Issues & Decisions

CPS Delta feature Exceptions

Wherever possible the Delta Feature will throw the same exceptions as defined in CPS core. If any new exception for the delta feature is required, the following will be updated here.

HTTP response codes for Delta between 2 Anchors API

The proposed API will be part of the CPS Data Interface. The following response codes will be returned by the API:

Request parameters:

Response Body/Delta Report Format

[
  {
    "action": "ADD",
    "xpath": "/bookstore/categories/[@code=3]",
    "target-data": {
      "code": 3,
      "name": "kidz"
    }
  },
  {
    "action": "REMOVE",
    "xpath": "/bookstore/categories/[@code=1]",
    "source-data": {
      "code": 1,
      "name": "Fiction"
    }
  },
  {
    "action": "UPDATE",
    "xpath": "/bookstore/categories/[@code=2]",
    "source-data": {
      "name": "Funny"
    },
    "target-data": {
      "name": "Comic"
    }
  }
]

Mechanism for Delta generation between Anchors

Additional Details

Format/Conventions to be used for Delta Report

The delta report format is based on two RFCs namely RFC 6902 and RFC 9144 . A detailed comparison of the RFCs can be found here.

RFC 6902 JSON Patch

JSON Patch defines a JSON document structure for expressing a sequence of operations to apply to a JavaScript Object Notation(JSON) document; it is suitable for use with the HTTP PATCH method.

But in it can also be used to get the differences between two JSON as the JSON patch document represents an array of objects. where each object represents a single operation(op), path and value.

The notation of "operation" is used because the same JSON Patch document can be used to perform HTTP Patch operations. But the operations field can be used as a reference to decide how the "action" field should function in the implementation of Delta report.

The operation "op" field in a JSON patch document can have following values:

[
     { "op": "test", "path": "/a/b/c", "value": "foo" },
     { "op": "remove", "path": "/a/b/c" },
     { "op": "add", "path": "/a/b/c", "value": [ "foo", "bar" ] },
     { "op": "replace", "path": "/a/b/c", "value": 42 },
     { "op": "move", "from": "/a/b/c", "path": "/a/b/d" },
     { "op": "copy", "from": "/a/b/d", "path": "/a/b/e" }
   ]

RFC 9144 Data Model

RFC 9144 YANG data model that defines RPCs intended to be used in conjunction with NETCONF [RFC6241] or RESTCONF [RFC8040]. These RFCs allow a client to request a server to compare two NMDA datastores and report any differences.

Comparison of RFC 9144 Data Model and RFC6902 JSON Patch format

• The core of the RFC 9144 solution is a new operator, <compare>, that compares the data tree contents of two datastores. The operation checks whether there are any differences in values or in data nodes that are contained in either datastore and then returns any differences as output. The output is returned in the format specified below.
• The RFC6902 approach to Delta report generation follows the JSON patch format. A JSON Patch document represents an array of objects, where each object contains exactly one operation, path and associated values. The operation can have following values: add, remove, replace, move, copy and test. The path represents the JSON patch format and the values contain the difference in source and target values.

{ "ietf-nmda-compare:output" : {
    "differences" : {
      "ietf-yang-patch:yang-patch" : {
        "patch-id" : "interface status",
        "comment" : "diff between intended (source) and operational",
        "edit" : [
          {
            "edit-id" : "1",
            "operation" : "replace",
            "target" : "/ietf-interfaces:interface=eth0/enabled",
            "value" : {
               "ietf-interfaces:interface/enabled" : "false"
            },
            "source-value" : {
               "ietf-interfaces:interface/enabled" : "true",
               "@ietf-interfaces:interface/enabled" : {
                 "ietf-origin:origin" : "ietf-origin:learned"
               }
             }
          },
          {
            "edit-id" : "2",
            "operation" : "create",
            "target" : "/ietf-interfaces:interface=eth0/description",
            "value" : {
              "ietf-interface:interface/description" : "ip interface"
            }
          }
        ]
      }
    }
  }
}

PATCH /my/data HTTP/1.1
   Host: example.org
   Content-Length: 326
   Content-Type: application/json-patch+json
   If-Match: "abc123"
 
   [
     { "op": "test", "path": "/a/b/c", "value": "foo" },
     { "op": "remove", "path": "/a/b/c" },
     { "op": "add", "path": "/a/b/c", "value": [ "foo", "bar" ] },
     { "op": "replace", "path": "/a/b/c", "value": 42 },
     { "op": "move", "from": "/a/b/c", "path": "/a/b/d" },
     { "op": "copy", "from": "/a/b/d", "path": "/a/b/e" }
   ]

CPS Delta Report Format

Based on the RFC documentations mentioned above, CPS Delta Report is designed in the format specified in the following code block. This Delta Report takes the features from both RFCs that are best suited for CPS and fulfill its requirements.

The format of Delta report has following key take aways from the above mentioned RFCs:

• The "op" field from RFC 6902 is replaced with "action" field because in CPS we want to report the action that was performed on the data. And this field can have 3 predefined values: ADD, REMOVE and UPDATE.
• The xpath is used in CPS to uniquely identify individual data nodes and is used in place of JSON path as defined in the two RFCs
• The source-data and target-data fields are used from RFC9144 to report the data that has been added, removed or updated. This approach properly categorizes the data instead of grouping it under the one "value" field as in RFC6902.

[
  {
    "action": "ADD",
    "xpath": "/bookstore/categories/[@code=3]",
    "target-data": {
      "code": "3,",
      "name": "kidz"
    }
  },
  {
    "action": "REMOVE",
    "xpath": "/bookstore/categories/[@code=1]",
    "source-data": {
      "code": "1,",
      "name": "Fiction"
    }
  },
  {
    "action": "UPDATE",
    "xpath": "/bookstore/categories/[@code=2]",
    "source-data": {
      "name": "Funny"
    },
    "target-data": {
      "name": "Comic"
    }
  }
]

Proposed Algorithm

The JSON data stored in CPS can be retrieved as Maps, And using this we can find the delta between two anchors retrieved as two separate Maps. The result can be stored in an JSON array with appropriate notations for action, xpath and payload. The following algorithm can be used to find the difference between the two maps and also recursively finds the difference between the child data nodes. The response is in form of a JSON Array which closely represents the JSON Patch format as described above.

Algorithm:

1. Create an empty JSON Array, to store the result
   • The JSON array will contain the following: action, xpath, payload as individual JSON objects each object representing a singular operation.
2. Fetch the data from two anchors and store in two separate Maps
3. Iterate over the entries of first map
4. For each entry of first map, check if the key is present in second map.
5. If the key is not found in second map, it means that the key-value pair has been removed from the second map. Create a JSON Object with DELETE action, the xpath to deleted node and payload. Add the Object to the JSON Array.
6. If the key is present in the second map, compare the values associated with the keys in both maps.
7. If the values are instance of Map, recursively call the comparision algorithm to compare all nested maps. Add all the necessary fields into a JSON object and put the object into the JSON Array
8. If the values are not equal, it means the key-value pair was updated. Create a JSON Object with UPDATE action, the xpath to updated nodes and payload. Add the Object to the JSON Array.
9. Iterate over the keys of second map and find the keys not present in first map. These keys were added as new entries. Create a JSON Object with ADD action, the xpath to added node and payload. Add the Object to the JSON Array.
10. Return the JSON Array and the updates to Kafka events

References/Future Changes

• CPS-2310: Refactoring of CPS Delta APIs