2017-08-30 15:43 GMT+02:00 Sylvain Bauza <sylvain.ba...@gmail.com>:

> Still on PTO for one day, but I'll try to answer. Sorry for the delay, was
> out traveling.
> 2017-08-21 23:09 GMT+02:00 Matt Riedemann <mriede...@gmail.com>:
>> I don't dabble in the RequestSpec code much, but in trying to fix bug
>> 1712008 [1] I'm venturing in there and have some questions. This is mostly
>> an email to Sylvain for when he gets back from vacation but I wanted to
>> dump it before moving forward.
> Heh :-)
>> Mainly, what is the difference between RequestSpec.force_hosts/force_nodes
>> and RequestSpec.requested_destination?
> force_[hosts/nodes] was a former key in the legacy filter_properties
> dictionary that was passed to the scheduler. When I designed the
> RequestSpec object, I just provided the same names for the fields that were
> related to those keys.
> requested_destination is a new field that wasn't previously in the legacy
> dictionaries but was just added when I wrote in Newton a new feature about
> verifying a target when moving.
> There is a main behavioural difference between those two RequestSpec
> fields, that is because the behaviours are different between a boot request
> and a move operation.
> When you boot an instance, you can pass (weirdly and terribly from an API
> point) a target for that instance by using the availability_zone flag. Some
> request  like nova boot inst1 --availability_zone nova:foo:bar will
> actually call the scheduler to just verify if host "foo" (and node "bar")
> is alive *without* verifying filters.
> When you move that instance by passing a target (something like "nova
> live-migrate inst1 foo2") will rather call the scheduler to verify only
> that host (and not the others) *by running filters*.
> TBC, if you 'force' a boot, it won't be a real forced boot because it will
> still call the scheduler but it will dumbly accept the proposed destination.
> If you want to *propose* a destination for a move, it will ask the
> scheduler to *really* verify that destination (or end up the migration if
> not possible). You can still want to *force* a move, but in that case, it
> just bypasses the scheduler.
>> When should one be used over the other? I take it that
>> requested_destination is the newest and coolest thing and we should always
>> use that first, and that's what the nova-api code is using, but I also see
>> the scheduler code checking force_hosts/force_nodes.
> MHO is that force_hosts is just a technical hack based on an unclear API
> contract about guarantting a boot request to succeed against a specific
> target. I'd rather propose a new API contract for providing a destination
> exactly based on the same semantics that the ones we have for move
> operations, ie. :
> - if a destination is proposed, run scheduler filters only against that
> destination and error out the boot request if the host can't sustain the
> request.
> - if a destination is proposed and a 'force' flag is used, then just pass
> the RPC request to the target compute service and magically expect
> everything will work.
> That said, there is a corner case now we have allocations made by the
> scheduler. If we go straight to the compute service, we won't create the
> allocation if a force operation is made. In that case, something (that has
> to be discussed in the spec mentioning the API change) has to reconcile
> allocations for the compute service. Maybe it's just a technical detail,
> but that has to be written in the spec tho.

Huh, haven't looked at the bug before writing the email. /me facepalms.
Yeah, the bug you wrote is the exact problem I mentioned above...

> Is that all legacy compatibility code? And if so, then why don't we handle
>> requested_destination in RequestSpec routines like
>> reset_forced_destinations() and to_legacy_filter_properties_dict(), i.e.
>> for the latter, if it's a new style RequestSpec with requested_destination
>> set, but we have to backport and call to_legacy_filter_properties_dict(),
>> shouldn't requested_destination be used to set force_hosts/force_nodes on
>> the old style filter properties?
>> Since RequestSpec.requested_destination is the thing that restricts a
>> move operation to a single cell, it seems pretty important to always be
>> using that field when forcing where an instance is moving to. But I'm
>> confused about whether or not both requested_destination *and*
>> force_hosts/force_nodes should be set since the compat code doesn't seem to
>> transform the former into the latter.
> I agree, that sounds important to me to have consistent behaviours between
> all nova operations that require a placement decision. I have that in my
> pipe for a very long time (I mean, writing the spec for modifying the API
> contract on instance creation) so I push it a bit more.
>> If this is all transitional code, we should really document the hell out
>> of this in the RequestSpec class itself for anyone trying to write new
>> client side code with it, like me.
> That sounds a very good short win to me.
> -Sylvain
>> [1] https://bugs.launchpad.net/nova/+bug/1712008
>> --
>> Thanks,
>> Matt
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