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

On Wed, Nov 30, 2011 at 03:21:07PM +0200, Itamar Heim wrote: > On 11/30/2011 11:29 AM, Daniel P. Berrange wrote: >> 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(a)us.ibm.com) wrote: >>>>>> On Tue, Nov 29, 2011 at 11:39:08AM -0800, Chris Wright wrote: >>>>>>> * Adam Litke (agl(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 >>>> 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. > > Indeed, but an API is not just the transport, it is also behavior > and error code numbering, etc. > trying to develop two different projects trying to do the same thing > sounds like quite the overhead to me. Agreed. > I prefer investing the time in making a lighter version of the > engine for a single node. > then you would also get the web admin and power user portal if you > want them. Could you flesh out some details on the effort required to implement a light-weight engine? I am concerned that this is more complex than it seems. Also, how would we avoid dual maintenance between the full engine and the light one when a new feature/api is added?