CPS core Get Data Node Inconsistencies

With recent developments in the CPS core API's a few inconsistencies that were observed with the Get Data Node API. The goal of this wiki is to provide the analysis of these inconsistencies, how these functional inconsistencies compare against the Query API and some possible solutions to them.

Issues & Decisions

Inconsistent response when retrieving data nodes.

When a get operation is performed on Root node xpath (/) it returns an array list containing all the data nodes under the root node xpath. But if the same operation is performed for a List Data node no array list is returned instead a 404 Not Found response is returned.

/v2/dataspaces/my-dataspace/anchors/multipleTop/node?xpath=/&descendants=3

[
    {
        "stores:bookstore": {
            "store-name": "My Bookstore",
            "store-owner": "James",
            "categories": [
                {
                    "name": "Test book",
                    "price": 100,
                    "stock": false,
                    "book-category": "SciFi"
                }
            ]
        }
    },
    {
        "stores:electronics-store": {
            "store-name": "My Electronics store",
            "category": [
                {
                    "status": true,
                    "address": "India",
                    "store-type": "electronics"
                }
            ]
        }
    }
]

/v2/dataspaces/my-dataspace/anchors/multipleTop/node?xpath=/bookstore/categories&descendants=3

{
    "status": "404 NOT_FOUND",
    "message": "DataNode not found",
    "details": "DataNode with xpath /bookstore/categories was not found for anchor multipleTop and dataspace my-dataspace."
}

Similarly, when the comparing the Get Data Node API to Query API, it was observed that when a Query is performed on a List data node then all the list items are returned, but when the same operation is performed using Get Data Node API it returns 404 Not Found as response.

Requesting an entire List using Query API

/v2/dataspaces/my-dataspace/anchors/multipleTop/nodes/query?cps-path=/bookstore/categories&descendants=3

[
    {
        "stores:categories": {
            "name": "Cook book",
            "price": 500,
            "stock": true,
            "book-category": "Cooking"
        }
    },
    {
        "stores:categories": {
            "name": "Test book",
            "price": 100,
            "stock": false,
            "book-category": "SciFi"
        }
    }
]

Reason for the inconsistency

When we look at the way the data is being stored in CPS DB we can see that when storing a List Data Node, the entire list is not stored under a single xPath referring to the list data node.

Instead, each individual list item is stored in the CPS DB with its along with its unique xPath which consists of the name of List Data node and the corresponding "key" leaf or "key" attribute.

And hence there is currently the entire list cannot be retrieved by the xPath for the List Data Node and only the individual list items can be retrieved.

In the above table we can see that the the individual list items of the list "Categories" are stored with respect to their unique xpaths and no xPath such as /bookstore/categories exists in the DB.

Possible Solutions

Implement a logic to retrieve all data nodes when a list is requested

One approach could be to implement logic to retrieve all the list items when a List data node is queried, similar to the approach taken in Query API.

This would require a custom logic in Get Data Node API which would fetch all the list items when a xpath for a list data node is sent in the get request. This would make the Get Data node API consistent with the Query API.

A sample request in this case would look something like:

/v2/dataspaces/my-dataspace/anchors/multipleTop/node?xpath=/bookstore/categories&descendants=3

Using a special character in the xPath for Get Data Node API

By using a special character as part of the attribute in xPath we can implement a logic to retrieve the entire List data node.

For instance, to retrieve the entire list we can use an asterisk symbol in the attributes part of the xPath . And a logic can be implemented where if the asterisk is present in the attribute part of the xPath then CPS should return the entire list data node.

/v2/dataspaces/my-dataspace/anchors/multipleTop/node?xpath=/bookstore/categories[@category=*]&descendants=3

In the above request our xPath looks something looks like this:

/bookstore/categories[@type=*]

[
    {
        "stores:categories": {
            "name": "Cook book",
            "price": 500,
            "stock": true,
            "book-category": "Cooking"
        }
    },
    {
        "stores:categories": {
            "name": "Test book",
            "price": 100,
            "stock": false,
            "book-category": "SciFi"
        }
    }
]

Redirect request to the Query API

Another proposed approach is to redirect the request to the Query API when a list data node is requested.

Note: The problem with this approach is to identify whether the request data node is a list or not, by using the xpath provided in the Get request. Also it is not feasible to redirect the request to Query API.