[This is preliminary documentation and subject to change.]
There are two phases of an installation process: acquisition and execution. A rollback phase may also occur if the installation is unsuccessful.
The exact installation procedure is determined during the acquisition phase. At the beginning of the acquisition phase, an application or a user instructs Microsoft® Windows® installer to install a feature or an application. The installer complies with this request by beginning the required action sequence. The actions query the database and generate a script that gives a step-by-step procedure for performing the installation. For information on installing an application, see Initializing an Application and Installing an Application.
During the execution phase, the installer passes the information to a process with elevated privileges to generate an installation script. The process with elevated privileges then runs the script. On a secure system, this gives a user with limited privileges the ability to instigate an installation without compromising system integrity.
If an installation is unsuccessful, the installer automatically restores the original state of the computer. When the installer processes the installation script it simultaneously generates a rollback script. In addition to the rollback script, the installer saves a copy of every file it deletes during the installation. These files are kept in a hidden, system directory called Config.msi. Once the installation is complete, the rollback script and the saved files are deleted.