GET & POST passing parameters - rest

Question

I've read many question on stackoverflow about passing parameters with GET and POST methods, but none of them satisfied my curiosity.
I use GETs to retrieve data passing parameters as path/query params and use POSTs to save data passing parameters as request body with Content-Type: application/json and sometimes request body + path/query params (depending on other APIs I've created for simmetry purpose only).
My question is when to use Content-Type: application/x-www-form-urlencoded. Say I want to call a POST without a request body of Content-Type: application/json but with params, do I have to use the application/x-www-form-urlencoded Content-Type or I can use path/query params as best practice?
What's the difference between sending data as path/query params and key-value params with Content-Type: application/x-www-form-urlencoded? Does the data size have to do with it?

Answer 1

What's the difference between sending data as path/query params and key-value params with Content-Type: application/x-www-form-urlencoded?
Consider this example
PUT /example?a=b&c=d
Content-Type: application/x-www-form-urlencoded
d=e&f=g
What's happening here? This is a request that the server replace its current representation of the resource identified by
/example?a=b&c=d
With the payload. In other words, after successful processing, we would expect
GET /example?a=b&c=d
to produce a response like
200 OK
Content-Type: application/x-www-form-urlencoded
d=e&f=g
POST /example?a=b&c=d
Content-Type: application/x-www-form-urlencoded
d=e&f=g
For POST, it's the same idea, except that instead of "replace the current representation", POST stands in for "process the payload according to the specific semantics of /example?a=b&c=d".
GETs to retrieve data passing parameters as path/query params
It might help to reframe your thinking here. We use GET to retrieve the current representation of a resource. We aren't passing parameters, we're passing a document (resource) identifier.
The fact that the origin server's implementation is going to parse that identifier is an implementation detail.
A URI Template is a compact sequence of characters for describing a
range of Uniform Resource Identifiers through variable expansion.
-- RFC 6570
I want to call a POST without a request body of Content-Type: application/json but with params, do I have to use the application/x-www-form-urlencoded Content-Type or I can use path/query params as best practice?
POST /example?a=b&c=d
Content-Type: application/json
{"d":"e","f":"g"}
Is a perfectly normal HTTP request.
Content-Type describes the payload only - it has nothing at all to do with the target URI, and how information might be encoded within it.