Re: Modeling conditional update based on existence of a dependent resource

On Tue, Jul 3, 2012 at 12:38 PM, Mike Kelly <[email protected]> wrote:
>
>
> On Tue, Jul 3, 2012 at 12:33 PM, Shaunak Kashyap <[email protected]>
> wrote:
>>
>>
>>
>> Hi folks,
>>
>>
>> I am looking to model an interaction in my REST application where the
>> client should only be allowed to modify a resource (say "B") if another
>> resource (say "A") exists.
>>
>> Using HTTP, if resource A exists, I would like PUT /uri/for/resource/B to
>> succeed. If resource B does not exist, I would like the same PUT request to
>> fail, responding with a status code that indicates to the client that
>> resource A does not exist. So what is a good status code to respond with in
>> the failure case? I considered 403 Forbidden but its semantics say that "the
>> request should not be repeated". This does not quite make sense in my
>> application since resource A might come into existence in the future, making
>> it okay for the client to re-issue the same PUT request for resource B.
>
>
> Would 404 not suffice here?
>
>>
>>
>> Alternatively (or additionally?), perhaps the client could issue a
>> conditional PUT on resource B which would only succeed if resource A
>> existed. How could I express such a condition in HTTP? I considered using
>> the If-Match request header but that seems to apply to representations of
>> the same resource (in my example, B) rather than of other resources (in my
>> example, A).
>
>
> You could use a Link header in the error response that has a relation type
> called something like "depends-on-existence", and point that link to
> resource A.

Or if you preferred the link to be in the body you could use
application/hal+json in the error response like so:

{
  "_links": {
    "depends-on-existence": { "href": "/uri/of/resource-A" }
  }
}

Cheers,
M