Joe Clarke <jcla...@cisco.com> writes:
> On 11/15/17 00:30, Juergen Schoenwaelder wrote:
>> Another thing to consider is that foo and foo2 allows an
>> implementation to support both during transition, with foo {semver
>> 1.x.y} and foo {semver 2.x.y} this may be harder.
>
> I'm not convinced this a bad thing. If a server supports multiple
> versions of a given module, which should a client use? Did the server
> vendor test each one?
>
> I suppose my gut reaction to Lou's question as to whether a server
> should support multiple versions was, "no." A client may have multiple
> versions loaded to support servers that support different versions. I
> may be convinced otherwise, but I feel that this will become untenable
> over time (even if module names change).
There are use cases for modules that are imported (i.e. not
implemented): it could be that a module author wants to use some
definitions from an old version of an imported module while, at the same
time, other definitions from a new version.
The semver-aware "import" statement should be able to deal with this.
Lada
>
> Joe
>
>>
>> /js
>>
>> On Tue, Nov 14, 2017 at 10:22:10PM +0100, Juergen Schoenwaelder wrote:
>>> On Wed, Nov 15, 2017 at 12:51:22AM +0800, Balazs Lengyel wrote:
>>>> Whenever a client OSS implements some higher level logic for a network
>>>> function, something that can not be implemented in a purely model driven
>>>> way, it is always dependent on a specific version of the Yang Module
>>>> (YAM). If the client finds that the module has been updated on the
>>>> network
>>>> node, it has to decide if it tries to handle it as it did the previous
>>>> version of the model or if it just stops to avoid problems. To make this
>>>> decision the client needs to know if the module was updated in a
>>>> backward
>>>> compatible way or not. This is not addressed with the current
>>>> versioning.
>>>
>>> The current rules aim at guaranteeing that definitions (with status
>>> current) remain backwards compatible. Do you have an example what the
>>> current rules fail to achieve this? Definitions with status deprecated
>>> or obsolete may not be present. But if they are present, they have the
>>> same semantics. This is the promise made to a client. (Note also that
>>> objects may be absent for reasons document in deviations or simply not
>>> accessible due to access control.)
>>>
>>>> While having PYANG based checks for backward compatibility is a very
>>>> good
>>>> idea, a comparison based check will never be a complete check. It is
>>>> quite possible to change just the behavior of an rpc/action/etc.
>>>> without
>>>> changing the YANG definition. This will only show up as a change of the
>>>> description statement that can not be analyzed by PYANG.
>>>
>>> The problem is to decide whether a change can break client
>>> expectations or not. Even 'bug fixes' can cause a client written to
>>> expect the old 'buggy' behaviour to fail. Also tricky are situations
>>> where behaviour was not clearly enough described and this is 'fixed'
>>> in a module update.
>>>
>>> Semantic versioning assumes that one always can clearly distinguish
>>> between incompatible updates and compatible updates. This may not be
>>> so clearly cut in practice, see above. (But then, we have the same
>>> judgement call at the end with today's update rules.)
>>>
>>>> When upgrading a network node we might introduce non-backward compatible
>>>> (NBC) changes. Today we need to introduce a new module for this. That
>>>> means during the upgrade process the node must convert stored
>>>> configuration instance data from ietf-routing to ietf-routing-2 format.
>>>> Instead of solving this data transformation/transfer problem just for a
>>>> few NBC data nodes, we will have to do it for the full model. This is
>>>> complicated. In many cases the transformation of a few NBC leafs can be
>>>> handled by good defaults or with a small script. Transferring the full
>>>> data set is more complicated. If we allow NBC updates in some cases this
>>>> problem is avoided.
>>>
>>> In XML land, this is mostly a change of the namespace (not of the
>>> prefix) if one keeps the same structure, no? In JSON land, the change
>>> of the module name more directly becomes visible in instance data; but
>>> this is all encoding details.
>>>
>>>> If we update the module from ietf-routing to ietf-routing-2 ? Do we keep
>>>> the prefix?
>>>
>>> I guess you mean the namespace, not the prefix. You can use any prefix
>>> you like.
>>>
>>>> In one sense it should be kept as it is the same module
>>>> "logically"; we also might have stored data including the prefix
>>>> (identityrefs, instance-identifiers). On the other hand having multiple
>>>> modules with the same prefix is a problem. The only good solution is to
>>>> allow incompatible updates in some cases.
>>>
>>> If we move towards allowing incompabile updates, then we need to have
>>> a mechanism to tell which versions of modules can work together and
>>> which combinations are affected by an incompatible update. We probably
>>> need to require strict import by revision or at least 'import by
>>> compatible revision' (whatever this means at the end).
>>>
>>>> CH 1)
>>>>
>>>> You write
>>>> "The YANG data modeling language [RFC7950] specifies strict rules for
>>>> updating..."
>>>> and again
>>>> "When the same YANG module name is kept, the new YANG module revision
>>>> must always be updated in a backward-compatible way."
>>>>
>>>> I strongly disagree. While we have strict rules about even small
>>>> modifications to existing schema, but you are allowed to
>>>> deprecate/obsolete big parts of the model, thereby possibly deleting
>>>> complete subtrees from the schema. That is anything but strict backward
>>>> compatibility.
>>>> I find this aspect of YANG inconsistent to the level that it would need
>>>> an
>>>> errata.
>>>
>>> Marking something deprecated / obsolete means you can not be sure this
>>> is implemented. But then, even definitions with status current may not
>>> be implemented (see deviations) or they may not be accessible to a
>>> client due to access control. However, if implemented and accessible,
>>> the guarantee today is that the semantics stay the same and don't
>>> change unexpectedly.
>>>
>>>> So practically the current rules allow backward incompatible changes
>>>> that
>>>> can only be detected by a line by line comparison of the yang modules.
>>>> In
>>>> a system with semantic versioning, you could determine backward
>>>> compatibility just by reading the version numbers.
>>>
>>> I do not see why you need a line by line comparison. With semantic
>>> versioning, you _hope_ the semantic version number is a good enough
>>> indicator. It might also be that your client is only using a subset
>>> that did not really change even though the semantic version number
>>> changed. Or the semantic version number indicates only minor changes
>>> that sill break your client.
>>>
>>>> CH 2.3)
>>>> As we need to create a new Yang Module (YAM) even for the smallest
>>>> incompatible modification, this increases the number of modules.
>>>
>>> So it seems to boil down to the question whether foo and foo2 is
>>> significantly more expensive than foo { semver 1.x.y } and foo {
>>> semver 2.x.y }. The main argument seems to be that the later keeps
>>> references that involve module names or namespaces unchanged (but
>>> they may or may not mean different things).
>>>
>>>> IMHO YANG package definition should be a separate issue, left out of
>>>> this
>>>> document. Andy has already provided some very good ideas about this
>>>> topic.
>>>
>>> I think it is necessary to think about how the semantic version
>>> numbers are used. See my remark above about imports. If we allow
>>> incompatible changes, than this has side effects and I think we are
>>> not done by just adding a semantic version number without going
>>> working throught the implications.
>>>
>>> /js
>>>
>>> --
>>> Juergen Schoenwaelder Jacobs University Bremen gGmbH
>>> Phone: +49 421 200 3587 Campus Ring 1 | 28759 Bremen | Germany
>>> Fax: +49 421 200 3103 <http://www.jacobs-university.de/>
>>
>
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