POST that doesn't create a resource - rest

Question

Assume the system manages users. Users are exposed via the following URL - /users. A particular user is exposed via the following URL - /users/{id}. Users have reports exposed via the following URL - /users/{id}/reports.
One operation consists of generating a report. The appropriate HTTP request is a POST on /users/{id}/reports. However, under certain conditions, a generated report would be exactly the same as the last generated report. Therefore, I thought that returning the last generated report in this case is a good approach.
I also know that in such case, no resource will be created.
Is there a correct RESTful way to handle this case? Maybe returning a special code?

Answer 1

Is there a correct RESTful way to handle this case? Maybe returning a special code?
Stepping back for a moment: a perfectly straight forward way to handle the "create" use case looks like
client POSTs a request to /users/1/reports
the origin server creates a new resource and calculates a new identifier for this resource (/users/1/reports/a)
the server returns a response that indicates that a new resource has been created, the location of that resource, and its current representation.
The indication that a new resource has been created is the status-code: 201.
The location of the newly created resource is described by the Location response header.
The location of the content is described by the Content-Location response header
The current representation is the message body of the response (no surprise).
HTTP/1.1 201 Created
Location: /users/1/reports/a
Content-Location: /users/1/reports/a
...
<representation of the report goes here>
In your case, where the resource already exists, then things look pretty much the same. To avoid implying that we have created a new resource, the status-code is changed to 200, and the Location header is dropped.
HTTP/1.1 200 OK
Content-Location: /users/1/reports/a
...
<representation of the report goes here>
If you prefer that the client retrieve the report representation using the identifier of the previously generated report, then you should use 303 See Other
It is primarily used to allow the output of a POST action to redirect the user agent to a selected resource, since doing so provides the information corresponding to the POST response in a form that can be separately identified, bookmarked, and cached, independent of the original request.
HTTP/1.1 303 See Other
Location: /users/1/reports/a
...
This pattern is commonly referred to as Post/Redirect/Get

Answer 2

i'd use 304 Not Modified in cases where the report is not modified. This should tell everyone, that the ressource didn't change since the last export and normally no further content is transmitted. This could also be used to instead refer to your older results if you cache those. Generally the 304 is not used for posts, but the use of a post to jus trigger the creation of a log can be considered a bit exotic as well.
If the client has performed a conditional GET request and access is allowed, but the document has not been modified, the server SHOULD respond with this status code. The 304 response MUST NOT contain a message-body, and thus is always terminated by the first empty line after the header fields.
RFC containing explanation of the 304 Status Code
If the creation worked i'd send a 201 created and use the location header as pointer to the new file.