Is it wrong to return 202 "Accepted" in response to HTTP GET?

I have a set of resources whose representations are lazily created. The computation to construct these representations can take anywhere from a few milliseconds to a few hours, depending on server load, the specific resource, and the phase of the moon.

The first GET request received for the resource starts the computation on the server. If the computation completes within a few seconds, the computed representation is returned. Otherwise, a 202 "Accepted" status code is returned, and the client must poll the resource until the final representation is available.

The reason for this behavior is the following: If a result is available within a few seconds, it needs to be retrieved as soon as possible; otherwise, when it becomes available is not important.

Due to limited memory and the sheer volume of requests, neither NIO nor long polling is an option (i.e. I can't keep nearly enough connections open, nor even can I even fit all of the requests in memory; once "a few seconds" have passed, I persist the excess requests). Likewise, client limitations are such that they cannot handle a completion callback, instead. Finally, note I'm not interested in creating a "factory" resource that one POSTs to, as the extra roundtrips mean we fail the piecewise realtime constraint more than is desired (moreover, it's extra complexity; also, this is a resource that would benefit from caching).

I imagine there is some controversy over returning a 202 "Accepted" status code in response to a GET request, seeing as I've never seen it in practice, and its most intuitive use is in response to unsafe methods, but I've never found anything specifically discouraging it. Moreover, am I not preserving both safety and idempotency?

So, what do folks think about this approach?

EDIT: I should mention this is for a so-called business web API--not for browsers.

If it's for a well-defined and -documented API, 202 sounds exactly right for what's happening.

If it's for the public Internet, I would be too worried about client compatibility. I've seen so many if (status == 200) hard-coded.... In that case, I would return a 200.

Also, the RFC makes no indication that using 202 for a GET request is wrong, while it makes clear distinctions in other code descriptions (e.g. 200).

> The request has been accepted for processing, but the processing has not been completed.

We did this for a recent application, a client (custom application, not a browser) POST'ed a query and the server would return 202 with a URI to the "job" being posted - the client would use that URI to poll for the result - this seems to fit nicely with what was being done.

The most important thing here is anyway to document how your service/API works, and what a response of 202 means.

From what I can recall - GET is supposed to return a resource without modifying the server. Maybe activity will be logged or what have you, but the request should be rerunnable with the same result.

POST on the other hand is a request to change the state of something on the server. Insert a record, delete a record, run a job, something like that. 202 would be appropriate for a POST that returned but isn't finished, but not really a GET request.

It's all very puritan and not well practiced in the wild, so you're probably safe by returning 202. GET should return 200. POST can return 200 if it finished or 202 if it's not done.

http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html

In case of a resource that is supposed to have a representation of an entity that is clearly specified by an ID (as opposed to a "factory" resource, as described in the question), I recommend staying with the GET method and, in a situation when the entity/representation is not available because of lazy-creation or any other temporary situation, use the 503 Service Unavailable response code that is more appropriate and was actually designed for situations like this one.

Reasoning for this can be found in the RFCs for HTTP itself (please verify the description of the 503 response code), as well as on numerous other resources.

Please compare with https://stackoverflow.com/questions/4642923/http-status-code-for-temporarily-unavailable-pages. Although that question is about a different use case, it actually relates to the exact same feature of HTTP.