There are two phases to a successful installation process: acquisition and execution. If the installation is unsuccessful, a rollback phase may occur.
At the beginning of the acquisition phase, an application or a user instructs the installer to install a feature or an application. The installer then progresses through the actions specified in the sequence tables of the installation database. These actions query the installation database and generate a script that gives a step-by-step procedure for performing the installation.
During the execution phase, the installer passes the information to a process with elevated privileges and runs the script.
If an installation is unsuccessful, the installer 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. Once the installation is complete, the rollback script and the saved files are deleted. For more information, see Rollback Installation.