Windows Installer Package

Internal Database Structure

MSI files are relational databases — Windows Installer stores components, features, file paths, and registry entries in SQL-like tables queryable with Orca, Microsoft's free table editor. The core tables include File (source media layout), Component (atomic install units), Feature (user-visible groupings), Registry (keys to write), and CustomAction (arbitrary logic executed during installation). Each row in the File table maps a cabinet entry to a target directory, while the Component table links files to registry entries and shortcuts through foreign keys. The LaunchCondition table gates installation on system prerequisites, and the Upgrade table coordinates major version transitions through product code relationships.

Custom Actions and Transforms

Custom actions range from Type 1 (DLL calls) through Type 51 (property assignments), giving package authors the ability to run arbitrary code during install sequences. This flexibility carries serious risk — a malicious custom action executes with the privilege level of the Windows Installer service, which often runs as SYSTEM. Deferred custom actions are particularly dangerous because they execute in the elevated server-side sequence after the user has already approved the installation. Transforms (.mst files) modify an MSI's database tables without altering the original package, and patches (.msp) deliver delta updates by replacing specific cabinet streams.

Execution Model

The msiexec.exe command-line tool drives installation through two phases: acquisition (UI sequence) and execution (server-side sequence). Administrative installs extract the package contents to a network share via msiexec /a, bypassing the execute sequence entirely. The Installer service maintains a rollback script during execution, allowing complete reversal if any step fails. This transactional model means a power failure mid-install leaves the system in its pre-install state rather than a corrupted intermediate. Group Policy can assign or publish MSI packages to domain-joined machines, enabling silent deployment across thousands of endpoints from a single administrative action.

Repair and Maintenance

Windows caches a stripped copy of each installed MSI in %WINDIR%\Installer. When a component goes missing or a registry key is deleted, the Installer service reads this cached database and restores only the affected rows — a targeted repair rather than full reinstallation. The cached copy lacks cabinet streams, so source media may be required for file-level repairs. Running msiexec /f triggers this self-healing process manually, with option flags controlling whether files, shortcuts, registry entries, or all three are verified and restored.