Wednesday, September 28, 2011

Self- Healing

  1. self healing is the inbuilt feature or ability of a windows insatller application to detect and repair any important resuorces which are required by the user to use the application and launch the application.
  2. Every resource is not checked during self healing as it would leads to exhaustive checking for resources which would lead to waste of time.
  3. When an windows installer MSI based application is launched (by clicking on an advertised shortcut or file type association),windows installer verifies whether the application is installed or not,if it is not installed windows installer installs the application(this is known as install on demand).if the application is already installed Windows Installer checks the existence of key path items. If there is a mismatch between the current system state and the value specified in the MSI package (e.g., a key file or registry is missing), then the related feature is re-installed. This process is also known as self-healing or self-repair.Both this happens at the feature level.
  4. As indicated in below figure, when an entry point is activated, the Component to which the entry point belongs is checked for which Feature it is attached to. Every component in that Feature is checked for non-existence of the keypaths. If any single keypath is missing, the entire feature is reinstalled.For example, say the Component in figure was installed on a computer. After a certain time period may be months,someone accidentally deletes the file a.dll. The next time the user launched the shortcut s.lnk, the files a.dll, c.dll, and the registry key would be checked for existence. If any of these three resources were missing, the entire Feature (which is made up of the Components component1,component2 and component3) would be reinstalled.

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