Varnish VCL - cache misses get an instant 302 response and are fetched in the background

miss_302.vcl

#
# Cache misses get an instant 302 response and are fetched in the background
# (Include this file in your VCL)
#

vcl 4.1;

import std;
import synthbackend;
import vha;

sub vcl_recv
{
    unset req.http.miss-bgfetch;
}

sub vcl_miss
{
    set req.http.miss-bgfetch = "true";
    return (fetch);
}

sub vcl_backend_fetch
{
    if (bereq.http.miss-bgfetch) {
        set bereq.backend = synthbackend.from_string("");
        return (fetch);
    }
}

sub vcl_backend_response
{
    if (bereq.http.miss-bgfetch) {
	# Insert a stale 302 response
	set beresp.status = 302;
        set beresp.http.Location = bereq.url;
        set beresp.http.Cache-Control = "private";
        set beresp.http.Retry-After = "1";

        set beresp.ttl = 0.001s;
        set beresp.grace = 24h;
        vha.set_insertion_time(now - 1s);

        return (deliver);
    }
}

sub vcl_deliver
{
    if (req.http.miss-bgfetch) {
        # Restart, cache hit on the 302, and trigger a bgfetch
        std.rollback(req);
        return (restart);
    }
}

# EOF