Re: [netmod] backward compatibility requirements in draft-verdt-netmod-yang-versioning-reqs-00

Christian Hopps <chopps@chopps.org> Sat, 21 July 2018 21:47 UTC

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From: Christian Hopps <chopps@chopps.org>
To: Andy Bierman <andy@yumaworks.com>
Cc: Christian Hopps <chopps@chopps.org>, NetMod WG <netmod@ietf.org>
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Date: Sat, 21 Jul 2018 17:47:45 -0400
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Subject: Re: [netmod] backward compatibility requirements in draft-verdt-netmod-yang-versioning-reqs-00
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Perhaps you are mistaking my intention here? I want a simple solution. I advocated for *not* introducing a new major version component. I don't want models changing all the time. I don't believe there is a large need for incompatible changes.

There are actual instances where small perhaps non-disruptive but incompatible changes are required. The example given to me for this type of change was when the original specification had an obvious bug (e.g., a range was incorrectly specified). I think for these there are non-standard solutions (e.g., a server could be configured to map a set (or all) clients to new module name/namespace if the operator/user decides its safe to do so). Naming conventions (e.g., a version suffix) can identify lineage to earlier "major" versions of a module as well.

Joe Clark mentioned one thing to me that I do think is missing and needed in YANG and that is some form of a "import after" so that module designers/users have the ability to select a version of the module that includes additions that have shown up in later revisions. I believe this can utilize the current revision (date) semantics.

It seems one place seeing a lot of "major" changes is with vendor models. I think internal development processes are driving these, and the vendors need to fix these processes, and not just make it easier to push these changes out to the operators/users. I expressed this multiple times to the design team this last week.

Thanks,
Chris.

Andy Bierman <andy@yumaworks.com>; writes:

> On Sat, Jul 21, 2018 at 9:00 AM, Christian Hopps <chopps@chopps.org>; wrote:
>
>> As I pointed out at the mic @102 this requirement derives directly from
>> the 1.x requirement of not changing the name of the module/namespace. If
>> you allow for changing the namespace/module name for "major" (i.e.,
>> incompatible) changes (i.e., like today) then this 3.1 requirement goes
>> away.
>>
>> I think the plan is to reword some of these to get closer to the intention
>> which I believe is to allow for smoother transition from one module to the
>> next while making incompatible but mostly non-impacting changes.
>>
>>
> You may find that reality interferes with your requirement.
> The term "incompatible" should be a clue.  If the change is genuinely
> incompatible
> then the underlying system change may be incompatible as well.
> Passive operations like <get> are not a problem, but configuration
> datastores
> are a problem.  There are good reasons that RFC 7950 says only 1 revision
> of a module can be advertised by a server.
>
> There is already a practical solution that is available today.
> A vendor can implement the server in a way that allows their customer to
> select the appropriate revision for their use-cases and tools environment.
>
> So how does YANG validation work?
> There is only 1 instance of <intended>.
> E.g., i there are 3 versions of module A and 2 versions of module B,
> then which versions does module C use for XPath validation that reference
> modules A and B?  Is a new version of XPath needed for YANG?
>
> It is possible to make a standard so complicated that nobody implements it.
>
>
> Thanks,
>> Chris.
>>
>>
> Andy
>
>
>> Andy Bierman <andy@yumaworks.com>; writes:
>>
>> Hi,
>>>
>>> I strongly object to requirement 3.1:
>>>
>>>
>>>     3.1  The solution MUST provide a mechanism to allow servers to
>>>             support existing clients in a backward compatible way.
>>>
>>>
>>>
>>> This is not what servers do today at all.
>>> They provide only one version of an implemented module, as specified in
>>> RFC
>>> 7950.
>>>
>>> It is a vendor and operator decision when to upgrade a server such that
>>> non-backward compatible changes are made. They must decide if/when it is
>>> ok
>>> based on the client applications in use.
>>>
>>> This requirement says you cannot make backward-incompatible changes
>>> which completely contradicts requirements 1.1 and 1.2.
>>>
>>> IMO requirement 3.1 should be removed, or change MUST to MAY
>>>
>>>
>>> Andy
>>> _______________________________________________
>>> netmod mailing list
>>> netmod@ietf.org
>>> https://www.ietf.org/mailman/listinfo/netmod
>>>
>>
>>