Restful API: Is it meaningful to send PUT without enclosed entity? - rest

Question

Consider the following scenario:
There are two existing entity: shopping card #1 and item #1. I want to add item #1 to shopping card #1. There are two possible ways to design a Restful api:
1:
Without body:
PUT http://myshoppingsite.com/api/shoppingcards/1/items/1
Host: myshoppingsite.com
2: With body (having enclosed-entity):
PUT http://myshoppingsite.com/api/shoppingcards/1/items/
Host: myshoppingsite.com
{itemId: "1"}
Actually, I cannot decide which one is better and is more meaningful in terms of restfulness. Any idea?
(Note: I believe the http method should be PUT because of idempotency, but this is not my question here.)
PS: The problem I have with the first design is that there is no such enclosed entity in the request. Linguistically put is a transitive verb, so I expect somebody puts something somewhere. I think the same story somehow is in the HTTP world.

Answer 1

PUT is a HTTP verb that is supposed to create or replace the target URI, so this makes your first option immediately wrong. This request should replace all the items in your shopping cart:
PUT http://myshoppingsite.com/api/shoppingcards/1/items/
Since you want to add something to your shopping cart, this is not an option. This leaves effectively two options. First: the common one:
POST http://myshoppingsite.com/api/shoppingcards/1/items/
POST can mean many things, but in the context of REST services it's often used to append something to a collection. However, you mention that you want idempotence. You have two options here, first you can still use POST and within the context of your API guarantee that the request will be idempotent. Using POST does not mean that it's per definition non-idempotent, it just means that the HTTP spec alone does not guarantee it. That does not prevent you from making the request idempotent.
The other option is indeed PUT:
PUT http://myshoppingsite.com/api/shoppingcards/1/items/1
Your have a concern with that though, because in the context of your API you say that the request body would end up empty.
The reason for this is that you attach special to the last /1 in the url, and I think this what's wrong. If you want to follow REST best practices, then urls should not have any special meaning.
I think a saner way to do this, if you insist on using PUT is to get rid of the notion of "an id". That concept only exists in your database and should not make its way to the API.
Instead, I imagine that your service has a list of products such as this one:
http://myshoppingsite.com/products/1
To add a product to a shopping cart using PUT, this request might look something like this:
PUT http://myshoppingsite/api/shoppingcards/1/items/[completely-arbitrary-string-or-perhaps-a-uuid]
Content-Type: application/json
{
"product" : "http://myshoppingsite.com/products/1",
"quantity" : 5
}
Personally, I would just use POST though.
Q: there should be some information about the product you are addisng, no? Yes and that information is part of Uril. Why is it bad?
I'm not saying it's bad, I'm saying it's not RESTful. Pick up any book about REST and you will see this confirmed. This might also be a good place to start reading more about what REST is:
http://martinfowler.com/articles/richardsonMaturityModel.html
I would personally say that very little people build true RESTful services. This is why I also want to specifically point out that I don't want to say this is bad or good for your specific API, it's simply not RESTful.
What if I don't care about these principles and want to keep my special-meaning url scheme?
Well that's a fair point, but then we've gone beyond the original question here. If you want to design an API where the last bit of the url is actually the 'representation' of the item in your shopping cart, then yes I agree that having it also in the body is redundant.
In that situation I'd say, keep the request body empty. Just don't call it REST I guess.