[netmod] YANG library structure

[Martin Bjorklund <[e-mail]>]

Hi,

There has been quite a lot of discussion about the YANG library
data model on the list.  The authors of draft-ietf-netconf-rfc7895bis
have tried to understand all arguments in the discussion, and provide
a solution.  Below are 3 solution proposals (we have discussed more,
but they are basically just variations on the same themes).

Absolute Requirements
---------------------

o  RFC 7950, Section 5.6.5 says:

     A server MUST NOT implement more than one revision of a module.

o  draft-ietf-netmod-revised-datastores says:

     The conventional configuration datastores [...] share exactly
     the same datastore schema

o  draft-ietf-netmod-revised-datastores says:

     The datastore schema for <operational> MUST be a superset of the
     combined datastore schema used in all configuration datastores
     except that YANG nodes supported in a configuration datastore MAY
     be omitted from <operational> if a server is not able to
     accurately report them.


These requirements (of course) still hold, and we think that the YANG
library document should explain what they imply for the data reported
as part of the library.  (For example, all conventional datastores
MUST have a reference to the same "schema").


Objectives (in no particular order)
-----------------------------------

1. As efficient as possible for a client to consume.

   Since the size of the yang library can be quite large, it should
   be possible for clients to cache the yang library information.

2. A dynamic datastore must be able to implement a module or feature
   that is not implemented in the conventional datastores.

3. It must be possible to NOT implement a module or feature in
   operational, even if it is implemented in some other datastore.

   This is required for transition purposes; a server that wants to
   implement <operational> should not have to implement all modules at
   once.

4. A given module can only be implemented in one revision in all
   datastores.  If a module is implemented in more than one
   datastores, the same revision is implemented in all these
   datastores.

5. Multiple revisions can be used for import, if import-by revision
   is used.

6. Nice to have: make it possible to be used by schema mount


It should be noted that because of 2 and 3 (and 6), the original data
model in RFC 7895 cannot be used.


Use Cases
---------

Here's a set of use cases that must be supported.

  C1. conventional + operational, all have the same schema

  C2. conventional + operational, ietf-hardware is not implemented in
      conventional

  C3. conventional + operational, some modules not yet implemented in
      operational, and some modules are partly implemented in
      operational.

  C4. conventional + operational + ephemeral, ephemeral has its own
      set of modules



Alt. A.
-------

  Each datastore refers to a schema, and each schema contains a flat
  list of all modules, features, etc.

    +--ro yang-library
       +--ro schema* [name]
       |  +--ro name                  string
       |  +--ro checksum              string
       |  +--ro module* [name]
       |  |  +--ro name         yang:yang-identifier
       |  |  +--ro revision?    revision-identifier
       |  |  +--ro namespace    inet:uri
       |  |  +--ro location*    inet:uri
       |  |  +--ro submodule* [name]
       |  |  |  +--ro name        yang:yang-identifier
       |  |  |  +--ro revision?   revision-identifier
       |  |  |  +--ro location*   inet:uri
       |  |  +--ro feature* [name]
       |  |  |  +--ro name    yang:yang-identifier
       |  |  +--ro deviation* [module]
       |  |     +--ro module    -> ../../name
       |  +--ro import-only-module* [name revision]
       |     +--ro name         yang:yang-identifier
       |     +--ro revision     union
       |     +--ro namespace    inet:uri
       |     +--ro location*    inet:uri
       |     +--ro submodule* [name]
       |        +--ro name        yang:yang-identifier
       |        +--ro revision?   revision-identifier
       |        +--ro location*   inet:uri
       +--ro datastore* [name]
       |  +--ro name      identityref
       |  +--ro schema    -> ../../schema/name
       +--ro checksum     string


  How does this solution handle the use cases above?

  C1: One schema, all datastores refer to this schema.

  C2: Two schemas, "conventional" and "operational".  They differ in
      just one element (ietf-hardware).  All other module information
      is entirely duplicated in both.

  C3: Two schemas, "conventional" and "operational".  They differ in
      the modules not implemented in operational, and operational also
      has some deviation modules with "not-implemented".

  C4: Three schemas, "conventional", "ephemeral", "operational".
      "operational" contains the union of all modules in the other
      two.


  Pro: simple on the client, simple on the server

  Con: verbose, since a single difference requires a complete, new,
       schema.


Alt. B.
-------

  Each datastore refers to a schema, and each schema contains a list
  of references to module-sets, and each module-set contains a flat
  list of all modules, features, etc.

  When combining module-sets into a schema there MUST NOT be any
  duplicate module definitions in the module-sets.


    +--ro yang-library
       +--ro module-set* [name]
       |  +--ro name                  string
       |  +--ro checksum              string
       |  +--ro module* [name]
       |  |  +--ro name         yang:yang-identifier
       |  |  +--ro revision?    revision-identifier
       |  |  +--ro namespace    inet:uri
       |  |  +--ro location*    inet:uri
       |  |  +--ro submodule* [name]
       |  |  |  +--ro name        yang:yang-identifier
       |  |  |  +--ro revision?   revision-identifier
       |  |  |  +--ro location*   inet:uri
       |  |  +--ro feature* [name]
       |  |  |  +--ro name    yang:yang-identifier
       |  |  +--ro deviation* [module]
       |  |     +--ro module    -> ../../name
       |  +--ro import-only-module* [name revision]
       |     +--ro name         yang:yang-identifier
       |     +--ro revision     union
       |     +--ro namespace    inet:uri
       |     +--ro location*    inet:uri
       |     +--ro submodule* [name]
       |        +--ro name        yang:yang-identifier
       |        +--ro revision?   revision-identifier
       |        +--ro location*   inet:uri
       +--ro schema* [name]
       |  +--ro name          string
       |  +--ro checksum      string
       |  +--ro module-set*   -> ../../module-set/name
       +--ro datastore* [name]
       |  +--ro name      identityref
       |  +--ro schema    -> ../../schema/name
       +--ro checksum      string

  How does this solution handle the use cases above?

  C1: One module-set, one schema, all datastores refer to this schema,
      the schema refers to the single module-set.

  C2: Two schemas, "conventional" and "operational", and two
      module-sets.  One module-set contains just "ietf-hardware" and
      the other everything else.  The "operational" schema refers to
      both module-sets, and the "conventional" to just the one without
      "ietf-hardware".

  C3: Two schemas, "conventional" and "operational", and three
      module-sets.  One module-set contains all modules fully
      implemented in both conventional and operational, one contains
      the modules implemented only in conventional, and one the
      modules and deviations for the partly implemented modules in
      operational.

  C4: Three schemas, "conventional", "ephemeral", "operational", but
      just two module-sets. "conventional" refers to one of the
      module-sets, and "ephemeral" to the other.  "operational" refers
      to both.

  Pro: less verbose

  Con: the client has to follow extra references and must combine the
       result from the references into a single schema.


Alt. C.
-------

  (This is the draft -02 version with just some name changes)

  Each datastore refers to a schema, and each schema contains a list
  of references to each module it includes.

    +--ro yang-library
       +--ro module* [id]
       |  +--ro id                  string
       |  +--ro name                yang:yang-identifier
       |  +--ro revision?           revision-identifier
       |  +--ro location*           inet:uri
       |  +--ro namespace           inet:uri
       |  +--ro feature*            yang:yang-identifier
       |  +--ro deviation* [module]
       |  |  +--ro module    -> ../../id
       |  +--ro conformance-type    enumeration
       |  +--ro submodule* [name]
       |     +--ro name        yang:yang-identifier
       |     +--ro revision?   revision-identifier
       |     +--ro location*   inet:uri
       +--ro schema* [name]
       |  +--ro name      string
       |  +--ro module*   -> ../../module/id
       +--ro datastore* [name]
       |  +--ro name          identityref
       |  +--ro schema    -> ../../schema/name
       +--ro checksum       string

  How does this solution handle the use cases above?

  C1: One schema, all datastores refer to this schema,
      the schema refers to all modules.

  C2: Two schemas, "conventional" and "operational", and the module
      list contains all modules.  The "operational" schema refers to
      all modules, and "conventional" to all modules except
      "ietf-hardware".

  C3: similar to C2, except there will be two entries in the module
      list for evenry module that is partly implemented in
      operational.

  C4: Three schemas, "conventional", "ephemeral", "operational", and
      the module list contains all modules.
      Each schema refers to the modules it supports.

  Pro: All modules available are listed in one place.

  Con: the client has to follow extra references and must combine the
       result from the references into a single schema.

       the least "direct" solution due to the module "id".

       probably a bit tricky to implement on the server.



/martin