[Engine-devel] API design and plan

[Ayal Baron]

----- Original Message -----
> On Tue, Dec 06, 2011 at 08:46:57AM -0600, Adam Litke wrote:
> > On Tue, Dec 06, 2011 at 02:58:59PM +0200, Dan Kenigsberg wrote:
> > > On Mon, Dec 05, 2011 at 11:34:18AM -0600, Adam Litke wrote:
> > > > Hi everyone.  On today's VDSM call we discussed the
> > > > requirements, design, and
> > > > plan for updating the API to include support for QMF and
> > > > single-host REST API.
> > > > All members present arrived at a general consensus on the best
> > > > way to design the
> > > > next-generation API.  I have tried to capture this discussion
> > > > in the oVirt wiki:
> > > > 
> > > >      http://ovirt.org/wiki/Vdsm_API
> > > > 
> > > > Please take a look at this page and let's discuss any changes
> > > > that may be needed
> > > > in order to adopt it as a working plan that we can begin to
> > > > execute.  Thanks!
> > > >
> > > 
> > > Very nice, I've fixed two bullets about the future of the
> > > xml-rpc.
> > 
> > Thanks... Updates look good to me.
> > 
> > > I think that we are missing something here: how do we model
> > > Vdsm-to-Vdsm
> > > communication, in a binding-blind way? I'm less worried about the
> > > storage-based mailbox used for lvextend requests: my problem is
> > > with
> > > migration command.
> > 
> > Ok, interesting...  Besides migration, are there other features
> > (current or
> > planned) that would involve P2P communication?  I want to ensure we
> > consider the
> > full problem space.
> 
> Well, I can imagine we would like a host in distress to migrate VMs
> to
> whomever can take them, without central management driving this
> process.
> (CAVE split brain)
> 
> At the momemt I cannot think of something that cannot be implemented
> by
> QMF events. Ayal?
> 
> > 
> > > Currently, the implementation of the "migrate" verb includes
> > > contacting
> > > the remote Vdsm over xml-rpc before issuing the libvirt
> > > migrateToURI2
> > > command ('migrationCreate' verb).
> > > 
> > > A Vdsm user who choose to use the REST binding, is likely to want
> > > this to
> > > be implemented this using a REST request to the destination. This
> > > means
> > > that the implementation of Vdsm depends on the chosen binding.
> > > 
> > > The issue can be mitigating by requiring the binding level to
> > > provide a
> > > "callback" for migrationCreate (and any other future Vdsm->world
> > > requests).
> > > This would complicate the beautiful png at
> > > http://ovirt.org/wiki/Vdsm_API#Design ... Does anyone have
> > > another
> > > suggestion?
> > 
> > Actually, I think you are blending the external API with vdsm
> > internals.  As a
> > management server or ovirt-engine, I don't care about the protocol
> > that vdsm
> > uses to contact the migration recipient.  As far as I am concerned
> > this is a
> > special case internal function call.  For that purpose, I think
> > xmlrpc is
> > perfectly well-suited to the task and should be used
> > unconditionally, regardless
> > of the bindings used to initiate the migration.
> > 
> > So I would propose that we modify the design such that we keep an
> > extremely thin
> > xmlrpc server active whose sole purpose is to service internal P2P
> > requests.
> 
> Interesting. We could avoid even that, if we could register a
> callback
> with libvirt, so that destination libvirtd called destination Vdsm to
> verify that all storage and networking resources are ready, before
> executing qemu. DanPB, can something like that be done? (I guess it
> is
> not realistic since we may need to pass vdsm-specific data from
> source
> to dest, and libvirt is not supposed to be a general purpose
> transport.)
> 
> Dan.

I don't understand the issue.  The whole point of the REST API is to be an easily consumable *single* node management API.  Once you start coordinating among different nodes then you need clustering and management (either distributed or centralized), in both cases it is fine to require having a bus in which case you have your method of communications between hosts to replace current xml-rpc.

Requiring an additional xml-rpc server sounds wrong to me.

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