Early Review of draft-ietf-i2rs-yang-network-topo-02
review-ietf-i2rs-yang-network-topo-02-yangdoctors-early-watsen-2017-03-17-00

See review at
https://www.ietf.org/mail-archive/web/yang-doctors/current/msg00032.html

Hi All,

It's been quiet on the list as a small group of us (Alex, Xufeng, Pavan, and
myself) went offline to discuss for a bit before bringing back to the group,
which I'm doing now.

Regarding resolving the modeling the issue, we went through nearly a dozen
ideas that we've narrowed down to two.  We discussed the pros/cons, but since
we each emphasize different values, we were unable to reach a consensus amongst
ourselves.  We're hoping that bringing the discussion here will bring more
perspectives and resolve this issue.

OPTION 1: separate /foo and /foo-state trees
--------------------------------------------

This option was/is described here: 
https://www.ietf.org/mail-archive/web/i2rs/current/msg04316.html.

PROS:
  a) does NOT break legacy clients (how we got here)
  b) consistent with convention used in many IETF modules
  c) able to show if/how opstate may differ from configured values

CONS:
  a) questionably valid YANG leafref usage
  b) complex server implementation (to handle require-instance false)
  c) eventually the module would need to migrate to the long-term
     solution, which would result in needing to also rewrite all
     modules that have augmented it (e.g., ietf-te-topology).
  d) leafref path expressions really only work for configuration data,
     though a clever server could have a special ability to peak at
     the opstate values when doing validations.  Of course, with
     require-instance is false, the value of leafref based validation
     checking is negated anyway, even for config true nodes, so this
     may not matter much.

OPTION 2: explicit client-option to also return tagged opstate data
-------------------------------------------------------------------

This option takes a couple forms.  The first is module-specific and
the second is generic.  In both cases, the idea is modeled after the
with-defaults solution (RFC6243), wherein the client passes a special
flag into <get-config> causing the server to also return opstate data,
having a special metadata flag set, intermingled with the configuration
data.

2A: Module-specific version

   module foo {
      import ietf-netconf { prefix nc; }
      import ietf-yang-metadata { prefix md; }
      md:annotation server-provided {
         type boolean;
      }
      container nodes {
         config true;
         list node {
            key "name";
            leaf name { type string; }
            leaf dependency {
               type leafref {
                 path "../node/name"
                 require-instance false;
               }
            }
         }
      }
      augment /nc:get-config/nc:input {
         leaf with-server-provided {
            type boolean;
         }
      }
   }

For instance:

  <get-config>
    <source>
      <running/>
    </source>
    <with-server-provided/>
   </get-config>

   <data>
     <nodes>
       <node>
         <name>overlay-node</name>
         <dependency>underlay-node</dependency>
       </node>
       <node foo:server-provided='true'>
         <name>underlay-node</name>
       </node>
     </nodes>
   </data>

PROS:
  a) does NOT break legacy clients (how we got here)
  b) having all data in one merged tree is simpler to process
     than two separate queries.
  c) module doesn't have to be rewritten for revised-datastores;
     the 'with-server-provided' switch would just not be passed
     by new opstate-aware clients.

CONS:
  a) inconsistent with convention used in many IETF modules
  b) unclear how to model 'with-server-provided' for RESTCONF
     (just use a description statement to define a query param?)
  c) unable to return the opstate value for any configured node
     (is it needed here?)
  d) requires server to support metadata, which is a relatively
     new concept and maybe not well supported by servers.
  e) only changes presentation-layer (doesn't change the fact
     that 'server-provided' data is not configuration), thus the
     leafref path expressions still don't work quite the way as
     desired, though a clever server could have a special ability
     to peak at the opstate values when doing validations. Of
     course, with require-instance is false, the value of leafref
     based validation checking is negated anyway, even for config
     true nodes, so this may not matter much.

2B: Generic version

The generic version is much the same, but rather than letting the
solution be limited to this one module, the idea is to generalize
it so it could be a server-level feature.  Having a generic RPC to
return data from more than one DS at a time was something that was
discussed ~1.5 years ago when we were kicking off the opstate effort.

The PROS and CONS are similar, but there are additional CONS in the
generic case.  The main ones being 1) how to simultaneously return
both the config and opstate values for a node (split at the leaves)
and 2) how to handle some YANG statements such as presence containers
and choice nodes.  For this reason, (2B) is NOT considered a viable
solution and is only here so that it's clear that it was discussed.

If there are any other options people want to suggest, please do so now!

Thanks,
Kent