You can return whatever HTTP response code you want, but if you care about knowing whether your site is working being about to look at the logs and see "That user requested a page that doesn't exist" being different to "That user requested a page that exists but had no results" is quite useful. In coding terms it's the difference between a null and an empty array.
You can do that with filtering, which should be a feature of every single logging tools.
Anyway, I agree that when you filter via queries, an empty list is more valid response than 404. That HTTP status should be returned IMHO when the requested (for example by id) item is not found (and of course with wrong paths, etc).
In this case I don't think the status should depend on the number of results. Here are you results, [] is a valid response body when there are no result. Returning 404 if there are no result (GET /books?title=a for instance) is misleading, the caller may think that /books is a non existent route and may conclude that books are reachable via another URI. To me, the querystring has no influence on the response status.
/books/1 could return 200 or 404 depending on the existence of the book#1, here it make sense because if /books/1 does not exist the API must tell it explicitly. However 404 belongs to the 4XX family which means "client error", is it an error to ask for a non existing book ? If you enter in a bookshop and ask for a book they don't have you did not "make a mistake". It's not like if you asked for a chainsaw. But in an API, especially with hypermedia, you are not supposed to request a resource that does not exist (unless the API provides a link to an existing resource that is was deleted before the caller try to reach it).
If you enter a bookshop and you ask for a book that does not exist then it's definitely your mistake.
If you ask for a book they don't have it's a different matter.
In any case, when you ask for a book in a library you are using their "search" endpoint. The equivalent to opening a books/1 url would be asking for a specific instance of a book by serial number or so. Then it's clear that you made a mistake uf you do that for an unexistent serial number...
A response code of 204 seems more appropriate but the problem is you're not allowed to send further information, which would make that descriptive response... not descriptive enough.
Yes, and this is obvious if /users/ exists and returns a 400 if the ID is required. That way you can tell the difference between /users/ being there and expecting and ID, and it not being there.
Of course it is technically possible, but doing so would violate the spec.
> The 404 (Not Found) status code indicates that the origin server did not find a current representation for the target resource or is not willing to disclose that one exists.
In the above case, the server _is_ returning a representation.
Another reason not to return a 404 in that case is that chances there will be monitor tooling in place that will treat a 404 as an "error" that will show up in your alerting, but would not be ideal; it will just be noise.
