I don't think AMQP is a good low level supported protocol as it's a very 
complex protocol to set up and support.
Also brokers are known to have their differences in standard implementation 
which means supporting them all is a mess.

It looks like the most accepted route is the libvirt route of having a c 
library abstracting away client server communication and having more advanced 
consumers build protocol specific bridges that may have different support 
standards.

On a more personal note, I think brokerless messaging is the way to go in ovirt 
because, unlike traditional clustering, worker nodes are not interchangeable so 
direct communication is the way to go, rendering brokers pretty much useless.

----- Original Message -----
> From: "Adam Litke" <a...@us.ibm.com>
> To: "Itamar Heim" <ih...@redhat.com>
> Cc: vdsm-devel@lists.fedorahosted.org
> Sent: Monday, July 9, 2012 9:56:17 AM
> Subject: Re: [vdsm] [RFC] An alternative way to provide a supported interface 
> -- libvdsm
> 
> 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 :)
> > >>
> > >
> > >
> > >_______________________________________________
> > >vdsm-devel mailing list
> > >vdsm-devel@lists.fedorahosted.org
> > >https://fedorahosted.org/mailman/listinfo/vdsm-devel
> > 
> > 
> > _______________________________________________
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> > vdsm-devel@lists.fedorahosted.org
> > https://fedorahosted.org/mailman/listinfo/vdsm-devel
> 
> --
> Adam Litke <a...@us.ibm.com>
> IBM Linux Technology Center
> 
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> 
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