WiX Toolset

Automatically download related bundles missing from cache

User stories

  • As a Setup developer I can have a BA that works with the engine to re-cache a related bundle such that my bundle can gracefully handle related bundles that are installed but missing from the cache.

Example scenario

  1. Install BundleA v1.0.
  2. Delete BundleA from the package cache (default for per-machine is C:\ProgramData\Package Cache).
  3. Run BundleA v1.1 which is supposed to do a major upgrade of BundleA v1.0 from step 1.
  4. BundleA v1.1 installs as if BundleA v1.0 is not installed on the machine.
  5. BundleA v1.1 and BundleA v1.0 are now both installed and the BA had no way to know this without manually checking for related bundles.


To help related bundles gracefully handle the scenario where they're missing from the cache, add the hash and file size to the ARP registration. Also, add a new attribute to the Bundle element which will be written to the ARP registration if specified.

<xs:attribute name="DownloadUrl" type="xs:string">
      The URL to use to download the bundle if missing from the package cache.

Add two new parameters to OnDetectRelatedBundle:

// Indicates whether the engine has the hash and file size of the related bundle to be able to recache it.
BOOL fCanBeRecached;
// The download url for the related bundle.
LPCWSTR wzDownloadUrl;

During Plan, related bundles that are missing from the cache will be skipped unless the hash and file size are available. That means that there will also be no message(s) sent to the BA for that bundle during planning, because the engine will only run packages from the cache and it will never put a file in the cache if it can't verify its hash and file size.

If the hash and file size are available, then the default action will depend on whether there's a download url for the bundle. If it is not available, then the default action will be to do nothing. Otherwise, the default action will be the same as if it were present in the cache.


Having the engine handle re-caching the related bundle is not strictly necessary, but it helps avoid an extra UAC prompt for per-machine bundles and helps the BA to avoid manually fixing the related bundle outside of the chain.

Burn has only cached itself into the package cache so far, so there might be some implementation details to work out when putting other bundles into the cache. For example, the DownloadUrl will likely point to a bundle with the attached container but the hash and file size will be for the stripped down bundle.

See Also