This is a mechanism whereby when heat processes a stack Server or Volume resource, the stack id, root stack id, stack resource uuid, stack resource name and the path in the stack can be passed to nova and cinder by heat as scheduler hints, to the configured schedulers for nova and cinder.
By default, passing the lifecycle scheduler hints is disabled. To enable it, set stack_scheduler_hints to True in heat.conf.
When heat processes a stack, and the feature is enabled, the stack id, root stack id, stack resource uuid, stack resource name, and the path in the stack (as a list of tuple, (stackresourcename, stackname)) will be passed to nova and cinder by heat as scheduler hints, to the configured schedulers for nova and cinder.
A heat provider may have a need for custom code to examine stack requests prior to performing the operations to create or update a stack. After the custom code completes, the provider may want to provide hints to the nova or cinder schedulers with stack related identifiers, for processing by any custom scheduler plug-ins configured for nova or cinder.