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:
>>>> Today I am releasing a proof of concept implementation of a REST API for
>>>> You can find the code on github. My goal is to eventually replace the
>>>> xmlrpc interface with a REST API. Once completed, ovirt-engine could
>>>> this new API. The major advantages to making this change are: 1) VDSM
>>>> a structured API that conceptually, structurally, and functionally aligns
>>>> the ovirt-engine REST API, 2) this new API can be made public, thus
>>>> 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
>> to decide on an API backwards-compat support policy. Would this be enough to
>> handle versioning issues? We shouldn't need anything like capabilities
>> 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
>>>> ovirt-engine wants to subscribe to asynchronous events. REST APIs do
>>>> typically support async events and instead rely on polling of resources.
>>>> investigating what options are available for supporting async events via
>>> 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
we settle on an API that can become _the_ first-class vehicle for interacting
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).
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