YANG Extension and Metadata Annotation for Immutable Flag

Abstract

This document defines a YANG extension named "immutable" to indicate that specific "config true" data nodes are not allowed to be created/deleted/updated. To indicate that specific instances of a list/leaf-list node cannot be changed after initialization, a metadata annotation with the same name is also defined. Any data node or instance marked as immutable is read-only to the clients of YANG-driven management protocols, such as NETCONF, RESTCONF and other management operations (e.g., SNMP and CLI requests).¶

2. Overview

The "immutable" concept only puts write access restrictions to read-write datastores. When a specific data node or instance is marked as "immutable", NACM cannot override this to allow create/delete/update access.¶

A particular data node or instance MUST have the same immutability in all read-write datastores. The immutable annotation information should be visible even in read-only datastores (e.g., <system>, <intended>, <operational>), however this only serves as information about the data node itself, but has no effect on the handling of the read-only datastore. The immutability property of a particular data node or instance MUST be protocol-independent and user-independent.¶

If a particular container/list/leaf-list node is marked as "immutable" without exceptions for "delete" in the schema, the server SHOULD NOT annotate its instances, as that provides no additional information. If a particular leaf/anydata/anyxml node is marked as "immutable" without exceptions for "delete" or "update" in the schema, the server SHOULD NOT annotate its instances, as that provides no additional information.¶

Servers MUST reject any attempt to the "create", "delete" and "update" access operations on an immutable data node or instance marked by the metadata annotation or YANG extension (except according to the exceptions argument). The error reporting is performed immediately at an <edit-config> operation time, regardless what the target configuration datastore is. For an example of an "invalid-value" error response, see Appendix A.1.2.¶

However the following operations SHOULD be allowed:¶

• Use a create, update, delete/remove operation on an immutable node/instance if the effective change is null. E.g. If a leaf has a current value of "5" it should be allowed to replace it with a value of "5".¶
• Create an immutable data node/instance with a same value initially set by the system if it doesn't exist in the datastore. E.g., explicitly configure a system-generated interface name and type in <running>;¶

Note that even if a particular data node is immutable without the exception for "delete", it still can be deleted with its parent node, e.g., /if:interfaces/if:interface/if:type leaf is immutable, but the deletion to the /if:interfaces/if:interface list entry is allowed; if a particular data node is immutable without the exception for "create", it means the client can never create the instance of it, regardless the handling of its parent node.¶

TODO: Is immutable inherited down the containment hierarchy? If it is, should we allow overriding the immutability of a particular contained element (i.e., to declare a contained data node as immutable=false inside an immutable container/list) ?¶

3. "Immutable" YANG Extension

The "immutable" YANG extension can be a substatement to a leaf, leaf-list, container, list, anydata or anyxml statement. It indicates that data nodes based on the parent statement MUST NOT be added, removed or updated except according to the exceptions argument. The server MUST reject any such write attempt.¶

The "immutable" YANG extension defines an argument statement named "exceptions" which gives a list of operations that users are permitted to invoke for the specified node.¶

The following values are supported for the "exceptions" argument:¶

• Create: allow users to create instances of the data node;¶
• Update: allow users to modify instances of the data node;¶
• Delete: allow users to delete instances of the data node.¶

4. "Immutable" Metadata Annotation

The "immutable" flag is used to indicate the immutability of a particular instantiated data node. It only applies to the list/leaf-list entries. The values are boolean types indicating whether the data node instance is immutable or not.¶

Any list/leaf-list instance annotated with immutable="true" is read-only to clients, which means that once an instance is created, the client cannot change it. If a list entry is annotated with immutable="true", any contained descendant instances of any type (including leafs, lists, containers, etc.) inside the specific instance is not allowed to be created, updated and deleted without the need to annotate descendant nodes instances explicitly.¶

Note that "immutable" metadata annotation is used to annotate instances of a list/leaf-list rather than schema nodes. For instance, a list node may exist in multiple instances in the data tree, "immutable" can annotate some of the instances as read-only, while others are not.¶

When the client retrieves a particular datastore, immutable data node instances MUST be annotated with immutable="true" by the server. If the "immutable" metadata annotation inside a list entry is not specified, the default "immutable" value for a list/leaf-list entry is false.¶

Different from the "immutable" YANG extension, deletion to an instance marked with immutable="true" metadata annotation SHOULD always be allowed unless the list/leaf-list data node in the schema has an im:immutable extension as substatement without a "delete" exception.¶

5. YANG Module

<CODE BEGINS> file="ietf-immutable@2022-04-18.yang"
// RFC Ed.: replace XXXX with RFC number and remove this note
  module ietf-immutable {
    yang-version 1.1;
    namespace "urn:ietf:params:xml:ns:yang:ietf-immutable";
    prefix im;

    import ietf-yang-metadata {
      prefix md;
    }

    organization
      "IETF Network Modeling (NETMOD) Working Group";

    contact
      "WG Web: <https://datatracker.ietf.org/wg/netmod/>

       WG List: <mailto:netmod@ietf.org>

       Author: Qiufang Ma
               <mailto:maqiufang1@huawei.com>

       Author: Qin Wu
               <mailto:bill.wu@huawei.com>

       Author: Balazs Lengyel
               <mailto:balazs.lengyel@ericsson.com>

       Author: Hongwei Li
               <mailto:flycoolman@gmail.com>";

    description
      "This module defines a metadata annotation named 'immutable'
       to indicate the immutability of a particular instantiated
       data node. Any instantiated data node marked with
       immutable='true' by the server is read-only to the clients
       of YANG-driven management protocols, such as NETCONF,
       RESTCONF as well as SNMP and CLI requests.

       The module defines the immutable extension that indicates
       that data nodes based ona data-dafinition statement cannot
       be added removed or updated except according to the
       exceptions argument.

       Copyright (c) 2022 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 Revised
       BSD License set forth in Section 4.c of the IETF Trust's
       Legal Provisions Relating to IETF Documents
       (https://trustee.ietf.org/license-info).

       This version of this YANG module is part of RFC HHHH
       (https://www.rfc-editor.org/info/rfcHHHH); 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.";

    revision 2022-04-18 {
      description
        "Initial revision.";
      reference
        "RFC XXXX: Immutable Metadata Annotation";
    }

    extension immutable {
      argument exceptions;
      description
        "The 'immutable' extension as a substatement to a data
         definition statement indicates that data nodes based on
         the parent statement MUST NOT be added, removed or
         updated by management protocols, such as NETCONF,
         RESTCONF or other management operations (e.g., SNMP
         and CLI requests) except when indicated by the
         exceptions argument.

         Immutable data MAY be marked as config true to allow
         'leafref', 'when' or 'must' constraints to be based
         on it.

         The statement MUST only be a substatement of the leaf,
         leaf-list, container, list, anydata, anyxml statements.
         Zero or one immutable statement per parent statement
         is allowed.
         No substatements are allowed.

         The argument is a list of operations that are
         permitted to be used for the specified node, while
         other operations are forbidden by the immutable extension.
         - create: allows users to create instances of the data node
         - update: allows users to modify instances of the data node
         - delete: allows users to delete instances of the data node

         To disallow all user write access, omit the argument;

         To allow only create and delete user access, provide
         the string 'create delete' for the 'exceptions' parameter.

         Providing all 3 parameters has the same affect as not
         using this extension at all, but can be used anyway.

         Equivalent YANG definition for this extension:

         leaf immutable {
           type bits {
             bit create;
             bit update;
             bit delete;
           }
           default '';
         }


         Adding immutable or removing values from the
         exceptions argument of an existing immutable statement
         are non-backwards compatible changes.
         Other changes to immutable are backwards compatible.";
    }

    md:annotation immutable {
      type boolean;
      description
        "The 'immutable' annotation indicates the immutability of an
         instantiated data node. Any data node instance marked as
         'immutable=true' is read-only to clients and cannot be
         updated through NETCONF, RESTCONF or CLI. It applies to the
         list and leaf-list entries. The default is 'immutable=false'
         if not specified for an instance.";
    }
  }

<CODE ENDS>

8. References

Appendix B. Changes between revisions

Note to RFC Editor (To be removed by RFC Editor)¶

v01 - v02¶

• clarify the relation between the creation/deletion of the immutable data node with its parent data node;¶
• Add a "TODO" comment about the inheritance of the immutable property;¶
• Define that the server should reject write attempt to the immutable data node at an <edit-config> operation time, rather than waiting until a <commit> or <validate> operation takes place;¶

v00 - v01¶

• Added immutable extension¶
• Added new use-cases for immutable extension and annotation¶
• Added requirement that an update that means no effective change should always be allowed¶
• Added clarification that immutable is only applied to read-write datastore¶
• Narrowed the applied scope of metadata annotation to list/leaf-list instances¶