api - PUT or POST when making idempotnet request for specific use-case -

i'm designing rest api web application.

my principles while designing api are:

• use client use cases perspective rather data model perspective. motivation: abstract db schema.
• each slash represent new use case/action.

lets in application have users, products, locations, product-news. use case user follows product-news location. if location empty user follows news product each location.
list of products , locations known.

what right method adding user follower of specific product-location combination?

i end following url:

/product/follow?product=<product_name>[&location=<location name>] 

the product , location names in query part because more flexible extend in future.

• the argument put: of course request idempotent - sending multiple times doesn't make other change sending once.
• the argument post: don't specify url, under resource set.

personally i'm closer put because use-case principle api (which consider right one) impotency rule seams corresponding one.

first identify resources. i'd that

/products/<product_name>/followers 

is list of users following product_name. can filter using query parameters:

/products/<product_name>/followers?location=<location_name> 

is list of users following product_name location_name.

a get request against such resource returns representation (json, xml, ...) of list of users.

post or put

a post request against such resource adds new user. request body of such post request contains details.

post /products/<product_name>/followers content-type: application/json  {   "user": "some user",   "location: "some location" } 

the server respond:

201 created location: /products/<product_name>/followers/some%20user 

if client knows uri, can use put:

put /products/<product_name>/followers/some%20user content-type: application/json  {   "location: "some location" } 

the server again respond:

201 created location: /products/<product_name>/followers/some%20user 

summary

if client able know uri, can use put. the url can known server, use post.