Return a synthetic response then fetch and cache object in Varnish?

Question

I'm wondering if my (possibly strange) use case is possible to implement in Varnish with VCL. My application depends on receiving responses from a cacheable API server with very low latencies (i.e. sub-millisecond if possible). The application is written in such a way that an "empty" response is handled appropriately (and is a valid response in some cases), and the API is designed in such a way that non-empty responses are valid for a long time (i.e. days).

So, what I would like to do is configure varnish so that it:

1. Attempts to look up (and return) a cached response for the given API call
2. On a cache miss, immediately return an "empty" response, and queue the request for the backend
3. On a future call to a URL which was a cache miss in #2, return the now-cached response

Is it possible to make Varnish act in this way using VCL alone? If not, is it possible to write a VMOD to do this (and if so, pointers, tips, etc, would be greatly appreciated!)

Answer 1

I don't think you can do it with VCL alone, but with VCL and some client logic you could manage it quite easily, I think.

In vcl_miss, return an empty document using error 200 and set a response header called X-Try-Again in the default case. In the client app, when receiving an empty response with X-Try-Again set, request the same resource asynchronously but add a header called X-Always-Fetch to the request. Your app does not wait for the response or do anything with it once it arrives. Also in vcl_miss, check for the presence of the same X-Always-Fetch header. If present, return (fetch) instead of the empty document. This will request the content from the back end and cache it for future requests.

I also found this article which may provide some help though the implementation is a bit clunky to me compared to just using your client code: http://lassekarstensen.wordpress.com/2012/10/11/varnish-trick-serve-stale-content-while-refetching/