Re: [netmod] New Version Notification for draft-verdt-netmod-yang-versioning-reqs-01.txt

Robert Wilton <> Thu, 25 October 2018 16:29 UTC

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To: Christian Hopps <>
Cc: Joe Clarke <>, "" <>, "" <>
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From: Robert Wilton <>
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Date: Thu, 25 Oct 2018 17:29:37 +0100
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Subject: Re: [netmod] New Version Notification for draft-verdt-netmod-yang-versioning-reqs-01.txt
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Hi Chris,

I think that there are two things driving this requirement:

What I regard as the key one, is that we want to be able to support the 
software that we have shipped.  In particular, we may need to fix bugs 
(perhaps at the operators request) to a YANG model that has already been 
released.  I.e. I think that there are some scenarios, where forking a 
YANG module, although undesirable is the right thing to do to include a 
fix.  I don't believe that features or deviations help solve this problem.

The two alternative solutions to being able to fix bugs, neither of 
which I think is pragmatic, that I can think of are:
(i) Vendors ensure that their YANG modules are perfect before they ship 
in a release.
(ii) If a bug is reported, operators are happy to wait until the bug has 
been fixed in the current development release, and will migrate to that 
latest release to pick up the fix.

The second thing driving this requirement is that vendors sometimes get 
asked for enhancements to existing releases, perhaps because the latest 
development release is too far out, or ask for an enhancement on the 
current train to be back ported to an older release.

So, aiming to have stable YANG modules, trying a lot harder to avoid 
non-backwards-compatible changes, and keeping new functionality to the 
head of the development I completely agree with you on.  But I still 
believe that there are some valid scenarios, that should be limited as 
much as possible, where it is necessary to make changes that sometimes 
break these rules, and having a limited scheme that clearly indicates 
where such breakages have occurred is probably better that the status 
quo of where the modules get changed, but the operator doesn't get any 
useful indication of what type of changes are being made.


On 25/10/2018 16:26, Christian Hopps wrote:
>> On Oct 20, 2018, at 1:55 PM, Joe Clarke <> wrote:
>> * New requirement 1.4 for supporting over-arching software releases
> [ I read this as supporting various different module versions based on a vendor's different software release trains. If this is wrong then the rest of this doesn't apply and I would just ask for the text to be update to clarify what it means. ]
> How many operators/users have asked for this or indicated it's a requirement for them?
> What problem is intractable without this requirement being met, and what is the cost of this requirement on the actual users?
> I have pushed back multiple times on this b/c I believe this "requirement" is really being pushed to make it easier for vendors (a small affected group) to develop their software at the cost of their users (the much larger affected group) who would then have to deal with multiple trains of the same module.
> Here is what I am afraid the vendors want here: A developer on release train X can easily change some data structure and then push the change into an automated system which generates a new YANG module definition and revs a version number -- all done! They don't have to deal with the inertia of making this change in their release train Y or Z and they don't have to treat modules as a stable API they are exporting, b/c they now have these new wonderful versions from this work. Meanwhile we the users now have to deal with N forks with all the various little incompatible changes random developers at the company wanted to make without having to coordinate with their coworkers/other internal teams. Now multiply this by M vendors. It's a nightmare. It shouldn't be what we are optimizing for, let alone making a requirement.
> We already have features and deviations why are they not enough to deal with functionality that is present or not in various software release/devices?
> FWIW I'm not against making it easier to develop software, but we have to be mindful if we are just pushing the cost (and magnifying it greatly) to other people in the community.
> Thanks,
> Chris.
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