Re: [netmod] Comment on draft-clacla-netmod-yang-model-update-02

[Juergen Schoenwaelder <[e-mail]>]

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: [phone]         Campus Ring 1 | 28759 Bremen | Germany
Fax:   [phone]         <http://www.jacobs-university.de/>