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Distinct Message Ratio (DMR)

The Distinct Message Ratio metric (DMR) represents a measure for the complexity/cohesion of data types in an interface. This metric is significantly modified from the original version [1]. It is the ratio between distinct messages and all messages that are inside an interface. DMR takes values between 0 and 1, where lower is considered better. Messages are either:

  • an input message: the combination of path parameters, query parameters, and the request body. The input data can be seen as a single data object which has the different parameters as sub-properties. Usually, each operation has exactly one input message.
  • an output message: the different responses of an operation, e.g. the 200 OK response and error schemes. Usually, an operation has exactly one valid response plus error messages as output.

In the example below are 3 messages from GET /pets

  • {limit}
  • {$ ref: "#/components/schemas/Pets"}
  • {$ ref: "#/components/schemas/Error"}

and 3 messages from GET /pets/{age}

  • {limit, age}
  • {$ ref: "#/components/schemas/Pets"}
  • {$ ref: "#/components/schemas/Error"}

To calculate DMR, we divide the count of distinct messages that occur only once in the whole API ({limit} and {limit, age}, i.e. 2) by the number of all messages (6):

DMR = 2 / 6 = 1 / 3

The fictional OpenAPI V3 specification below describes two paths each containing one operation.

paths:
    /pets:
        get:
            parameters:
                - name: limit
                  in: query
                  schema:
                    type: integer
            responses:
                '200':
                    description: An array of pets
                    content:
                        application/json:
                            schema:
                                $ref: "#/components/schemas/Pets"
                default:
                    description: unexpected error
                    content:
                        application/json:
                            schema:
                                $ref: "#/components/schemas/Error"

    /pets/{age}:
        get:
            parameters:
                - name: limit
                  in: query
                  schema:
                    type: integer
                - name: age
                  in: path
                  required: true
                  schema:
                    type: integer
            responses:
                '200':
                    description: An array of pets with specified age
                    content:
                        application/json:
                            schema:
                                $ref: "#/components/schemas/Pets"
                default:
                    description: unexpected error
                    content:
                        application/json:
                            schema:
                                $ref: "#/components/schemas/Error"

components:
    schemas:
        Pet:
            properties:
                age:
                    type: integer
                name:
                    type: string

        Pets:
            type: array
            items:
                $ref: "#/components/schemas/Pet"

        Error:
            properties:
                message:
                    type: string

Source

[1] Baski, D., & Misra, S. (2011). Metrics suite for maintainability of extensible markup language Web Services. IET software, 5(3), 320-341.