Wouldn't it be easier to add a retry=true to the request, at least if your implementing exponential back off in an existing codebase? Then you just hash the request (minus the retry field) in your cache layer and send the saved response again. You'll need a little extra code to update timestamps on the client side but the server side can be implemented as middleware in most frameworks. Expire the cache after a reasonable total timeout and make the client do a new read if they try to retry after cache has cleared it.