The CachingScheduler has been deprecated since Pike [1].
It does not use the placement service and as more of nova
relies on placement for managing resource allocations,
maintaining compabitility for the CachingScheduler is
exorbitant.
The release note in this change goes into much more detail
about why the FilterScheduler + Placement should be a
sufficient replacement for the original justification
for the CachingScheduler along with details on how to migrate
from the CachingScheduler to the FilterScheduler.
Since the [scheduler]/driver configuration option does allow
loading out-of-tree drivers and the scheduler driver interface
does have the USES_ALLOCATION_CANDIDATES variable, it is
possible that there are drivers being used which are also not
using the placement service. The release note also explains this
but warns against it. However, as a result some existing
functional tests, which were using the CachingScheduler, are
updated to still test scheduling without allocations being
created in the placement service.
Over time we will likely remove the USES_ALLOCATION_CANDIDATES
variable in the scheduler driver interface along with the
compatibility code associated with it, but that is left for
a later change.
[1] Ia7ff98ff28b7265058845e46b277317a2bfc96d2
Change-Id: I1832da2190be5ef2b04953938860a56a43e8cddf
25dadb94db