[bct]

> what resource is it acting on?

The distance between the two cities.

> You have to come up with some added, unnecessary, implementation-exposing abstraction like "CityPair".

This is a really weird thing to say. No, you don't. I have no idea why you would think that.

> What are the four CRUD operations for it? [...] In which case "delete" could be ill-defined

Not every method has to be valid for every resource. It's a total non-issue that you can't delete a distance.

(And POST/GET/PUT/DELETE is a very different concept from CRUD.)

[SilasX]

>>what resource is it acting on?

>The distance between the two cities.

Don't be cute. What server resource is it acting on? The value of the distances is (potentially) computed by some encapsulated algorithm -- you're not acting on that resource. The server resources that are touched are the two cities, and then whatever it does behind the scenes.

>This is a really weird thing to say. No, you don't. I have no idea why you would think that.

Because I'm not GETing a city, so I can't use the GET operation on the city resource; I have to make another resource to GET. Fine, it doesn't have to be a city pair: but there is a many-to-many mapping, which in REST-favoring frameworks (e.g. Rails), requires a separate table. By reducing everything to CRUD, you must create a new resource (type) for each new operation.

>(And POST/GET/PUT/DELETE is a very different concept from CRUD.)

That's a non-standard definition of "very different", considering that POST is create, GET is retrieve, PUT is update, and DELETE is delete.

[jon_moore]

GET /cityDistance?city1=Chicago&city2=Austin

Also, while POST can mean "create", it can also mean "append" or "process some arbitrary request."

[SilasX]

>GET /cityDistance?city1=Chicago&city2=Austin

Yep! Works like a charm, until you have to expose a URI pointing to every combination of cities (or indeed, combination of any parameter set).

And I know (like I said before) you can fall back on "no, just tell the user where to put the parameters and you won't have to do that!" ... which is just re-inventing the RPC -- and satisfying users that don't want to navigate a long session just to find the URI they want, every time they send a request.

>Also, while POST can mean "create", it can also mean "append" or "process some arbitrary request."

Used correctly, it doesn't mean (that the sender is requesting that you) "process some arbitrary request"; it should only be used for non-idempotent operations. Close enough to summarize as "create" (appending is certainly creating something in this context!), and generally, for something to have different effects when repeated, you have to create something. PUT/update and DELETE/delete are idempotent specifically because the changes they make aren't creations.

In any case it's clearly an abuse of the term "very different concept from" in the GGP's comment "And POST/GET/PUT/DELETE is a very different concept from CRUD".