There are times when repairing or servicing a product that source media is required. As more product installs move to web-based installs, these prompts for source are harder to solve. In order to replace missing, corrupted, or older files, the source is required because those files have to come from somewhere.

Burn, the bootstrapper application in the Windows Installer XML toolset, aims to solve that by caching setup packages on disk – specifically in %ProgramData% or %LocalAppData% depending on whether a package is installed per-machine or per-user, respectively. Whenever a user repairs a bundle, the cached packages are used to mitigate source prompts. And because this location is registered with Windows Installer, even if Windows Installer package (MSI or MSP) requires source it will be resolved automatically.

For more reasons and examples of why source is required, please read the following links:

• Resolving Prompts for Source
• Source Resolution during Patch Uninstall
• Adding New Components to Existing Features Installs the Feature Tree
• ResolveSource Requires Source
• Installing Visual Studio 2010 Service Pack 1 may prompt for source

Caching packages is optional in Burn, but caching packages does help solve a number of potential issues that may be difficult – and can certainly be annoying – to resolve. And since source resolution cannot prompt during silent installs, caching improves the robustness of deployments.