Preflight redirect caching

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#1st generation

Request arrives in fastly, backend is set to point to our preflight app, response comes back, we decorate the request with headers (including a flag to indicate preflight has successfully happened) then restart. This time it goes through to our router, or cached response if it exists Note - code examples simplified to exclude error handling etc

vcl_recv () {
   if (req.http.preflighted) {
    // set backend to router then
    return(fetch);
   } else {
     // set backend to preflight then
     return (pass)
    }
}

vcl_fetch () {
   if (req.backend == preflight) {
      req.http.thing = beresp.http.thing
      return(restart);
   }
}

2nd generation

Same as above, but now preflight sometimes serves redirects, so in vcl_fetch we deliver these straight to the end user

vcl_recv () {
   if (req.http.preflighted) {
    // set backend to router then
    return(fetch);
   } else {
     // set backend to preflight then
     return (pass)
    }
}

vcl_fetch () {
   if (req.backend == preflight) {
    if (beresp.status == 301 || beresp.status == 302) {
			return (deliver);
		}
    req.http.thing = beresp.http.thing
    return(restart);
   }
}

3rd Generation

Same as above, but now we'd like to cache the redirects. The code below works - we get a cache hit - but it has a very damaging effect on uncachable preflight requests. For some reason an extra restart is triggered which causes the request to go to preflight twice. We've tried playing about with vcl_hit and vcl_miss to no avail

vcl_recv () {
   if (req.http.preflighted) {
    // set backend to router then
    return(fetch);
   } else {
     // set backend to preflight then
     // lookup rather than pass because some requests are cachable
     return (lookup)
    }
}

vcl_hit () {
   if (req.backend == preflight) {
    return(deliver);
   }
}

vcl_fetch () {
   if (req.backend == preflight) {
    if (beresp.status == 301 || beresp.status == 302) {
			return (deliver);
		}
    req.http.thing = beresp.http.thing
    return(restart);
   }
}