I am currently using Enterprise Architect to model the API. For these projects, you have users who are given a budget to purchase certain products. The model is not yet complete, but I am adding it here as a reference.
In EA, I added some additional stereotypes: path, resource, and view + the new ParamQuery data type.
To model the API, you create a new class diagram and start from the root of your API as the class of the stereotype "Application". Then you add your resources and paths. For each resource you add the necessary operations GET, POST, PUT, etc. If QueryParams are passed in a string for GET operations, you add them as parameters for the operation, but as a QueryParam type (you must create it for the first time). For the body of the POST operations message, you add a parameter to the type of class that it represents.
Then you add the "view" classes used by the Resources.

Chris aelbrecht
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