On Fri, Jul 06, 2012 at 03:53:08PM +0300, Itamar Heim wrote:
> On 07/06/2012 01:15 AM, Robert Middleswarth wrote:
> >On 07/05/2012 04:45 PM, Adam Litke wrote:
> >>On Thu, Jul 05, 2012 at 03:47:42PM -0400, Saggi Mizrahi wrote:
> >>>
> >>>----- Original Message -----
> >>>>From: "Adam Litke" <a...@us.ibm.com>
> >>>>To: "Saggi Mizrahi" <smizr...@redhat.com>
> >>>>Cc: "Anthony Liguori" <anth...@codemonkey.ws>, "VDSM Project
> >>>>Development" <vdsm-devel@lists.fedorahosted.org>
> >>>>Sent: Thursday, July 5, 2012 2:34:50 PM
> >>>>Subject: Re: [RFC] An alternative way to provide a supported
> >>>>interface -- libvdsm
> >>>>
> >>>>On Wed, Jun 27, 2012 at 02:50:02PM -0400, Saggi Mizrahi wrote:
> >>>>>The idea of having a supported C API was something I was thinking
> >>>>>about doing
> >>>>>(But I'd rather use gobject introspection and not schema
> >>>>>generation) But the
> >>>>>problem is not having a C API is using the current XML RPC API as
> >>>>>it's base
> >>>>I want to disect this a bit to find out exactly where there might be
> >>>>agreement
> >>>>and disagreement.
> >>>>
> >>>>C API is a good thing to implement - Agreed.
> >>>>
> >>>>I also want to use gobject introspection but I don't agree that using
> >>>>glib
> >>>>precludes the use of a formalized schema.  My proposal is that we
> >>>>write a schema
> >>>>definition and generate the glib C code from that schema.
> >>>>
> >>>>I agree that the _current_ xmlrpc API makes a pretty bad base from
> >>>>which to
> >>>>start a supportable API.  XMLRPC is a perfectly reasonable
> >>>>remote/wire protocol
> >>>>and I think we should continue using it as a base for the next
> >>>>generation API.
> >>>>Using a schema will ensure that the new API is well-structured.
> >>>There major problems with XML-RPC (and to some extent with REST as
> >>>well) are high call overhead and no two way communication (push
> >>>events). Basing on XML-RPC means that we will never be able to solve
> >>>these issues.
> >>I am not sure I am ready to conceed that XML-RPC is too slow for our
> >>needs.  Can
> >>you provide some more detail around this point and possibly suggest an
> >>alternative that has even lower overhead without sacrificing the
> >>ubiquity and
> >>usability of XML-RPC?  As far as the two-way communication point, what
> >>are the
> >>options besides AMQP/ZeroMQ?  Aren't these even worse from an overhead
> >>perspective than XML-RPC?  Regarding two-way communication: you can
> >>write AMQP
> >>brokers based on the C API and run one on each vdsm host.  Assuming
> >>the C API
> >>supports events, what else would you need?
> >I personally think that using something like AMQP for inter-node
> >communication and engine - node would be optimal.  With a rest interface
> >that just send messages though something like AMQP.
> I would also not dismiss AMQP so soon
> we want a bug with more than a single listener at engine side
> (engine, history db, maybe event correlation service).
> collectd as a means for statistics already supports it as well.
> I'm for having REST as well, but not sure as main one for a consumer
> like ovirt engine.

I agree that a message bus could be a very useful model of communication between
ovirt-engine components and multiple vdsm instances.  But the complexities and
dependencies of AMQP do not make it suitable for use as a low-level API.  AMQP
will repel new adopters.  Why not establish a libvdsm that is more minimalist
and can be easily used by everyone?  Then AMQP brokers can be built on top of
the stable API with ease.  All AMQP should require of the low-level API are
standard function calls and an events mechanism.

> >
> >Thanks
> >Robert
> >>>>>The current XML-RPC API contains a lot of decencies and
> >>>>>inefficiencies and we
> >>>>>would like to retire it as soon as we possibly can. Engine would
> >>>>>like us to
> >>>>>move to a message based API and 3rd parties want something simple
> >>>>>like REST so
> >>>>>it looks like no one actually wants to use XML-RPC. Not even us.
> >>>>I am proposing that AMQP brokers and REST APIs could be written
> >>>>against the
> >>>>public API.  In fact, they need not even live in the vdsm tree
> >>>>anymore if that
> >>>>is what we choose.  Core vdsm would only be responsible for providing
> >>>>libvdsm
> >>>>and whatever language bindings we want to support.
> >>>If we take the libvdsm route, the only reason to even have a REST
> >>>bridge is only to support OSes other then Linux which is something
> >>>I'm not sure we care about at the moment.
> >>That might be true regarding the current in-tree implementation.
> >>However, I can
> >>almost guarantee that someone wanting to write a web GUI on top of
> >>standalone
> >>vdsm would want a REST API to talk to.  But libvdsm makes this use
> >>case of no
> >>concern to the core vdsm developers.
> >>
> >>>>>I do think that having C supportability in our API is a good idea,
> >>>>>but the
> >>>>>current API should not be used as the base.
> >>>>Let's _start_ with a schema document that describes today's API and
> >>>>then clean
> >>>>it up.  I think that will work better than starting from scratch.
> >>>>  Once my
> >>>>schema is written I will post it and we can 'patch' it as a community
> >>>>until we
> >>>>arrive at a 1.0 version we are all happy with.
> >>>+1
> >>Ok.  Redoubling my efforts to get this done.  Describing the output of
> >>list(True) takes awhile :)
> >>
> >
> >
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Adam Litke <a...@us.ibm.com>
IBM Linux Technology Center

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