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It is my personal opinion that there is no necessary condition between
'having the possibility of leaving a resource without service provider' and
'API users want to create resources without service providers'.
While being able to change the 'provider' associated with a resource is a
reasonable use case, albeit not one we might want to implement now,
unassociated resources as a valid use case need to be motivated better. The
possibility Eugene describes in his post - a service provider being deleted
- is probably not a 'regular' use case, but more the consequence of a
potentially error-prone operation.

Sticking to the concept that Neutron should hide as much as possible
details regarding backing technologies - and from what I gather providers
map 1:1 with backend technologies - I would say that the default action of
associating resource with a 'default' service provider still makes sense; I
am not convinced by the idea of having a 'No_op' service provider; I think
if an API user wants to create a resource, but does not want it to be
'implemented', the way to go would be to put it administratevely down,
regardless of the provider used.

Going back to the original issue - removal of a provider, I think we should
consider whether such an action is legal or not.
If it's not legal, then we might think about adding a check at startup
time: if service instances mapped to unknow providers are found, Neutron
may fail to start up.
It is also nteresting the case when the provider name is not changed, but
the driver is changed. I have reason to believe this might lead to a bit of
mayhem; it *might* be manageable, but probably for Havana it might be just
worth documenting that this kind of operation should just not be performed.

Salvatore


On 29 July 2013 16:34, Eugene Nikanorov <enikanorov at mirantis.com> wrote:

> Hi folks,
>
> Recently we've been discussing with Nachi Ueno some specific use case of
> deployments with multiple providers for particular advanced service.
>
> What If admin wants to remove certain provider from configuration file?
> The following handling was proposed:
> 1) Before restarting neutron-server to apply new configuration, admin
> undeploys all resources that were deployed with provider being removed.
> That's needed to gracefully cleanup devices used by provider being removed.
>
> 2) Removal of a provider should not result in resource deletion.
> E.g. user's logical resources are preserved, and they can be associated
> with other service providers later on.
>
> 3) Having (1) and (2) it's obvious that after those steps are performed,
> users are left with pure logical resources which don't have physical
> representation.
> I think such resources are no worse then deployed ones, e.g. could be
> worked with just the same way as those which have a provider associated.
>
> The conclusion from (3) is that users themselves may want to create such
> resources.
> The workflow would be to create resource with no provider, configure it,
> and then deploy at once.
>
> And I have an API question regarding such use case.
> Currently user doesn't specify provider to create resource. For this to
> continue to work we introduced the notion of 'default provider'. So, If
> user doesn't specify provider - the default one is used.
> Then if we want users to be able to create unassociated resource, what
> kind of provider they need to pass to 'create' API call?
>
> One important consideration that appeared while I was implementing
> multiple providers for lbaas is that a resource always needs to be handled
> by some provider, even if not associated with any. That is needed to
> preserve consistency of DB operations, because in current model (for lbaas
> at least) plugin sets object (pool, vip, etc) status to PENDING_DELETE, and
> then driver deletes it.
> If resource is unassociated with the driver, then 'Noop' driver must be
> used for this logic to work properly.
>
> One of the solution to this would be to have special name for the provider
> to indicate 'Noop' provider. The reason for having special name is that
> it's better to not specify Noop in configuration as it is essential basic
> provider that should always be present.
>
> Please share your thoughts.
>
> Thanks,
> Eugene.
>
>
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