On 9/30/2019 12:27 PM, Dan Smith wrote:
>> 4. Does the cell conductor need access to the API DB?
> Technically it should not be allowed to talk to the API DB for
> "separation of concerns" reasons. However, there are a couple of
> features that still rely on the cell conductor being able to upcall to
> the API database, such as the late affinity check.

In case you haven't seen this yet, we have a list of operations
requiring "up-calls" from compute/cell-conductor to the API DB in the
docs here:

https://docs.openstack.org/nova/latest/user/cellsv2-layout.html#operations-requiring-upcalls

Some have been fixed for awhile and some are still open because they are
not default configuration we normally deal with (cross_az_attach=False)
or hit in CI* runs (reschedules).

I think the biggest/hardest problem there to solve is the late affinity
check which long-term should be solved with placement but no one is
working on that. The reschedule stuff related to getting AZ/aggregate
info is simpler but involves some RPC changes so it's not trivial and
again no one is working on fixing that.

I think for those reasons CERN is running without a superconductor mode
and can hit the API DB from the cells. Devstack superconductor mode is
the ideal though for the separation of concerns Dan pointed out.

*Note we do hit the reschedule issue sometimes in multi-cell jobs:

http://logstash.openstack.org/#dashboard/file/logstash.json?query=message%3A%5C%22CantStartEngineError%3A%20No%20sql_connection%20parameter%20is%20established%5C%22%20AND%20tags%3A%5C%22screen-n-cond-cell1.txt%5C%22&from=7d

--

Thanks,

Matt