Applications that rely on network resources for installation-on-demand are susceptible to source failures if the source location should change for any reason or become damaged. The Windows Installer provides source resiliency for features that are installed on-demand by using a source list. The source list contains the locations searched by the installer for installation packages. The entries in this list can be network locations, Uniform Resource Locators (URLs), or compact discs. If one of these sources fails, the installer can quickly and seamlessly try the next.
The application developer does not need to incorporate any special information into the installer package to ensure source resiliency. Once the application is installed, the installer has the behavior of adding the last successfully used source as an entry in the source list. By default, this source is the location from which the installer package is initially installed, and is the same as the SourceDir property.
The installer begins searching for a source by checking the most recently used source location in the source list. If this search fails, the installer searches the list of network sources, then media sources, and finally URL sources. System administrators can change this search order using the SearchOrder system policy. If these searches fail, the installer may present a Browse Dialog so that the user can search for the source manually. The browse dialog box cannot be displayed if the user interface level is set to None. For details, see User Interface Levels.
Commonly, the installer should only display a browse dialog box if the current user is an administrator or if the installation does not require elevated privileges. An administrator can control the display of the browse dialog box to users with the DisableBrowse and AllowLockDownBrowse policies. An administrator also controls whether users can install applications from sources located on media by using the DisableMedia and AllowLockDownMedia policies. The use of these policies depends on the Windows Installer version. For details, see the following: