On Wed, Nov 30, 2011 at 12:13:24PM +0200, Yaniv Kaul wrote:
> On 11/30/2011 11:41 AM, Daniel P. Berrange wrote:
> >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 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...
> >>
> >>>><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)
> >Yes, this is what I was inclined to suggest too. Polling is such a
> >misleading description :-P
> >
> >This isn't really an either/or choice - you could trivially just
> >pass a query parmeter to /event to indicate whether you want an
> >immediate  404 reply, or to wait for 'N' seconds, or infinity
> >There's no reason it needs to consume a thread in VDSM permanently,
> >though I don't think that's a huge problem even if it does keep a
> >permanent thread open, since Linux threading is so lightweight.
> >
> >In more detail, I'd was going to suggest
> >
> >   1. One or more location for registering/unregistering interest
> >      in events in various different resources/objects
> >
> >         /some/path/register-event?foo=bar
> >         /some/path/unregister-event?foo=bar
> >         /other/path/register-event?eek=oooh
> >
> >   2. A common location for receiving all events
> >
> >          /event?wait=NNNN   (NNN = 0 =>  immediate 404,
> >                              NNN>= 0 =>  wait time,
> >                              omitted parameter =>  inifinity)
> >
> >   3. Added  bonus points to either
> >
> >        - Support persistent keepalive connections so that
> >          you can just send further GET /event requests over
> >          the same socket FD after receving each reply
> >
> >        - Used HTTP chunked encoding. Each event is sent back
> >          as a separate chunk. The server can just carry
> >          on sending you chunks forever as new events occur,
> >          so the HTTP reply is in effect infinitely long.
>              - the server would respond with the event status in a
> 100-continue response, thus 'lingering' the response for as much as
> the client needs. Less overhead than chunks, just as problematic
> with crappy proxies that do not properly support either chunking or
> 100-continue properly (but should not be an issue if it's within
> SSL).

In this hostile world, I think it is reasonable for us to consider HTTPS
to be the only serious option for production deployments, and discount
plain HTTP for anything except adhoc dev/test usage. So I say we don't
have to worry abvout broken proxies here.

Regards,
Daniel
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