The VTAM® shadow resource function enables VTAM to dynamically change what it defined previously as CDRSCs into same-domain resources during takeover.

A shadow resource is an alternate definition of a network resource. For example, an LU is defined to a backup host both as a same-domain resource and as a cross-domain resource. Under normal operation, the LU is a cross-domain resource, and the cross-domain definition is used. The same-domain resource definition, which is not used, is the shadow resource. Whichever definition is not being used is considered to be the shadow resource.

If the host that owns the LU fails, the backup host assumes ownership of the LU, the same-domain definition becomes the active definition, and the cross-domain definition becomes the shadow resource. Resource takeover can be accomplished without disrupting existing active sessions. When the original host becomes available, the backup host relinquishes ownership of the LU. The backup host then activates its cross-domain definition for the LU, and its same-domain definition once again is the shadow resource.

When the LU is owned by the backup host, other hosts that need to access the LU must be able to communicate with and route requests to the backup host.