Re: [netmod] [Netconf] Retrieving Information Pointed by leafref

Per Hedeland <per@tail-f.com> Tue, 10 October 2017 06:04 UTC

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To: Igor Bryskin <Igor.Bryskin@huawei.com>
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From: Per Hedeland <per@tail-f.com>
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Date: Tue, 10 Oct 2017 08:04:19 +0200
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Subject: Re: [netmod] [Netconf] Retrieving Information Pointed by leafref
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I already showed you an XPath filter expression that will do what you
originally asked for, i.e. retrieve the data for a tunnel and the
associated 'explicit-path's in a single request, based on the
(simplified) data model that you provided. What you are describing now
seems like just variations on the theme - as I wrote, it is the client
that decides, via the XPath expression, which leafref-leafs that should
be "dereferenced" for selection of additional data.

I'm afraid I don't have the time to craft XPath expressions for each and
every one of your use cases (and I wouldn't trust them to be correct
without testing anyway). I also provided a link to the definition of the
XPath capability in the NETCONF spec. XPath itself is well-established
technology, not specific to NETCONF or YANG. I suggest that you consult
the specification at http://www.w3.org/TR/1999/REC-xpath-19991116 and/or
one of the many tutorials or even books dedicated to the subject - I
don't have any specific recommendations for the latter, but maybe
someone else does.

--Per

On 2017-10-10 00:11, Igor Bryskin wrote:
> Hi Per,
> 
> This is a good news, but, please, help us out.
> Consider, we have a node - "te-tunnel" - which among other attributes has two key leafref lists:
> 1) each member of the 1st list points to a "connection" supporting the te-tunnel. All connections supporting all te-tunnels are stored in a single list of connections.
> 2) each member of the 2nd list points to a supporting "te-tunnel" - the te-tunnel in question depends on. All te=tunnels including the te-tunnel in question, are stored in a single list of te-tunnels.
> 
> The question: how the client can retrieve via a single request all attributes of the te-tunnel in question along with all parameters of all connections supporting the te-tunnel, but with just pointers to supporting te-tunnels (so that the interested client can use the pointers to retrieve full data via subsequent separate requests) ?
> 
> Likewise, how the client can ask for full data of the te-tunnel and all supporting te-tunnels and just pointers for supporting connections?
>
> I really appreciate your help,
> 
> Igor
> 
>   
> -----Original Message-----
> From: Per Hedeland [mailto:per@tail-f.com]
> Sent: Monday, October 09, 2017 5:21 PM
> To: Igor Bryskin
> Cc: mbj@tail-f.com; xufeng.liu.ietf@gmail.com; netconf@ietf.org; netmod@ietf.org
> Subject: Re: [Netconf] [netmod] Retrieving Information Pointed by leafref
> 
> Just to be clear: what we're suggesting is that you can use the
> already-existing standard NETCONF XPath capability to achieve the desired
> result - see https://tools.ietf.org/html/rfc6241#section-8.9
> 
> --Per
> 
> On 2017-10-09 21:52, Igor Bryskin wrote:
>> I agree. For example, a leafref may point not to a singls entity, but to a list of entities, and the client might want to expand all of them into the joint get response.
>>
>> Igor
>>
>> *From:*Per Hedeland
>> *To:*Martin Bjorklund,
>> *Cc:*Igor Bryskin,xufeng.liu.ietf@gmail.com,netconf@ietf.org,netmod@ietf.org,
>> *Date:*2017-10-09 15:12:22
>> *Subject:*Re: [Netconf] [netmod] Retrieving Information Pointed by leafref
>>
>> On 2017-10-09 19:13, Martin Bjorklund wrote:
>>> Igor Bryskin <Igor.Bryskin@huawei.com> wrote:
>>>>
>>>> Hi Per,
>>>>
>>>> Basically, what we need is a way for a client to request something
>>>> like this:
>>>>
>>>> get <XPath> joint with <XPath1, XPath2, ..., XPathn>
>>>
>>> ... which is what Per's expression does!  Note that "|" in XPath means
>>> "union".
>>>
>>> But as Per explained, it only works in some cases (when the leafref
>>> acts a "single pointer").
>>
>> Well, that particular expression works only in that case - but since it
>> is effectively the client that (perhaps based on the data model) decides
>> what the leafref-leafs "mean" (in this case the single key of a single
>> list), other cases can be handled the same way. E.g. multiple
>> leafref-to-key leafs that together give the keys of a multi-key list
>> just amounts to a slightly hairier XPath filter...
>>
>> --Per
>>
>>>> with a server interpreting the request as follows:
>>>> if a node pointed by XPath contains a pointer (e.g. key leafref)
>>>> matching one of the XPath from the "joint with" list, then the server
>>>> must provide the entire body of the node pointed by the pointer,
>>>> otherwise, just the pointer (as it happens today, that is, when no
>>>> "joint with" list specified).
>>>>
>>>> We think that this would allow for the client to optimize the number
>>>> of request-response iterations depending on application/use case.
>>>>
>>>> Regards,
>>>> Igor
>>>
>>>
>>>
>>> /martin
>>>
>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> -----Original Message-----
>>>> From: Per Hedeland [mailto:per@tail-f.com]
>>>> Sent: Monday, October 09, 2017 12:06 PM
>>>> To: Xufeng Liu
>>>> Cc: Igor Bryskin; netconf@ietf.org; netmod@ietf.org
>>>> Subject: Re: [Netconf] [netmod] Retrieving Information Pointed by
>>>> leafref
>>>>
>>>> I understand your use case, but a leaf of type leafref does not in
>>>> general identify a single node in the data tree - the leafref path
>>>> could
>>>> be for a non-key leaf, and/or the path could traverse list nodes,
>>>> and/or
>>>> the "target" list could have multiple keys and thus multiple
>>>> leafref-leafs be required to identify a specific list entry.
>>>>
>>>> Thus it seems to me that your use case is not a reasonable basis for a
>>>> new protocol operation. My XPath foo isn't very good either, but I do
>>>> believe Robert's suggestion of using an XPath filter could be a way
>>>> forward. I *think* the filter expression would be something along the
>>>> lines of
>>>>
>>>>    /te/tunnels/tunnel[name='foo'] |
>>>>    /te/explicit-paths/explicit-path[name=/te/tunnels/tunnel[name='foo']/paths/path/explicit-path]
>>>>
>>>> --Per
>>>>
>>>> On 2017-10-09 15:42, Xufeng Liu wrote:
>>>>> Hi Per,
>>>>>
>>>>>    
>>>>>
>>>>> *From:* Igor Bryskin [mailto:Igor.Bryskin@huawei.com]
>>>>> *Sent:* Sunday, October 8, 2017 7:04 PM
>>>>> *To:* Igor Bryskin <Igor.Bryskin@huawei.com>om>; per@tail-f.com;
>>>>> *xufeng.liu.ietf@gmail.com
>>>>> *Cc:* netconf@ietf.org; netmod@ietf.org
>>>>> *Subject:* Re: [Netconf] [netmod] Retrieving Information Pointed by
>>>>> *leafref
>>>>>
>>>>>    
>>>>>
>>>>>
>>>>> Hi Joel,
>>>>>
>>>>> Thanks, I think I didnt explain our problem correctly.
>>>>>
>>>>> In our case we have a leafref pointing to a te tunnel name, which
>>>>> happens to be a key to lookup the (axilary) tunnel.  We need a way to
>>>>> include the entire tunnel body (not just a name) into the get
>>>>> response. This is to optimize the number of iterations between the
>>>>> client and the server. As Xufeng put it something similar to SQL join,
>>>>>
>>>>> Igor
>>>>>
>>>>> *From:*Igor Bryskin
>>>>>
>>>>> *To:*per@tail-f.com,xufeng.liu.ietf@gmail.com,
>>>>>
>>>>> *Cc:*netconf@ietf.org,netmod@ietf.org,
>>>>>
>>>>> *Date:*2017-10-08 17:36:47
>>>>>
>>>>> *Subject:*Re: [Netconf] [netmod] Retrieving Information Pointed by
>>>>> *leafref
>>>>>
>>>>>    
>>>>>
>>>>> Hi Per,
>>>>>
>>>>> In a nutshell we would lika for a netconf client to have a way to
>>>>> instruct the server on whether in response to the get request the
>>>>> server needs to provide the entire body of a datastore node pointed
>>>>> to by a leafref or just a pointer to said node, so that the node's
>>>>> body could be retrieved by a subsequent separate request. This is
>>>>> requested by implementors who want to optimise rhe number of
>>>>> interactions between a client and its server.
>>>>>
>>>>> Cheers,
>>>>> Igor
>>>>>
>>>>> *From:*Per Hedeland
>>>>>
>>>>> *To:*Xufeng Liu,
>>>>>
>>>>> *Cc:*netconf@ietf.org,'NetMod WG',
>>>>>
>>>>> *Date:*2017-10-08 14:01:27
>>>>>
>>>>> *Subject:*Re: [Netconf] [netmod] Retrieving Information Pointed by
>>>>> *leafref
>>>>>
>>>>>    
>>>>>
>>>>> On 2017-10-06 23:11, Xufeng Liu wrote:
>>>>>> During the design team discussion for TE and MPLS YANG modeling, we
>>>>>> have received a request from implementers: How to minimize the number
>>>>>> of NETCONF/RESTCONF RPCs to improve operation efficiency?
>>>>>> Especially for the case when the operator or client software needs to
>>>>>> retrieve the object contents pointed by a leafref.
>>>>>>
>>>>>> For example, given the following simplified TE tunnel model,
>>>>>>
>>>>>> +--rw te
>>>>>>
>>>>>>        +--rw explicit-paths
>>>>>>
>>>>>>        |  +--rw explicit-path* [name]
>>>>>>
>>>>>>        |     +--rw name                      string
>>>>>>
>>>>>>        |        +--rw explicit-route-object* [index]
>>>>>>
>>>>>>        |           +--rw index                   uint32
>>>>>>
>>>>>>        |           +--rw explicit-route-usage?   identityref
>>>>>>
>>>>>>        +--rw tunnels
>>>>>>
>>>>>>        |  +--rw tunnel* [name]
>>>>>>
>>>>>>        |  |  +--rw name                   string
>>>>>>
>>>>>>        |  |  +--rw paths
>>>>>>
>>>>>>        |  |  |  +--rw path* [name]
>>>>>>
>>>>>> |  |  |     +--rw explicit-path?  ->
>>>>>> |  |  |     ../../../../../explicit-paths/explicit-path/name
>>>>>>
>>>>>> when the client tries to retrieve a tunnels information based on the
>>>>>> tunnel name, the get operation returns a list of leafrefs pointing
>>>>>> to the paths of the tunnel.
>>>>>
>>>>> Sorry, I'm afraid I don't follow. Can you explain exactly what your
>>>>> "get" request is (protocol and payload), and where the "list of
>>>>> leafref's" (whatever that may be) occurs in the reply?
>>>>>
>>>>> */[Xufeng] The get operation is the NETCONF/RESTCON <get> protocol
>>>>> *operation, or the <get-data> operation described in
>>>>> *https://tools.ietf.org/html/draft-dsdt-nmda-netconf-01 and the GET
>>>>> *operations
>>>>> on {+restconf}/ds/<datastore> described in
>>>>> https://tools.ietf.org/html/draft-ietf-netconf-nmda-restconf-00./*
>>>>>
>>>>> */ /*
>>>>>
>>>>> */We have a list of leafref values because in this example model, each
>>>>> *tunnel contains a list of paths, each of them contains a leafref. The
>>>>> *get returns a value for each instance of such a leafref,
>>>>> which (as a string value) will be used as a constraint (foreign key)
>>>>> to retrieve the instance of an explicit-path in the model above./*
>>>>>
>>>>>
>>>>>
>>>>> JFYI, in case there is some fundamental misunderstanding here: a leaf
>>>>> of
>>>>> type leafref has a single value - *one* of those that satisfy the
>>>>> leafref
>>>>> constraint, in case there are multiple "candidates".
>>>>>
>>>>> --Per
>>>>>
>>>>>> The client needs to issue at
>>>>>> least one more get operation to retrieve the path information about
>>>>>> the given tunnel. The request is to combine these two operations into
>>>>>> one.
>>>>>>
>>>>>> In the RDBMS SQL world, join can be used when SQL select is
>>>>>> performed, but NETCONF/YANG currently does not have this capability.
>>>>>>
>>>>>> Wed like to ask whether such a request is considered reasonable.
>>>>>>
>>>>>> If the request is reasonable, the next question is how to
>>>>>> proceed. This seems to be a protocol issue rather than YANG modeling
>>>>>> issue. Is it acceptable to add a new operation to achieve such a
>>>>>> <get-data> operation with expanded leafrefs?
>>>>>>
>>>>>> Comments and suggestions are appreciated.
>>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>> - Xufeng
>>>>>>
>>>>>>
>>>>>>
>>>>>> _______________________________________________
>>>>>> netmod mailing list
>>>>>> netmod@ietf.org <mailto:netmod@ietf.org>
>>>>>> https://www.ietf.org/mailman/listinfo/netmod
>>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> Netconf mailing list
>>>>> Netconf@ietf.org <mailto:Netconf@ietf.org>
>>>>> https://www.ietf.org/mailman/listinfo/netconf
>>>>>
>>>>
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