I've opened a wiki page [1] for the stable API and extracted some of the "TODO" 
points so we don't forget. Everyone can feel free to add more stuff.

[1] http://ovirt.org/wiki/VDSM_Stable_API_Plan

Rest of the comments inline
----- Original Message -----
> From: "Adam Litke" <a...@us.ibm.com>
> To: "Saggi Mizrahi" <smizr...@redhat.com>
> Cc: "VDSM Project Development" <vdsm-devel@lists.fedorahosted.org>, "Barak 
> Azulay" <bazu...@redhat.com>, "Itamar
> Heim" <ih...@redhat.com>, "Ayal Baron" <aba...@redhat.com>, "Anthony Liguori" 
> <aligu...@redhat.com>
> Sent: Monday, June 18, 2012 12:23:10 PM
> Subject: Re: [virt-node] RFC: API Supportability
> On Mon, Jun 18, 2012 at 11:02:25AM -0400, Saggi Mizrahi wrote:
> > The first thing we need to decide is API supportabiliy. I'll list
> > the questions
> > that need to be answered. The decision made here will have great
> > effect on
> > transport selection (espscially API change process and versioning)
> > so try and
> > think about this without going to specfic technicalities (eg. "X
> > can't be done
> > on REST").
> Thanks for sending this out.  I will take a crack at these
> questions...
> I would like to pose an additional question to be answered:
> - Should API parameter and return value constraints be formally
> defined?
>   If so, how?
> Think of this as defining an API schema.  For example: When creating
> a VM, which
> parameters are required/optional?  What are the valid formats for
> specifying a
> VM disk?
>  What are all of the possible task states?
Has to be part of response to the call that retrieves the state. This will 
allow us to change the states in a BC manner.
>  Is there a
> maximum length
> for the storage domain description?
I totally agree, how depends on the transport of choice but in any case I think 
the definition should be done in a declarative manner (XML\JSON) using concrete 
types (important for binding with C\Java) and have some *code to enforce* that 
the input is correct. This will prevent clients from not adhering to the schema 
exploiting python's relative lax approach to types. We already had issues with 
the engine wrongly sending numbers as strings and having this break internally 
because of some change in the python code made it not handle the conversion 
very well.
> > New API acceptance process
> > ==========================
> >  - What is the process to suggest new API calls?
> New APIs should be proposed on a mailing list.  This allows everyone
> to
> participate in the conversation and preserves the comments.  If the
> API is
> simple, a patch the provides a concrete example of implementation is
> recommended.  Once the API design is agreed upon, patches can be
> submitted via
> the standard method (gerrit) to implement a new experimental API
> based on the
> design.  The submitter of the patches should reply to the design
> discussion
> thread to notify participants of the available code.
> >  - Who can ack such a change?
> API changes should be subject to wider approval than a simple change
> to an
> internal component.  I believe that the +1,-1 system works well here
> and we
> should seek approvals from all participants in the design discussion
> if
> possible.
I will add that you need a +1 from at least 2 maintainers for an API change.
Also someone has to test that the change did not break old clients.
> >  - Does someone have veto rights?
> Anyone can NACK an API design.  Same rules as for normal code.
> >  - Are there experimental APIs?
> Yes!  Dave Allan has mentioned that from his experience in libvirt,
> it would be
> very nice to have experimental APIs that can be improved before being
> baked into
> the supportable API.  I definitely agree.  In fact, all new APIs
> should go
> through a period of being experimental.  Experimental API functions
> should
> begin with '_'.  Once deemed stable, the '_' can be removed.
I don't like this specific mangling scheme but I do agree that we need 
experimental calls.
I also think that you need a mechanism to "turn them on" similar to `import 
__future__` in python so that you make sure API user knows it's experimental.
> > 
> > API deprecation process
> > =======================
> >  - Do we allow deprecation?
> I would like to allow deprecation because it grants us an avenue to
> clean up the
> API from time to time.  That being said, I am not aware of a clean
> way to do
> this without breaking old clients too badly.  At a minimum, an API
> would need to
> be deprecated for at least 2 years before it can be removed.  How
> will this
> decision influence the initial API design?  Are there features that
> we can build
> into an API that can ease the burden of deprecation on API consumers?
Deprecation is tricky. We also need a mechanism for a client to know that his 
version of the API no longer exists so it can check for that at host connection 
and fail if the client is to old.
To do that we could either have API group versions and expose which versions 
are supported in full.
We could also take the opengl route of querying for call. But this might be to 
fine grained.
> >  - When can an API call be deprecated?
> As a matter of last resort.  Deprecation must last for an agreed-upon
> minumum
> time period (for supportability) before an API may be removed.
> >  - Who can ack such a change?
> >  - Does someone have veto rights?
> Same as above.
> > 
> > API change process
> > ==================
> >  - Can calls be modified or no symbol can ever repeat in a
> >  different form
> I would suggest libvirt's model here.  APIs can be extended by
> passing flags to
> enable new behavior.  New parameters can be added to an API as long
> as they are
> optional.
And the default value preserves original behavior.
> >  - When can an API call be deprecated?
> See above.
> >  - Who can ack such a change?
> >  - Does someone have veto rights?
> Same as above.
> > API versioning
> > ==============
> >  - Is the API versioned as a whole, is it per subsystem (storage,
> >  networking,
> >    etc..) or is each call versioned by itself.
> The API should be versioned as a whole.  This version should be the
> same as the
> software release version.  Additionally, the API should support a
> list of
> capabilities that can be used to express features that can not be
> expressed by
> the presense of a function signature.  For example, the supported
> storage domain
> types.  The ovirt-engine rest API has a decent capabilities API.
What about plugins\extensions eg. gluster support. Reuse the capabilities 
> >  - What happens when old clients connects to a newer server.
> The API is append-only (with two exceptions) so there should never be
> a problem
> with this.
> Exceptions:
>  - Experimental functions may be missing
>  - Deprecated functions may have been removed
> >  - What happens when a new client connects to an older sever
I think new clients should check for version at start (similar to old client 
connecting to new server) and declare that interaction as not our problem. 
Might be tricky though if the host was updated mid conversation.
> This is a bit harder.  We need to handle the following cases:
> 1) Server wants to use an API call that may not exist on an old
> version.
> An attempt to call a non-existing function should result in a
> standardized error
> that the client can handle appropriately.  In such a case, a client
> that wants
> to support older servers would need to handle the error and perform
> the needed
> operation using an alternate method.  To ease this process for
> clients, our API
> should have a registry of allowed methods.  In REST this would be
> RSDL.  It
> could also be exported as a component of the capabilities API.
> 2) Server wants to supply unrecognized parameters to a function.
> Anytime a new parameter is added to a function, a corresponding flag
> should be
> specified to enable handling of that parameter.  In this way, an old
> server can
> return an error for 'Unknown flag'.
> Have a missed any cases?
> >  - How will versioning be expressed in the bindings?
> The API should have a call to return the overall version.  Also, the
> capabilities call should list all noteworthy features that are
> present.
If the API is versioned as a whole you could theoretically expose which range 
of versions you support.
> >  - Do we retrict newer clients from using old APIs when talking
> >  with a new
> >    server?
> No.  A new client that wants to be the most compatible across vdsm
> versions may
> choose to use an old API (even if a flashier one is available).
> --
> Adam Litke <a...@us.ibm.com>
> IBM Linux Technology Center
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