=============== BackfillMachine

In Crimson, backfill is implemented with Boost State Chart <https://www.boost.org/doc/libs/1_86_0/libs/statechart/doc/>_.

.. //TODO: Once the implementation is settled: .. * Explain exceptional states once we finish working on this code .. * Explain example happy path flow (code walkthorugh?) .. * https://tracker.ceph.com/issues/68728

A sample of the recent state model:

.. note:: Cancelled and Crushed states are not included in the following graph in order to make it easier to follow:

      * **Any** state is able to transit into ``Crushed``.

      * **Any** state (except from ``Initial`` and ``Waiting``) can transit into ``Cancelled``

.. image:: crimson_backfillmachine.svg

In similarly to :doc:/dev/peering a copy of the latest BackfillMachine state model can be genereated using the gen_state_diagram.py <https://github.com/ceph/ceph/blob/master/doc/scripts/gen_state_diagram.py>_