On Tue, Nov 29, 2011 at 12:54:44PM -0800, Chris Wright wrote:
> * Adam Litke (a...@us.ibm.com) wrote:
> > On Tue, Nov 29, 2011 at 11:39:08AM -0800, Chris Wright wrote:
> > > * Adam Litke (a...@us.ibm.com) wrote:
> > > >
> > > > https://github.com/aglitke/vdsm-rest/
> > > >
> > > > Today I am releasing a proof of concept implementation of a REST API
> > > > for vdsm.
> > > > You can find the code on github. My goal is to eventually replace the
> > > > current
> > > > xmlrpc interface with a REST API. Once completed, ovirt-engine could
> > > > switch to
> > > > this new API. The major advantages to making this change are: 1) VDSM
> > > > will gain
> > > > a structured API that conceptually, structurally, and functionally
> > > > aligns with
> > > > the ovirt-engine REST API, 2) this new API can be made public, thus
> > > > providing an
> > > > entry point for direct virtualization management@the node level.
> > >
> > > Adam, this looks like a nice PoC. I didn't see how API versioning is
> > > handled. Any VDSM developers willing to review this work?
> > Thanks for taking a look. I am not handling versioning yet. I think we
> > can add
> > a version field to the root API object. As for compatibility, we'll just
> > have
> > to decide on an API backwards-compat support policy. Would this be enough
> > to
> > handle versioning issues? We shouldn't need anything like capabilities
> > because
> > the API is discoverable.
> Right, that seems sensible.
> Did you find cases where RPC to REST resource mapping was difficult?
I haven't yet fully implemented the current vdsm API but I suspect that certain
calls (like the ones you mention below) will require some extensions to what I
have available currently. The main missing piece is probably events and a nice
polling API. Another big piece of work will be to rebase onto the newly
redesigned storage model.
> I could see something like migrate() plus migrateStatus() and
> migrateCancel() needing to add some kind of operational state that to the
> resource. And something like monitorCommand() which has both a possible
> side-effect and some freefrom response...
Hopefully monitorCommand will not be too bad, since vdsm should be asking
libvirt for the VM details when they are needed. Of course we'll need to be
testing to make sure we aren't keeping state around. Also, I would expect
monitorCommand to 'taint' the VM in the eyes of the vdsm API (as it does for
> > > <snip>
> > > > ovirt-engine wants to subscribe to asynchronous events. REST APIs do
> > > > not
> > > > typically support async events and instead rely on polling of
> > > > resources. I am
> > > > investigating what options are available for supporting async events
> > > > via REST.
> > >
> > > I think typical is either polling or long polling. If it's a single
> > > resource, then perhaps long polling would be fine (won't be a large
> > > number of connections tied up if it's only a single resource).
> > Not sure if this is what you are referring to, but I was thinking we could
> > do a
> > batch-polling mechanism where an API user passes in a list of task UUIDs
> > and/or
> > event URIs. The server would respond with the status of these resources in
> > one
> > response. I have some ideas on how we could wire up asynchronous events on
> > the
> > server side to reduce the amount of actual work that such a batch-polling
> > operation would require.
> Oh, I just meant this:
> Polling (GET /event + 404 loop)
> Long polling (GET + block ... can chew up a thread connection)
Yep. And we can talk later about building an API for efficient, repeated
polling. I wonder if the ovirt-engine guys could weigh in as to whether a REST
interface with event polling would be acceptable to them. It is critical that
we settle on an API that can become _the_ first-class vehicle for interacting
Adam Litke <a...@us.ibm.com>
IBM Linux Technology Center
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