---
upgrade:
  - |
    The ``caching_scheduler`` scheduler driver, which was deprecated in the
    16.0.0 Pike release, has now been removed. Unlike the default
    ``filter_scheduler`` scheduler driver which creates resource allocations
    in the placement service during scheduling, the ``caching_scheduler``
    driver did not interface with the placement service. As more and more
    functionality within nova relies on managing (sometimes complex) resource
    allocations in the placement service, compatibility with the
    ``caching_scheduler`` driver is difficult to maintain, and seldom tested.
    The original reasons behind the need for the CachingScheduler should now
    be resolved with the FilterScheduler and the placement service, notably:

    * resource claims (allocations) are made atomically during scheduling to
      alleviate the potential for racing to concurrently build servers on the
      same compute host which could lead to failures
    * because of the atomic allocation claims made during scheduling by the
      ``filter_scheduler`` driver, it is safe [1]_ to run multiple scheduler
      workers and scale horizontally

      .. [1] There are still known race issues with concurrently building some
        types of resources and workloads, such as anything that requires
        PCI/NUMA or (anti-)affinity groups. However, those races also existed
        with the ``caching_scheduler`` driver.

    To migrate from the CachingScheduler to the FilterScheduler, operators can
    leverage the ``nova-manage placement heal_allocations`` command:

    https://docs.openstack.org/nova/latest/cli/nova-manage.html#placement

    Finally, it is still technically possible to load an out-of-tree scheduler
    driver using the ``nova.scheduler.driver`` entry-point. However,
    out-of-tree driver interfaces are not guaranteed to be stable:

    https://docs.openstack.org/nova/latest/contributor/policies.html#out-of-tree-support

    And as noted above, as more of the code base evolves to rely on resource
    allocations being tracked in the placement service (created during
    scheduling), out-of-tree scheduler driver support may be severely impacted.

    If you rely on the ``caching_scheduler`` driver or your own out-of-tree
    driver which sets ``USES_ALLOCATION_CANDIDATES = False`` to bypass the
    placement service, please communicate with the nova development team in
    the openstack-dev mailing list and/or #openstack-nova OFTC IRC channel
    to determine what prevents you from using the ``filter_scheduler`` driver.
