackaged into an MSI.
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