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Versions: (draft-verdt-netmod-yang-schema-comparison) 00

Network Working Group                                          R. Wilton
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Standards Track                          March 17, 2020
Expires: September 18, 2020


                         YANG Schema Comparison
              draft-ietf-netmod-yang-schema-comparison-00

Abstract

   This document specifies an algorithm for comparing two revisions of a
   YANG schema to determine the scope of changes, and a list of changes,
   between the revisions.  The output of the algorithm can be used to
   help select an appropriate revision-label or YANG semantic version
   number for a new revision.  This document defines a YANG extension
   that provides YANG annotations to help the tool accurately determine
   the scope of changes between two revisions.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 18, 2020.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology and Conventions . . . . . . . . . . . . . . . . .   4
   3.  Generic YANG schema tree comparison algorithm . . . . . . . .   4
     3.1.  YANG module revision scope extension annotations  . . . .   5
   4.  YANG module comparison algorithm  . . . . . . . . . . . . . .   5
   5.  YANG schema comparison algorithms . . . . . . . . . . . . . .   6
     5.1.  Standard YANG schema comparison algorithm . . . . . . . .   6
     5.2.  Filtered YANG schema comparison algorithm . . . . . . . .   6
   6.  Comparison tooling  . . . . . . . . . . . . . . . . . . . . .   7
   7.  Module Versioning Extension YANG Modules  . . . . . . . . . .   7
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     10.1.  YANG Module Registrations  . . . . . . . . . . . . . . .  11
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     11.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Warning, this is an early (-00) draft with the intention of scoping
   the outline of the solution, hopefully for the WG to back the
   direction of the solution.  Refinement of the solution details is
   expected, if this approach is accepted by the WG.

   This document defines a solution to Requirement 2.2 in
   [I-D.verdt-netmod-yang-versioning-reqs].  Complementary documents
   provide a complete solution to the YANG versioning requirements, with
   the overall relationship of the solution drafts described in
   [I-D.verdt-netmod-yang-solutions].

   YANG module 'revision-labels'
   [I-D.verdt-netmod-yang-module-versioning] and the use of YANG
   semantic version numbers [I-D.verdt-netmod-yang-semver] can be used
   to help manage and report changes between revisions of individual
   YANG modules.

   YANG packages [I-D.rwilton-netmod-yang-packages] along with YANG
   semantic version numbers can be used to help manage and report
   changes between revisions of YANG schema.





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   [I-D.verdt-netmod-yang-module-versioning] and
   [I-D.rwilton-netmod-yang-packages] define how to classify changes
   between two module or package revisions, respectively, as backwards
   compatible or non-backwards-compatible.
   [I-D.verdt-netmod-yang-semver] refines the definition, to allow
   backwards compatible changes to be classified as 'minor changes' or
   'editorial changes'.

   'Revision-label's and YANG semantic version numbers, whilst being
   generally simple and helpful in the mainline revision history case,
   are not sufficient in all scenarios.  For example, when comparing two
   revisions/versions on independent revision branches, without a direct
   ancestor relationship between the two revisions/versions.  In this
   cases, an algorithmic comparison approach is beneficial.

   In addition, the module revision history's 'nbc-changes' extension
   statement, and YANG semantic version numbers, effectively declare the
   worst case scenario.  If any non-backwards-compatible changes are
   restricted to only parts of the module/schema that are not used by an
   operator, then the operator is able to upgrade, and effectively treat
   the differences between the two revisions/versions as backwards
   compatible because they are not materially impacted by the non-
   backwards-compatible changes.

   Hence, this document defines algorithms that can be applied to
   revisions of YANG modules or versions of YANG schema (e.g., as
   represented by YANG packages), to determine the changes, and scope of
   changes between the revisions/versions.

   For many YANG statements, programmatic tooling can determine whether
   the changes between the statements constitutes a backwards-compatible
   or non-backwards-compatible change.  However, for some statements, it
   is not feasible for current tooling to determine whether the changes
   are backwards-compatible or not.  For example, in the general case,
   tooling cannot determine whether the change in a YANG description
   statement causes a change in the semantics of a YANG data node.  If
   the change is to fix a typo or spelling mistake then the change can
   be classified as an editorial backwards-compatible change.
   Conversely, if the change modifies the behavioral specification of
   the data node then the change would need to be classified as either a
   non editorial backwards-compatible change or a non-backwards-
   compatible change.  Hence, extension statements are defined to
   annotate a YANG module with additional information to clarify the
   scope of changes in cases that cannot be determined by algorithmic
   comparison.

   Open issues are tracked at <https://github.com/netmod-wg/yang-ver-dt/
   issues>, tagged with 'schema-comparison'.



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2.  Terminology and Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document uses terminology introduced in the YANG versioning
   requirements document [I-D.verdt-netmod-yang-versioning-reqs].

   This document makes of the following terminology introduced in the
   YANG Packages [I-D.rwilton-netmod-yang-packages]:

   o  YANG schema

   In addition, this document defines the terminology:

   o  Change scope: Whether a change between two revisions is classified
      as non-backwards-compatible, backwards-compatible, or editorial.

3.  Generic YANG schema tree comparison algorithm

   The generic schema comparison algorithm works on any YANG schema.
   This could be a schema associated with an individual YANG module, or
   a YANG schema represented by a set of modules, e.g., specified by a
   YANG package.

   The algorithm performs a recursive tree wise comparison of two
   revisions of a YANG schema, with the following behavior:

      The comparison algorithm primarily acts on the parts of the schema
      defined by unique identifiers.

      Each identifier is qualified with the name of the module that
      defines the identifier.

      Identifiers in different namespaces (as defined in 6.2.1 or RFC
      7950) are compared separately.  E.g., 'features' are compared
      separately from 'identities'.

      Within an identifier namespace, the identifiers are compared
      between the two schema revisions by qualified identifier name.
      The 'renamed-from' extension allow for a meaningful comparison
      where the name of the identifier has changed between revisions.
      The 'renamed-from' identifier parameter is only used when an
      identifier in the new schema revision cannot be found in the old
      schema revision.



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      YANG extensions, features, identities, typedefs are checked by
      comparing the properties defined by their YANG sub-statements
      between the two revisions.

      YANG groupings, top-level data definition statements, rpcs, and
      notifications are checked by comparing the top level properties
      defined by their direct child YANG sub-statements, and also by
      recursively checking the data definition statements.

      The rules specified in section 3 of
      [I-D.verdt-netmod-yang-module-versioning] determine whether the
      changes are backwards-compatible or non-backwards-compatible.

      The rules specified in section 3.2 of
      [I-D.rwilton-netmod-yang-packages] determine whether backwards-
      compatible changes are 'minor' or 'editorial'.

      For YANG description", "must", and "when" statements, the
      "backwards-compatible" and "editorial" extension statements can be
      used to mark instances when the statements have changed in a
      backwards-compatible or editorial way.  Since by default the
      comparison algorithm assumes that any changes in these statements
      are non-backwards-compatible.  XXX, more info required here, since
      the revisions in the module history probably need to be available
      for this to work in the general branched revisions case.

      Submodules are not relevant for schema comparison purposes, i.e.
      the comparison is performed after submodule resolution has been
      completed.

3.1.  YANG module revision scope extension annotations

4.  YANG module comparison algorithm

   The schema comparison algorithm defined in Section 3 can be used to
   compare the schema for individual modules, but with the following
   modifications:

      Changes to the module's metadata information (i.e. module level
      description, contact, organization, reference) should be checked
      (as potential editorial changes).

      The module's revision history should be ignored from the
      comparison.

      Changes to augmentations and deviations should be sorted by path
      and compared.




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5.  YANG schema comparison algorithms

5.1.  Standard YANG schema comparison algorithm

   The standard method for comparing two YANG schema versions is to
   individually compare the module revisions for each module implemented
   by the schema using the algorithm defined in Section 4 and then
   aggregating the results together:

   o  If all implemented modules in the schema have only changed in an
      editorial way then the schema is changed in an editorial way

   o  If all implemented modules in the schema have only been changed in
      an editorial or backwards-compatible way then the schema is
      changed in a backwards-compatible way

   o  Otherwise if any implemented module in the schema has been changed
      in a non-backwards-compatible way then the schema is changed in a
      non-backwards-compatible way.

   The standard schema comparison method is the RECOMMENDED scheme to
   calculate the version number change for new versions of YANG
   packages, because it allows the package version to be calculated
   based on changes to implemented modules revision history (or YANG
   semantic version number if used to identify module revisions).

5.2.  Filtered YANG schema comparison algorithm

   Another method to compare YANG schema, that is less likely to report
   inconsequential differences, is to construct full schema trees for
   the two schema versions, directly apply a version of the comparison
   algorithm defined in Section 3.  This may be particular useful when
   the schema represents a complete datastore schema for a server
   because it allows various filtered to the comparison algorithm to
   provide a more specific answer about what changes may impact a
   particular client.

   The full schema tree can easily be constructed from a YANG package
   definition, or alternative YANG schema definition.

   Controlled by input parameters to the comparison algorithm, the
   following parts of the schema trees can optionally be filtered during
   the comparison:

      All "grouping" statements can be ignored (after all "use"
      statements have been processed when constructing the schema).





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      All module and submodule metadata information (i.e. module level
      description, contact, organization, reference) can be ignored.

      The comparison can be restricted to the set of features that are
      of interest (different sets of features may apply to each schema
      versions).

      The comparison can be restricted to the subset of data nodes,
      RPCs, notifications and actions, that are of interest (e.g., the
      subset actually used by a particular client), providing a more
      meaningful result.

      The comparison could filter out backwards-compatible 'editorial'
      changes.

   In addition to reporting the overall scope of changes at the schema
   level, the algorithm output can also optionally generate a list of
   specific changes between the two schema, along with the
   classification of those individual changes.

6.  Comparison tooling

   'pyang' has some support for comparison two module revisions, but
   this is currently limited to a linear module history.

   TODO, it would be helpful if there is reference tooling for schema
   comparison.

7.  Module Versioning Extension YANG Modules

   YANG module with extension statements for annotating NBC changes,
   revision label, status description, and importing by version.

   <CODE BEGINS> file "ietf-yang-rev-annotations@2019-11-11.yang"
   module ietf-yang-rev-annotations {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-yang-rev-annotations";
     prefix rev-ext;

     organization
       "IETF NETMOD (Network Modeling) Working Group";
     contact
       "WG Web:   <https://datatracker.ietf.org/wg/netmod/>
        WG List:  <mailto:netmod@ietf.org>

        Author:   Robert Wilton
                  <mailto:rwilton@cisco.com>";




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     description
       "This YANG 1.1 module contains extensions to annotation to YANG
        module with additional metadata information on the nature of
        changes between two YANG module revisions.

        XXX, maybe these annotations could also be included in
        ietf-yang-revisions?

        Copyright (c) 2019 IETF Trust and the persons identified as
        authors of the code.  All rights reserved.

        Redistribution and use in source and binary forms, with or
        without modification, is permitted pursuant to, and subject
        to the license terms contained in, the Simplified BSD License
        set forth in Section 4.c of the IETF Trust's Legal Provisions
        Relating to IETF Documents
        (http://trustee.ietf.org/license-info).

        This version of this YANG module is part of RFC XXXX; see
        the RFC itself for full legal notices.

        The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
        NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED',
        'MAY', and 'OPTIONAL' in this document are to be interpreted as
        described in BCP 14 (RFC 2119) (RFC 8174) when, and only when,
        they appear in all capitals, as shown here.";

     // RFC Ed.: update the date below with the date of RFC publication
     // and remove this note.
     // RFC Ed.: replace XXXX (inc above) with actual RFC number and
     // remove this note.

     revision 2019-11-11 {
       description
         "Initial version.";
       reference
         "XXXX: YANG Schema Comparison";
     }

     extension backwards-compatible {
       argument revision-date-or-label;
       description
         "Identifies a revision (by revision-label, or revision date if
          a revision-label is not available) where a
          backwards-compatible change has occurred relative to the
          previous revision listed in the revision history.

          The format of the revision-label argument MUST conform to the



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          pattern defined for the ietf-yang-revisions
          revision-date-or-label typedef.

          The following YANG statements MAY have zero or more
          'rev-ext:non-backwards-compatible' statements:
              description
              must
              when

          Each YANG statement MUST only a have a single
          non-backwards-compatible, backwards-compatible, or editorial
          extension statement for a particular revision-label, or
          corresponding revision-date.";

       reference
         "XXXX: YANG Schema Comparison;
          Section XXX, XXX";
     }

     extension editorial {
       argument revision-date-or-label;
       description
         "Identifies a revision (by revision-label, or revision date if
          a revision-label is not available) where an editorial change
          has occurred relative to the previous revision listed in the
          revision history.

          The format of the revision-label argument MUST conform to the
          pattern defined for the ietf-yang-revisions
          revision-date-or-label typedef.

          The following YANG statements MAY have zero or more
          'rev-ext:non-backwards-compatible' statements:
              description

          Each YANG statement MUST only a have a single
          non-backwards-compatible, backwards-compatible, or editorial
          extension statement for a particular revision-label or
          corresponding revision-date.";

       reference
         "XXXX: YANG Schema Comparison;
          Section XXX, XXX";
     }

     extension renamed-from {
       argument yang-identifier;
       description



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         "Specifies a previous name for this identifier.

          This can be used when comparing schema to optimize handling
          for data nodes that have been renamed rather than naively
          treated them as data nodes that have been deleted and
          recreated.

          The argument 'yang-identifier' MUST take the form of a YANG
          identifier, as defined in section 6.2 of RFC 7950.

          Any YANG statement that takes a YANG identifier as its
          argument MAY have a single 'rev-ext:renamed-from'
          sub-statement.

          TODO, we should also facilitate identifiers being moved into
          other modules, e.g. by supporting a module-name qualified
          identifier.";

       reference
         "XXXX: YANG Schema Comparison;
          Section XXX, XXX";
     }
   }
   <CODE ENDS>

8.  Contributors

   This document grew out of the YANG module versioning design team that
   started after IETF 101.  The following individuals are (or have been)
   members of the design team and have worked on the YANG versioning
   project:

   o  Balazs Lengyel

   o  Benoit Claise

   o  Bo Wu

   o  Ebben Aries

   o  Jason Sterne

   o  Joe Clarke

   o  Juergen Schoenwaelder

   o  Mahesh Jethanandani




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   o  Michael Wang

   o  Qin Wu

   o  Reshad Rahman

   o  Rob Wilton

   The ideas for a tooling based comparison of YANG module revisions was
   first described in [I-D.clacla-netmod-yang-model-update].  This
   document extends upon those initial ideas.

9.  Security Considerations

   The document does not define any new protocol or data model.  There
   are no security impacts.

10.  IANA Considerations

10.1.  YANG Module Registrations

   The following YANG module is requested to be registered in the "IANA
   Module Names" registry:

   The ietf-yang-rev-annotations module:

      Name: ietf-yang-rev-annotations

      XML Namespace: urn:ietf:params:xml:ns:yang:ietf-yang-rev-
      annotations

      Prefix: rev-ext

      Reference: [RFCXXXX]

11.  References

11.1.  Normative References

   [I-D.rwilton-netmod-yang-packages]
              Wilton, R., Rahman, R., Clarke, J., Sterne, J., and W. Bo,
              "YANG Packages", draft-rwilton-netmod-yang-packages-03
              (work in progress), February 2020.








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   [I-D.verdt-netmod-yang-module-versioning]
              Claise, B., Clarke, J., Rahman, R., Wilton, R., Lengyel,
              B., Sterne, J., and K. D'Souza, "Updated YANG Module
              Revision Handling", draft-verdt-netmod-yang-module-
              versioning-01 (work in progress), October 2019.

   [I-D.verdt-netmod-yang-semver]
              Claise, B., Clarke, J., Rahman, R., Wilton, R., Lengyel,
              B., Sterne, J., and K. D'Souza, "YANG Semantic
              Versioning", draft-verdt-netmod-yang-semver-01 (work in
              progress), October 2019.

   [I-D.verdt-netmod-yang-solutions]
              Wilton, R., "YANG Versioning Solution Overview", draft-
              verdt-netmod-yang-solutions-03 (work in progress),
              February 2020.

   [I-D.verdt-netmod-yang-versioning-reqs]
              Clarke, J., "YANG Module Versioning Requirements", draft-
              verdt-netmod-yang-versioning-reqs-02 (work in progress),
              November 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7895]  Bierman, A., Bjorklund, M., and K. Watsen, "YANG Module
              Library", RFC 7895, DOI 10.17487/RFC7895, June 2016,
              <https://www.rfc-editor.org/info/rfc7895>.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <https://www.rfc-editor.org/info/rfc7950>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8407]  Bierman, A., "Guidelines for Authors and Reviewers of
              Documents Containing YANG Data Models", BCP 216, RFC 8407,
              DOI 10.17487/RFC8407, October 2018,
              <https://www.rfc-editor.org/info/rfc8407>.

   [RFC8525]  Bierman, A., Bjorklund, M., Schoenwaelder, J., Watsen, K.,
              and R. Wilton, "YANG Library", RFC 8525,
              DOI 10.17487/RFC8525, March 2019,
              <https://www.rfc-editor.org/info/rfc8525>.



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11.2.  Informative References

   [I-D.clacla-netmod-yang-model-update]
              Claise, B., Clarke, J., Lengyel, B., and K. D'Souza, "New
              YANG Module Update Procedure", draft-clacla-netmod-yang-
              model-update-06 (work in progress), July 2018.

   [I-D.ietf-netmod-yang-instance-file-format]
              Lengyel, B. and B. Claise, "YANG Instance Data File
              Format", draft-ietf-netmod-yang-instance-file-format-08
              (work in progress), March 2020.

   [RFC8340]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
              BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
              <https://www.rfc-editor.org/info/rfc8340>.

   [semver]   "Semantic Versioning 2.0.0", <https://www.semver.org>.

Author's Address

   Robert Wilton
   Cisco Systems, Inc.

   Email: rwilton@cisco.com



























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