HTTP Status Code for Resource not yet available suggests returning a 409 conflict response, rather than a 404 response, in the case that a resource doesn't exist because it is in the middle of being generated.

From the w3 spec:

10.4.10 409 Conflict

The request could not be completed due to a conflict with the current state of the resource. This code is only allowed in situations where it is expected that the user might be able to resolve the conflict and resubmit the request. The response body SHOULD include enough

information for the user to recognize the source of the conflict. Ideally, the response entity would include enough information for the user or user agent to fix the problem; however, that might not be possible and is not required.

Conflicts are most likely to occur in response to a PUT request. For example, if versioning were being used and the entity being PUT included changes to a resource which conflict with those made by an earlier (third-party) request, the server might use the 409 response to indicate that it can't complete the request. In this case, the response entity would likely contain a list of the differences between the two versions in a format defined by the response Content-Type.

This is slightly awkward, since the 409 code is "only allowed in situations where it is expected that the user might be able to resolve the conflict and resubmit the request." I suggest the response body includes a message (possibly in some a response format matching the rest of your API) like, "This resource is currently being generated. It was initiated at [TIME] and is estimated to complete at [TIME]. Please try again later."

Note that I would only suggest the 409 approach if it is highly likely that the user who is requesting the resource is also the user who initiated generation of that resource. Users not involved with the generation of the resource would find a 404 error less confusing.

HTTP Status Code for Resource not yet available suggests returning a 409 conflict response, rather than a 404 response, in the case that a resource doesn't exist because it is in the middle of being generated.

From the w3 spec:

10.4.10 409 Conflict

The request could not be completed due to a conflict with the current state of the resource. This code is only allowed in situations where it is expected that the user might be able to resolve the conflict and resubmit the request. The response body SHOULD include enough

information for the user to recognize the source of the conflict. Ideally, the response entity would include enough information for the user or user agent to fix the problem; however, that might not be possible and is not required.

Conflicts are most likely to occur in response to a PUT request. For example, if versioning were being used and the entity being PUT included changes to a resource which conflict with those made by an earlier (third-party) request, the server might use the 409 response to indicate that it can't complete the request. In this case, the response entity would likely contain a list of the differences between the two versions in a format defined by the response Content-Type.

This is slightly awkward, since the 409 code is "only allowed in situations where it is expected that the user might be able to resolve the conflict and resubmit the request." I suggest the response body includes a message (possibly in some a response format matching the rest of your API) like, "This resource is currently being generated. It was initiated at [TIME] and is estimated to complete at [TIME]. Please try again later."

Note that I would only suggest the 409 approach if it is highly likely that the user who is requesting the resource is also the user who initiated generation of that resource. Users not involved with the generation of the resource would find a 404 error less confusing.