[Engine-devel] Proposed next-generation vdsm API

[Ayal Baron]

----- Original Message -----
> On Thu, Dec 01, 2011 at 02:02:27AM -0500, Ayal Baron wrote:
> > 
> > 
> > ----- Original Message -----
> > > 
> > > 
> > > > -----Original Message-----
> > > > From: Adam Litke [mailto:agl at us.ibm.com]
> > > > Sent: Thursday, December 01, 2011 0:41 AM
> > > > To: vdsm-devel at lists.fedorahosted.org; engine-devel at ovirt.org
> > > > Cc: Daniel P. Berrange; Chris Wright; Dan Kenigsberg; Itamar
> > > > Heim
> > > > Subject: Proposed next-generation vdsm API
> > > > 
> > > > Recently we've had some very productive discussions concerning
> > > > the
> > > > VDSM
> > > API.  I
> > > > want to attempt to refocus the discussion around an emerging
> > > > proposal
> > > and see if
> > > > we can agree on a sensible path forward.
> > > > 
> > > > Based on the discussion, I have identified the following
> > > > requirements
> > > that
> > > > a new API for vdsm should have:
> > > > 
> > > > 1.) Single API that can be consumed by ovirt-engine and ISVs
> > > >  - We don't want to maintain multiple parallel APIs
> > > >  - To develop a vendor ecosystem, we must have a robust
> > > >  external
> > > >  API to
> > > >    vdsm
> > > > 
> > > > 2.) Full vdsm capabilities are exposed without requiring
> > > > ovirt-engine
> > > >  - ovirt components should be modular and independently useful
> > > >  - Some deployments might want to manage nodes without
> > > >  ovirt-engine
> > > > 
> > > > 3.) Standardized protocol with low overhead
> > > >  - Required for widespread adoption
> > > > 
> > > > 4.) Support for asynchronous tasks and events
> > > >  - Needed by ovirt-engine and other consumers
> > > > 
> > > > Based on these requirements, the following proposal has started
> > > > to
> > > emerge:
> > > > 
> > > > Create a REST API that will provide all of the functionality
> > > > that
> > > > is
> > > currently
> > > > available via the xmlrpc interface (with the goal of
> > > > deprecating
> > > > xmlrpc
> > > once it
> > > > becomes mature enough).  To support advanced clustering
> > > > features
> > > > that
> > > > ovirt-engine is planning, we'll write an QMF broker that can
> > > > proxy
> > > > the
> > > REST API
> > > > onto a message bus.  ovirt-engine will interact with vdsm
> > > > exclusively
> > > over this
> > > > bus but the REST API will be the principle API and the entry
> > > > point
> > > > for
> > > ISV apps.
> > > > A REST API provides a light-weight and standard way to access
> > > > all
> > > > of the
> > > vdsm
> > > > functionality.
> > > > 
> > > > The REST API will handle events by exposing a new 'events'
> > > > collection at
> > > the api
> > > > root.  REST users will use some sort of polling to collect
> > > > these
> > > > events.
> > > The
> > > > details of this interface are being worked on.  Several ways
> > > > for
> > > minimizing the
> > > > impact of polling have been discussed.  The QMF broker can
> > > > expose a
> > > > publish/subscribe model for events as appropriate.
> > > > 
> > > > Is this model an acceptable way to improve the vdsm API?  I
> > > > would
> > > > like
> > > to hear
> > > > the opinions of ovirt-engine developers, vdsm developers, and
> > > > other
> > > > stakeholders.  Thanks for providing feedback on this proposal!
> > > 
> > > Why things non native to REST and wrap it in QMF, rather than do
> > > the
> > > reverse?
> > > Or just to them in parallel, since it sounds like both are going
> > > to
> > > be
> > > first class citizens?
> > 
> > This was more my understanding from our discussion on IRC
> > yesterday.
> 
> I'm afraid I did not follow that discussion to your conclusions...
> 
> > REST API - everything that is relevant for single node management
> > QMF - same API as above + multi-node relevant API calls.  I don't
> > see any
> > reason for doing weird things over REST to support the latter.
> 
> A QMF broker runs on the vdsm host and talks to the REST API.  It
> connects to a
> bus and exposes an API to ovirt-engine on this bus using a vdsm-base
> schema.
> ovirt-engine wants additional clustering functionality.  This API
> should be
> implemented completely between the QMF broker and ovirt-engine using
> a separate
> vdsm-cluster schema.
> 
> > In fact, I don't even see any real reason for going through the
> > REST API when
> > using QMF.
> 
> Because we want to avoid the proliferation of APIs.  I would prefer a
> mostly
> vertical chain of API components to a vdsm with several independent
> APIs (which
> are sure to diverge or be neglected by individual
> developers/patches).
> 
> > If you take a look at today's API you will see that there is
> > nothing there
> > that limits it to XML-RPC and we could easily expose all the calls
> > using REST
> > or anything else.  In python, exposing a new verb in the various
> > APIs can be
> > automatic so this would require very little maintenance.  Any
> > multi-node or
> > transport specific calls can be decorated as such and would be
> > automatically
> > ignored/picked up by the relevant API layer.  This way, we could
> > also easily
> > enable using different bus protocols assuming a customer already
> > has a
> > deployment as was suggested yesterday.
> 
> I don't think this will be as automatic as you suggest in practice.
>  It sounds
> like it will increase code complexity, obfuscation, and maintenance
> burden.

Even today we don't really have any reliance on the transport.
The beauty of REST is its simplicity, but with that come limitations. If we require the qmf API to go through REST then we're basically limiting the qmf to the REST shortcomings or we implement things that bend REST the wrong way to accomodate the extra requirements.
For any "simple" call I'm fine with qmf going through REST if it simplifies maintenance, but for things like events, I believe there is no justification for going through REST.

> 
> --
> Adam Litke <agl at us.ibm.com>
> IBM Linux Technology Center
> 
>