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

[Itamar Heim]

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 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.

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.
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.