> On 26 Aug 2016, at 14:38, Martin Bjorklund <[email protected]> wrote:
>
> Ladislav Lhotka <[email protected]> wrote:
>> Martin Bjorklund <[email protected]> writes:
>>
>>> Hi,
>>>
>>> [replying to the first post in this (old) thread; the thread got a bit
>>> off-topic]
>>>
>>> Balazs Lengyel <[email protected]> wrote:
>>>> Hello,
>>>>
>>>> As I understand it, Schema-mount today does not support an important
>>>> use-case which we definitely need, but others also indicated they
>>>> want.
>>>>
>>>> I want to specify off-line in design time which models will be mounted
>>>> where. Many of my nodes know in design-time what their model structure
>>>> will be, so I want a way to be able to document this in YANG.
>>>
>>> I'd like to understand this use case in more detail. You say that
>>> that a "node knows in design-time" that YANG models it will use. Thus
>>> it seems natural to be able to specify which other modules to mount in
>>> the YANG module itself.
>>
>> Did you have a chance to look into my slides from Berlin? I sketched some
>> solutions there.
>
> Well, that's a *solution*. I'd like to understand the *problem* first ;)
>
>>> The problem I see with this is that it is very limited to the use case
>>> where each implementation defines its own YANG modules. Suppose you
>>> have such a model, for example:
>>>
>>> module A {
>>> ...
>>> mount ... {
>>> mount-module B;
>>> mount-module C;
>>> }
>>> }
>>>
>>> [pseudo-code for module A that specifies that it mounts B and C]
>>>
>>> Now, suppose you take this module A to another implementation, and it
>>> needs to augment something into module C; essentially this means that
>>> it would like to mount B, C and new module D. This will not be
>>> possible unless it modifies module A.
>>
>> Not necessarily, D could contain an augment with a target specified by a
>> schema node identifier that uses nodes from both A and C.
>
> Not if B and D are defined as standalone modules, e.g. if B is
> ietf-interfaces and D is ietf-ip.
Right, we cannot have everything.
>
>> Imagine that instead of "ietf-ip" augmenting "ietf-interfaces" it would
>> be the other way around: "ietf-interfaces" mounts "ietf-ip". Then the
>> augment in ietf-ipv6-router-advertisements
>>
>> augment "/if:interfaces-state/if:interface/ip:ipv6" { ... }
>>
>> needn't be changed.
>
> This would be an entirely different kind of mount! The current mount
> doesn't work like that; it doesn't give you visibility into the
> combined models - by design.
Of course this is a different kind of mount.
>
>
>> The major problem with this IMO is that it needs a new YANG statement.
>>
>>>
>>>> In
>>>> today's proposal the only way to find the Yang-Mounts is to read it
>>>> from the live node.
>>>>
>>>> * OAM integrators or operators want to be able to write CLI scripts
>>>> and Netconf messages without accessing (expensive) real nodes. For
>>>> this they need to know the mounts
>>>> * We want to generate some fancy documentation from YANG automatically
>>>> in design-time.
>>>
>>> In a way this is no different from the situation today - in order to
>>> learn what YANG modules a device supports you need to connect and
>>> parse the <hello> message (or ietf-yang-library data in the near
>>> future). This info could also be made available off-line in a file.
>>
>> Yup. So we just need some machine-readable description of the structure
>> of mounted schemas.
>>
>>>
>>> It should certainly be possible to define a file format that a device
>>> somehow could "publish" off-line that contained all the info that is
>>> available at run-time. This file format could be exactly the same as
>>> you would get if you did a NETCONF <get/> operation with some filter
>>> to only retreive the ietf-yang-library data at the mount points.
>>
>> Well, if you have multiple levels of mounts, it will get messy: you
>> wouldn't know which yang-library data belongs to which mount point.
>
> Why not - imagine that you do a full <get/> on such a device, it will
> return the nested yang library data just fine.
Well, yes, if you are prepared to parse and process arbitrary data for which
you don't know the schema. I think the schema and instance data (= <get/>
reply) are different things and we should treat them so: the complete schema
description should IMO have a well-known (meta-)schema, even if we express it
with YANG.
Lada
>
>
> /martin
>
>
>> I believe some variation on YSDL could work, and as I wrote in another
>> thread, we could also incorporate datastores: after defining the
>> (multilevel) schemas, each datastore will get one assigned - and they
>> can also share them where needed.
>>
>> Lada
>>
>>>
>>>
>>>
>>> /martin
>>>
>>>
>>>>
>>>> * Many use cases need the possibility to mount schemas, but do not
>>>> need the added complexity of schema changes in run-time.
>>>> Notwithstanding the case of "YANG Features", for me the model schema
>>>> is a mostly static description of a nodes capabilities. Most of the
>>>> time I do not want to worry about the node changing its schema on
>>>> the fly.
>>>>
>>>>
>>>> For this I propose 2 YANG extensions
>>>>
>>>> extension schema-mount {
>>>> description "Indicates that a YANG Module is to be mounted into
>>>> another module.
>>>> The argument specifies the name of the module to be mounted.";
>>>> argument mounted-module;
>>>> }
>>>>
>>>> extension schema-mount-target {
>>>> description "Specifies the target node under which a YANG module is to
>>>> be mounted.
>>>> The statement can only be used inside a schema-mount statement.
>>>> The argument follows the same rules as an augment statement's target.
>>>> argument target-node;
>>>> }
>>>>
>>>> The two extension statements can be placed in a separate module or the
>>>> mounted module.
>>>>
>>>> I don't insist on the solution, but I need the off-line/design-time
>>>> specification of yang-mount to be possible. IMHO the design-time mount
>>>> use-case is more important than the dynamic-mount.
>>>>
>>>> regards Balazs
>>>> --
>>>> Balazs Lengyel Ericsson Hungary Ltd.
>>>> Senior Specialist
>>>> Mobile: +36-70-330-7909 email: [email protected]
>>>
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>>
>> --
>> Ladislav Lhotka, CZ.NIC Labs
>> PGP Key ID: E74E8C0C
--
Ladislav Lhotka, CZ.NIC Labs
PGP Key ID: E74E8C0C
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