[nova] Validation for requested host/node on server create

melanie witt melwittt at gmail.com
Wed May 22 23:33:17 UTC 2019

On Wed, 22 May 2019 17:13:48 -0500, Matt Riedemann <mriedemos at gmail.com> 
> It seems we've come to an impasse on this change [1] because of a
> concern about where to validate the requested host and/or
> hypervisor_hostname.
> The change is currently validating in the API to provide a fast fail 400
> response to the user if the host and/or node don't exist. The concern is
> that the lookup for the compute node will be done again in the scheduler
> [2]. Also, if the host is not provided, then to validate the node we
> have to iterate the cells looking for the given compute node (we could
> use placement though, more on that below).
> I've added this to the nova meeting agenda for tomorrow but wanted to
> try and enumerate what I see are the options before the meeting so we
> don't have to re-cap all of this during the meeting.
> The options as I see them are:
> 1. Omit the validation in the API and let the scheduler do the validation.
> Pros: no performance impact in the API when creating server(s)
> Cons: if the host/node does not exist, the user will get a 202 response
> and eventually a NoValidHost error which is not a great user experience,
> although it is what happens today with the availability_zone forced
> host/node behavior we already have [3] so maybe it's acceptable.
> 2. Only validate host in the API since we can look up the HostMapping in
> the API DB. If the user also provided a node then we'd just throw that
> on the RequestSpec and let the scheduler code validate it.
> Pros: basic validation for the simple and probably most widely used case
> since for non-baremetal instances the host and node are going to be the same
> Cons: still could have a late failure in the scheduler with NoValidHost
> error; does not cover the case that only node (no host) is specified
> 3. Validate both the host and node in the API. This can be broken down:
> a) If only host is specified, do #2 above.
> b) If only node is specified, iterate the cells looking for the node (or
> query a resource provider with that name in placement which would avoid
> down cell issues)
> c) If both host and node is specified, get the HostMapping and from that
> lookup the ComputeNode in the given cell (per the HostMapping)
> Pros: fail fast behavior in the API if either the host and/or node do
> not exist
> Cons: performance hit in the API to validate the host/node and
> redundancy with the scheduler to find the ComputeNode to get its uuid
> for the in_tree filtering on GET /allocation_candidates.
> Note that if we do find the ComputeNode in the API, we could also
> (later?) make a change to the Destination object to add a node_uuid
> field so we can pass that through on the RequestSpec from
> API->conductor->scheduler and that should remove the need for the
> duplicate query in the scheduler code for the in_tree logic.
> I'm personally in favor of option 3 since we know that users hate
> NoValidHost errors and we have ways to mitigate the performance overhead
> of that validation.

Count me in the option 3 boat too, for the same reasons. Rather avoid 
NoValidHost and there's mitigation we can do for the perf issue.


> Note that this isn't necessarily something that has to happen in the
> same change that introduces the host/hypervisor_hostname parameters to
> the API. If we do the validation in the API I'd probably split the
> validation logic into it's own patch to make it easier to test and
> review on its own.
> [1] https://review.opendev.org/#/c/645520/
> [2]
> https://github.com/openstack/nova/blob/2e85453879533af0b4d0e1178797d26f026a9423/nova/scheduler/utils.py#L528
> [3] https://docs.openstack.org/nova/latest/admin/availability-zones.html

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