RESTful API - Correct behaviour when spurious/not requested parameters are passed in the request

Question

We are developing a RESTful api that accepts query parameters in the request in the form of JSON encoded data.

We were wondering what is the correct behaviour when <

Answer 1

Imagine I have the following JSON schema:

{
   "frequency": "YEARLY",
   "date": 23,
   "month": "MAY",
}

The frequency attribute accepts "WEEKLY", "MONTHLY" and "YEARLY" value. The expected payload for "WEEKLY" frequency value is:

{
   "frequency": "WEEKLY",
   "day": "MONDAY",
}

And the expected payload for "MONTHLY" frequency value is:

{
   "frequency": "MONTHLY",
   "date": 23,
}

Give the above JSON schema, typically I will have need a POJO containing frequency, day, date, and month fields for deserialization.

If the received payload is:

{
   "frequency": "MONTHLY",
   "day": "MONDAY",
   "date": 23,
   "year": 2018
}

I will throw an error on "day" attribute because I will never know the intention of the sender:

1. frequency: "WEEKLY" and day: "MONDAY" (incorrect frequency value entered), or
2. frequency: "MONTHLY" and date: 23

For the "year" attribute, I don't really have choice. Even if I wish to throw an error for that attribute, I may not be able to. It's ignored by the JSON serialization/deserialization library as my POJO has no such attribute. And this is the behavior of GSON and it makes sense given the design decision.

Navigating the Json tree or the target Type Tree while deserializing

When you are deserializing a Json string into an object of desired type, you can either navigate the tree of the input, or the type tree of the desired type. Gson uses the latter approach of navigating the type of the target object. This keeps you in tight control of instantiating only the type of objects that you are expecting (essentially validating the input against the expected "schema"). By doing this, you also ignore any extra fields that the Json input has but were not expected.

As part of Gson, we wrote a general purpose ObjectNavigator that can take any object and navigate through its fields calling a visitor of your choice.

Extracted from GSON Design Document