[Engine-devel] Announcing a proof of concept REST API for VDSM

Wed Nov 30 09:29:23 UTC 2011

On Wed, Nov 30, 2011 at 09:14:16AM +0200, Itamar Heim wrote:
> On 11/29/2011 11:36 PM, Adam Litke wrote:
> > On Tue, Nov 29, 2011 at 12:54:44PM -0800, Chris Wright wrote:
> >> * Adam Litke (agl at us.ibm.com) wrote:
> >>> On Tue, Nov 29, 2011 at 11:39:08AM -0800, Chris Wright wrote:
> >>>> * Adam Litke (agl at 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 at 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
> > libvirt).
> >
> >>>> <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
> 
> cc-ing engine-devel...
> 
> > 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
> > with vdsm.
> 
> i have two points for consideration around this:
> 1. as the api between ovirt-engine and vdsm, I had a preference for the 
> bus like nature of QMF, as it would allow multiple ovirt-engine to load 
> balance handling of messages from the queue, and multiple consumers for 
> some messages (say, history service picking up the stats in parallel to 
> engine picking them, rather than copying them from engine).

I tend to agree, using a bus like QMF between ovirt-engine and vdsm is
an inherantly more scalable network architecture, since it avoids the
need to have a direct point-to-point connection between the engine and
every single node, instead you can build a resilient grid by stategically
deploying QPid brokers.

As compared to REST, it is also much better at coping with async events,
since you can have a push, rather than pull, model which VDSM just puts
events onto the bus as they're generated, to be lazily consumed by any
remote nodes when desired.

NB, I don't mean to imply that there should not *also* be a REST API for
the node level. A REST API has the very compelling property that it is
trivially accessible from anything with HTTP client support, which is
basically everything in existance today.

> 2. as node level api, i think a lightweight ovirt-engine managing a 
> single node and exposing the exact same REST API and behavior of the 
> multi-node ovirt engine would be easier to cosnume for someone that 
> wants to interact with a single node same way they would interact with 
> ovirt-engine.

If the REST API is well specified, you wouldn't need to necessarily have
a lightweight ovirt-engine deployed at the node level, you could just
have VDSM implement the same REST specification natively, as is impl
in the engine. Or perhaps provide a some shered code that can be plugged
into both VDSM & the enegine to facilitate provision of the same REST
interface.

Regards,
Daniel
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