----- Original Message ----- From: "Ori Liel" <oliel(a)redhat.com&gt; To: engine-devel(a)ovirt.org Cc: meyering(a)redhat.com, eglynn(a)redhat.com, gjansen(a)redhat.com, jhernand(a)redhat.com, "Einav Cohen" <ecohen(a)redhat.com&gt;, "Roy Golan" <rgolan(a)redhat.com&gt;, "Michael Kublin" <mkublin(a)redhat.com&gt; Sent: Wednesday, April 11, 2012 11:32:51 AM Subject: Hot-plug of disks and nics On the subject of activate/deactivate disks and nics ("hot-plug"), I'd like to hear opinions about the correct modelling. I see two possible options (I will use disks as an example, but the chosen modelling should be the same for nics and disks). Option 1 - update: ================= PUT .../api/vms/{vm:id}/disks/{disk:id} <disk> <activated>true</activated> </disk> Pros: ----- * RESTful, no need for new action Cons: ----- * Inconsistent with other flows. We do not normally update status fields to perform actions. For example to run a VM, we do not update it's status to 'activated', we run an action (start). * Enables user to update disk properties AND activate/deactivate in the same operation. Updating certain disk properties is forbidden while the VM is up, but activating/deactivating the disk is allowed always. This requires business-logic in REST-API: if the user activates the disk and changes properties on it - REST-API must first change the properties and then activate the disk (if disk is activated first, update properties will fail). If, on the other hand, the user *deactivates* the disk and changes properties on it - REST-API must first deactivate the disk and then change the properties (changing properties while disk is still active will fail). This bug-prone logic is only necessary because when activate/deactivate is merged with update. Option 2 - action ================= .../api/vms/{vm:id}/disks/{disk:id}/attach

Pros: ---- * Less RESTful Cons: ----- * Consistent with other flows * Does not have the drawback of the additional business-logic described above Opinions? Thanks, Ori.

> * Inconsistent with other flows. We do not normally update status > fields to perform actions. For example to run a VM, we do not update > it's status to 'activated', we run an action (start). I think this point is the crux of the matter. IMO the consistency between activation implemented as direct state manipulation versus the state change occurring as a side-effect of an action, would be a deal-breaker. IIRC one of the reasons we avoided that kind of direct state manipulation first time round is that it doesn't lend itself to multi-state transitions, e.g. activation that traversed multiple intermediate states, say: quiescent->pending->activating->activated That example is a bit contrived and unrealistic, but in any case the principal is that the client-visible state machine for a resource may not necessarily transition directly from the initial to the terminal state.

Also an action as opposed to a direct state manipulation makes it more natural to express side-effects of the activation, and to implement in-progress status querying or cancelation. For those reasons, I'm thinking that using actions consistently accross the board is better than exposing an action in one case while allowing a direct state manipulation in another. So my vote would be for option #2.

----- Original Message ----- From: "Jim Meyering" <meyering(a)redhat.com&gt; To: "Ori Liel" <oliel(a)redhat.com&gt; Cc: engine-devel(a)ovirt.org, eglynn(a)redhat.com, gjansen(a)redhat.com, jhernand(a)redhat.com, "Einav Cohen" <ecohen(a)redhat.com&gt;, "Roy Golan" <rgolan(a)redhat.com&gt;, "Michael Kublin" <mkublin(a)redhat.com&gt; Sent: Wednesday, April 11, 2012 1:00:47 PM Subject: Re: Hot-plug of disks and nics Ori Liel wrote: > On the subject of activate/deactivate disks and nics ("hot-plug"), I'd > like to hear opinions about the correct modelling. I see two possible > options (I will use disks as an example, but the chosen modelling > should be the same for nics and disks). > > Option 1 - update: > ================= > PUT > .../api/vms/{vm:id}/disks/{disk:id} > <disk> > <activated>true</activated> > </disk> > > Pros: > ----- > * RESTful, no need for new action > > Cons: > ----- > * Inconsistent with other flows. We do not normally update status > fields to perform actions. For example to run a VM, we do not update > it's status to 'activated', we run an action (start). > * Enables user to update disk properties AND activate/deactivate in > the same operation. Updating certain disk properties is forbidden > while the VM is up, but activating/deactivating the disk is allowed > always. This requires business-logic in REST-API: if the user > activates the disk and changes properties on it - REST-API must first > change the properties and then activate the disk (if disk is activated > first, update properties will fail). If, on the other hand, the user > *deactivates* the disk and changes properties on it - REST-API must > first deactivate the disk and then change the properties (changing > properties while disk is still active will fail). This bug-prone logic > is only necessary because when activate/deactivate is merged with > update. > > > > Option 2 - action > ================= > > .../api/vms/{vm:id}/disks/{disk:id}/[activate] > > [Con]: > ---- > * Less RESTful > > [Pros]: > ----- > * Consistent with other flows > * Does not have the drawback of the additional business-logic described above I too prefer #2. IMHO, we need not be slaves to consistency or RESTfulness, but here, consistency wins hands down when strict conformance to REST guidelines evokes a paragraph worth of "Cons".