CPS Delta between an anchor and JSON payload API proposal

Overview

The second API proposed under the Delta feature is to generate a delta between configuration stored under an Anchor and JSON payload provided by the user. All the responses and exceptions thrown by this API will be similar to the API to generate delta between 2 anchors to maintain consistency between the API's.

Issues & Decisions

HTTP response codes for Delta between Anchor and Payload API

Proposed API

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

Request parameters:

Alternative proposed API

This alternative API takes in an additional query parameter as an input, where in the user can provide the Schema Context in form of a Yang file. This yang file will only be used to generate the data nodes from the json payload and will not be persisted in CPS DB.

Request parameters:

Request body:

Response Body/Delta Report Format

[
  {
    "action": "ADD",
    "xpath": "/bookstore/categories/[@code=3]",
    "target-data": {
      "code": 3,
      "name": "kidz"
    }
  },
  {
    "action": "DELETE",
    "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"
    }
  }
]

Alternative approach for schema free delta report.

Problem Statement

The problem with 2nd Delta API is that it needs to compare any JSON payload to data stored under an anchor. This JSON payload can be from any source and regardless of its schema a delta should be generated between the payload and the data under the anchor.

The preferable approach here would be to parse the JSON payload into DataNodes and then compare the generated DataNodes against the DataNodes under the anchor. This would allow us to have a delta report which is in line with CPS and would enable us to provide crucial detail about the data nodes that is their xpath.

But since the second API accepts any JSON string as an input, one cannot be always sure that the payload will have the same schema as the anchor. And hence the approach of parsing to DataNodes can only happen when the schemas are identical.

In cases where schemas are not identical, there as soon as we attempt to parse the payload to DataNode, CPS will throw an exception terminating the parsing.

The most feasible alternative is to make use of JSON Nodes along with JSON path. Where JSON Nodes and JSON Path are used as a replacement for DataNodes and xPaths respectively.

Proposed Solutions

Use of JSON Nodes and JSON Path

Another alternative approach can be use of JSON Nodes and JSON Path as a replacement for DataNodes and xPaths respectively. In this approach, we can use JSON Paths to identify if a JSON Node has been added or deleted and if two identical JSON Paths exist in source anchor and the JSON payload then a comparison of JSON Nodes will be performed.

In order to achieve this the overall approach will be divided into two parts,

First, we determine if the JSON payload has the same schema as the data within the anchor. To do so the payload will be parsed against the schema information retrieved from the anchor and DataNodes will be formulated using this schema. If the parsing of JSON payload to DataNodes fails, then this would mean the schema differs and we switch to the alternative approach. If the parsing is successful we can go ahead with generating delta between the DataNodes.

If the parsing fails then, the data from source anchor needs to be fetched as a JSON string. And then both the JSON payload and the data from the anchor will be parsed to JSON Nodes and their respective JSON paths.

Once this is done a separate algorithm will be used to find the delta between JSON Nodes.

JSON Path

The most suitable candidate to be used as replacement for xpath are JSON path, a sample JSON path looks like this: 

$.store.book[0].title

Here:

• $ symbol refers to the root object or element.
• . operator is the dot-child operator, which you use to denote a child element of the current element.
• [ ] is the subscript operator, which you use to denote a child element of the current element (by name or index).

In order to utilize the JSON Path a new algorithm will need to be implemented to generate JSON paths from the available JSON Nodes.

Also, the existing delta algorithm can then be modified and reused for JSON path and JSON nodes.

Separation of Added Nodes from the JSON payload

Another approach to determine the delta between the anchor data and JSON payload can be implemented as follows.

• Step 1: The anchor data and JSON payload are fetched as JSON strings.
• Step 2: The JSON payload is compared to the JSON string from the anchor and all the newly added data nodes in JSON payload are extracted to a separate JSON string, while simultaneously removing this newly added data from the JSON payload.
• Step 3: After step 2 the JSON payload will be split into two JSON strings, one containing the Unmodified, Updated and Deleted data and second containing only the Added data.
• Step 4: The JSON string from anchor and the JSON string containing the Unmodified, Updated and Deleted data can then be parsed into DataNodes, as at this point, we can be sure that all the data in the particular JSON string will belong to the schema, which can then be used by the existing algorithm to find the delta. This part of delta report will contain the action, xpath and source/target data.
• Step 5: The added data can then be put in the delta report, but since this approach assumes that all the added data nodes are not part of the schema, it is not possible to determine their xpath and hence the field for xpath can be redacted for added data nodes in this approach.

Exapmle:

{
  "test:bookstore": {
    "bookstore-name": "Chapters/Easons",
    "categories": [
      {
        "code": "01/1",
        "name": "SciFi",
        "books": [
          {
            "authors": [
              "Iain M. Banks"
            ],
            "lang": "en/it",
            "price": "895",
            "pub_year": "1994",
            "title": "Feersum Endjinn/Endjinn Feersum"
          },{
            "authors": [
              "Ursula K. Le Guin"
            ],
            "lang": "en",
            "price": "1099",
            "pub_year": "1999",
            "title": "Far Horizons"
          }
        ]
      },{
        "name": "Horror",
        "code": "03",
        "books": [
          {
            "authors": [
              "ABC"
            ],
            "lang": "en",
            "price": "699",
            "pub_year": "1995",
            "title": "Horror Book"
          }
        ]
      }
    ],
  }
}

{
  "test:bookstore": {
    "bookstore-name": "Chapters/Easons",
    "categories": [
      {
        "code": "01/1",
        "name": "SciFi",
        "books": [
          {
            "authors": [
              "Iain M. Banks"
            ],
            "lang": "en/it",
            "price": "895",
            "pub_year": "1994",
            "title": "Feersum Endjinn/Endjinn Feersum"
          },
          {
            "authors": [
              "Ursula K. Le Guin"
            ],
            "lang": "en",
            "price": "1099",
            "pub_year": "1999",
            "title": "Far Horizons"
          }
        ]
      },
      {
        "name": "kids",
        "code": "02",
        "books": [
          {
            "authors": [
              "Philip Pullman"
            ],
            "lang": "en",
            "price": "699",
            "pub_year": "1995",
            "title": "The Golden Compass"
          }
        ]
      },
      {
        "name": "Horror",
        "code": "03",
        "books": [
          {
            "authors": [
              "ABC"
            ],
            "lang": "en",
            "price": "699",
            "pub_year": "1995",
            "title": "Horror Book"
          }
        ]
      }
    ],
    "bookstore-phone": "123",
    "bookstore-emp": { }
  },
  "test:bookstore2": {
    "bookstore-name": "Test"
  }
}

{
  "test:bookstore" : {
    "categories" : [ {
      "name" : "kids",
      "code" : "02",
      "books" : [ {
        "authors" : [ "Philip Pullman" ],
        "lang" : "en",
        "price" : "699",
        "pub_year" : "1995",
        "title" : "The Golden Compass"
      } ]
    } ],
    "bookstore-phone" : "123",
    "bookstore-emp" : { }
  },
  "test:bookstore2" : {
    "bookstore-name" : "Test"
  }
}

{
  "test:bookstore": {
    "bookstore-name": "Chapters/Easons",
    "categories": [
      {
        "code": "01/1",
        "name": "SciFi",
        "books": [
          {
            "authors": [
              "Iain M. Banks"
            ],
            "lang": "en/it",
            "price": "895",
            "pub_year": "1994",
            "title": "Feersum Endjinn/Endjinn Feersum"
          },
          {
            "authors": [
              "Ursula K. Le Guin"
            ],
            "lang": "en",
            "price": "1099",
            "pub_year": "1999",
            "title": "Far Horizons"
          }
        ]
      },
      {
        "name": "Horror",
        "code": "03",
        "books": [
          {
            "authors": [
              "ABC"
            ],
            "lang": "en",
            "price": "699",
            "pub_year": "1995",
            "title": "Horror Book"
          }
        ]
      }
    ]
  }
}

Mechanism for Delta generation between Anchor and Payload