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

["Xufeng Liu" <[e-mail]>]

Hi Rob,

 

Thanks for the proposal. I think that the subtree does help, to a certain extend. The approach is worth mentioning to the implementers, though the remaining XPath is still complicated and could be worse for a more complex model. The good thing about the approach is that this is what we already have today. I agree with Tom on that NETCONF should have a better solution, but that may require protocol changes and need to cover more generic cases as Per described.

 

Thanks,

- Xufeng

 

From: Netconf [mailto:[e-mail]] On Behalf Of Robert Wilton
Sent: Wednesday, October 11, 2017 6:49 AM
To: t.petch <[e-mail]>; Igor Bryskin <[e-mail]>; [e-mail]; [e-mail]
Subject: Re: [Netconf] [netmod] Retrieving Information Pointed by leafref

 

I've also been thinking about this problem a bit more :-)

The XPath solution works, but the expression isn't particularly nice to write, and I suspect that implementations may struggle to implement it efficiently (if they support XPath filtering at all).

A nicer solution here might be to allow the XPath filters to be combined with a subtree filter along the lines of a more advanced type of "Attribute Match Expression" sec 6.2.2 of rfc6241.

E.g. rather than this XPath filter:

/te:te/te:tunnels/te:tunnel[te:name='foo'] |
/te:te/te:connections/te:connection[te:name=/te:te/te:tunnels/te:tunnel[te:name='foo']/te:connections/te:connection/te:name]
 

Here is example of what a subtree filter combined with an XPath filter could potentially look like (which of course isn't valid NETCONF/YANG today):

<filter type="subtree-xpath">
  <te:te xmlns:te="...">
    <te:tunnels te:name="foo"/>
    <te:connections>
      <filter type="xpath" select="te:name = ../te:tunnels/te:tunnel[te:name='foo']/te:connections/te:connection/te:name)"/>
    </te:connections>
  </te:te>
</filter>


Any opinions on whether this would be beneficial or is this just reinventing the wheel?

Thanks,
Rob



On 11/10/2017 10:41, t.petch wrote:

Igor
 
Thinking laterally, this is a problem that DNS encountered a few decades
ago and solved, by allowing the server to include additional information
not specifically requested that the server can see is going to be needed
for the next step, so if the client asks only about a CNAME, then the
server can provide the relevant IP address as well.
 
I suspect that the current rules for Netconf do not allow the server to
send anything not explicitly requested, which is a shame (IMO).
 
The DNS approach works very well, in fact I do not think we would
survive without it.
 
Tom Petch
 
----- Original Message -----
From: "Igor Bryskin"  <mailto:[e-mail]> <[e-mail]>
Sent: Tuesday, October 10, 2017 2:35 PM
 
Hi Rob,
 
This helps a lot. What you wrote will work.
 
The only difference is that if we would have the "joimt with" clause as
we proposed, the server would be able to tailor the te-tunnel
presentation to the client's requirements, e.g. substituting the
connection pointers with connection bodies, while, according to your
suggestion, the server will provide the te-tunnel body as is, and then
augment it with the cobbection information, thus, leaving
for the client to "shuffle " the received data. But I do agree, this
would be a minor inconvinience for the client, the important thing is
that the client will get all the data in one piece.
 
Thanks a lot,
Igor
 
c
 
From:Robert Wilton
To:Igor Bryskin,
Cc:Per Hedeland,[e-mail],[e-mail] <mailto:[e-mail],[e-mail]> ,
Date:2017-10-10 06:41:04
Subject:Re: [netmod] [Netconf] Retrieving Information Pointed by leafref
 
Hi Igor,
 
On 09/10/2017 23:11, 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) ?
I think that it might be something like this (for tunnel name foo):
 
   /te/tunnels/tunnel[name='foo'] |
 
/te/connections/connection[name=/te/tunnels/tunnel[name='foo']/connectio
ns/connection/name]
 
E.g. in English, this should equate to something like:
 
Return all information for tunnel foo AND ALSO
Return all information for all connections where the connection name
matches one of the connections listed in tunnel foo.
 

 
Likewise, how the client can ask for full data of the te-tunnel and

all supporting te-tunnels and just pointers for supporting connections?
If my xpath above is right, then this would be something roughly like
this:
 
   /te/tunnels/tunnel[name='foo'] |
 
/te/tunnels/tunnel[name=/te/tunnels/tunnel[name='foo']/supporting-tunnel
s/supporting-tunnel/name]
 
 
I'm an XPath novice, so the expressions might be wrong.
 
https://www.freeformatter.com/xpath-tester.html might be useful. E.g. if
you can construct a simple XML instance tree of your data, you could
validate whether the XPath expression works.
 
I hope that this is of some help,
Rob
 
 

 
I really appreciate your help,
 
Igor
 
 
-----Original Message-----
From: Per Hedeland [mailto:[e-mail]]
Sent: Monday, October 09, 2017 5:21 PM
To: Igor Bryskin
Cc: [e-mail] <mailto:[e-mail]> ; [e-mail] <mailto:[e-mail]> ; [e-mail] <mailto:[e-mail]> ;

[e-mail] <mailto:[e-mail]> 

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,[e-mail],[e-mail],[e-mail] <mailto:[e-mail],[e-mail],[e-mail]> ,

*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  <mailto:[e-mail]> <[e-mail]> 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:[e-mail]]
Sent: Monday, October 09, 2017 12:06 PM
To: Xufeng Liu
Cc: Igor Bryskin; [e-mail] <mailto:[e-mail]> ; [e-mail] <mailto:[e-mail]> 
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']/pat
hs/path/explicit-path]

 
--Per
 
On 2017-10-09 15:42, Xufeng Liu wrote:

Hi Per,
 
 
 
*From:* Igor Bryskin [mailto:[e-mail]]
*Sent:* Sunday, October 8, 2017 7:04 PM
*To:* Igor Bryskin  <mailto:[e-mail]> <[e-mail]>; [e-mail] <mailto:[e-mail]> ;
*[e-mail] <mailto:*[e-mail]> 
*Cc:* [e-mail] <mailto:[e-mail]> ; [e-mail] <mailto:[e-mail]> 
*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:*[e-mail],[e-mail] <mailto:[e-mail]> ,
 
*Cc:*[e-mail],[e-mail] <mailto:[e-mail]> ,
 
*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:*[e-mail],'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
 
 
 
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