Why not? I wouldn’t say „I performed the search and there’s 0 results“ is an error condition. It’s just the result of a search, and everything went fine.
Hm, maybe? I guess it depends on what we mean by search; if myapp.com/search?someproduct finds that there are 0 matches then yeah that's probably a 200, but if myapp.com/products/123456 fails because no product has id 123456 then that's a textbook 404.
It’s both nonsense, cause what you see here is a double conversion from an arbitrary problem domain into http domain and back again. Using a specific http code together with an app-domain code could make sense iff you wanted an intermediate host (a proxy etc) to perform some additional operation based on that status. Otherwise http status doesn’t speak the call language and can be just OK. (400/500 should still be handled by a client).
Back-and-forth conversion is a very poor idea. It works for what was “internet resources” initially (basically files and folders), but later people stretched that on application data models and that creates constant issues because people naturally can’t understand the mapping, cause there’s none. This is not a good idea. Talk to http hosts with http and talk to your client with a language you designed specifically for talking to it. 200 vs non-200 is http level and orthogonal to in-service statuses.
