[This is preliminary documentation and subject to change.]
With traditional installation technology, it was necessary to exit an application and rerun setup to perform an installation task. This commonly occurred whenever a user wanted a feature or product they had not chosen during the first run of setup. This often made the process of product configuration inefficient because it required the user to anticipate which functionality they required before they ever used the product.
Just-in-time installation makes it possible to offer functionality to users and applications in the absence of the files themselves. This notion is known as advertising. The Microsoft® Windows® installer has the capability of advertising functionality and to make just-in-time installations of application features or entire products. Whenever a user or application activates an advertised feature or product the installer proceeds with an installation of the needed components. This speeds up and shortens the configuration process because additional functionality can be accessed without having to exit and rerun another setup procedure.
When a product uses the installer, a user can choose at setup time which features or applications to install and which to advertise. Then if while running an application the user requests an advertised feature that has not yet been installed, the application will call the installer to enact a just-in-time feature level installation of the necessary files. If the user activates an advertised product that has not yet been installed, the operating system will call the installer to enact a just-in-time product level installation.
For more information about how the installer enacts product and feature level installations see the section Advertising Applications and Features.
Advertisement and just-in-time installation can also facilitate system management by enabling administrators to designate applications as required or optional for different groups of users. There are two types of advertising known as 'assigning' and 'publishing.' If an administrator assigns an application to a group, then these users can install the application on demand. If however the administrator publishes the application to the group, no entry points will appear to these users and a just-in-time installation will only be activated if another application activates the published application.
For more information about the two types of advertising see the section Assigning and Publishing.
For information on starting an application for the first time, see Initializing An Application.