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

Andy Bierman <andy@yumaworks.com> Sat, 21 July 2018 16:25 UTC

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From: Andy Bierman <andy@yumaworks.com>
Date: Sat, 21 Jul 2018 09:25:19 -0700
Message-ID: <CABCOCHSkpn_=04qJP9m6TUA+doCjk0=BFG6jX9T4awj+CO-QdA@mail.gmail.com>
To: Christian Hopps <chopps@chopps.org>
Cc: 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 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
>>
>
>