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

Andy Bierman <> Sat, 21 July 2018 16:25 UTC

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From: Andy Bierman <>
Date: Sat, 21 Jul 2018 09:25:19 -0700
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To: Christian Hopps <>
Cc: NetMod WG <>
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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 <>; 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
then the underlying system change may be incompatible as well.
Passive operations like <get> are not a problem, but configuration
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.

> Chris.

> Andy Bierman <>; 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