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

Andy Bierman <andy@yumaworks.com> Mon, 23 July 2018 14:08 UTC

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From: Andy Bierman <andy@yumaworks.com>
Date: Mon, 23 Jul 2018 07:08:03 -0700
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To: Robert Wilton <rwilton@cisco.com>
Cc: Christian Hopps <chopps@chopps.org>, NetMod WG <netmod@ietf.org>
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Subject: Re: [netmod] backward compatibility requirements in draft-verdt-netmod-yang-versioning-reqs-00
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On Mon, Jul 23, 2018 at 6:24 AM, Robert Wilton <rwilton@cisco.com>; wrote:

>
>
> On 23/07/2018 12:54, Andy Bierman wrote:
>
>
>
> On Mon, Jul 23, 2018 at 2:50 AM, Robert Wilton <rwilton@cisco.com>; wrote:
>
>> Hi Chris, Andy,
>>
>>
>> On 21/07/2018 17:00, Christian Hopps 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.
>>>
>> Not sure that I agree.
>>
>> I think that you have made an assumption here that the server will
>> continue to support both old and new major revision (with different name)
>> of the module at the same time.  However, these is nothing in the existing
>> YANG upgrade rules that requires that.
>>
>> Ultimately, there is a choice whether supporting older module versions is
>> the servers problem or the clients problem, or perhaps a combination of the
>> two.
>>
>> The aim of requirement 3.1 is to ensure that there is a standard
>> mechanism available so that server implementations that want the
>> flexibility of supporting older client versions have a standard way of
>> doing so.  My intention is that this part of the solution would be optional
>> to implement and hence decided by the market, which is why the text in the
>> requirement is "to allow servers" rather than "to require servers".
>>
>>
>
> API versioning is usually done on the message exchanges.
> Trying to do the same for datastore contents is not going to work.
>
> A YANG schema can be considered an API.  Particularly looking at say the
> OpenConfig YANG schema.  I doubt all implementations will store all their
> configuration and operational state in a central place.
>
> You can write the word MUST in all caps as many times as you want,
> but that will not change anything.
>
>
> I disagree.  Marking a requirement as a MUST means that requirement has to
> be met for a solution to be considered viable.
>
> I previously had some text in the draft explaining how RFC 2119 text
> translates to evaluating the requirements, but was asked to take it out
> because it is obvious.  Perhaps it should go back in ...
>
>
So you can answer the question how YANG validation works when multiple
revisions of a
module are implemented?  I do not doubt that you can find a leaf somewhere
that can
be changed.  Not at all convinced the validation rules can be rewritten to
account
for actual incompatible changes, like changing the type of data node or
replacing nodes
with completely different configuration.  Even less convinced that this
complexity is
worth the cost.



Thanks,
> Rob
>
>
Andy


>
>
>> Thanks,
>> Rob
>>
>
> Andy
>
>
>>
>>
>>
>>> 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.
>>>
>>> Thanks,
>>> Chris.
>>>
>>> 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
>>>>
>>>
>>> _______________________________________________
>>> netmod mailing list
>>> netmod@ietf.org
>>> https://www.ietf.org/mailman/listinfo/netmod
>>> .
>>>
>>>
>>
>
>