varnish4 vs varnish3 - grace behaviour

Question

I'm currently working on a migration project from varnish3 to varnish4 and I am facing a behaviour that I don't understand.

To put it simply, with the same configuration file used both in Varnish3 and Varnish4, I don't have the same results concerning hits and misses.

This seems to be related to the grace attribute, but I don't figure out how it works.

So below is an example which describes the problem into details:

My varnish3 configuration:

# ----------------------------------------------
backend default {
    .host = "nginx";
    .port = "80";
}

sub vcl_fetch {
     set beresp.grace= 5s;
     set beresp.ttl = 1s;
}

sub vcl_deliver {
    if (obj.hits > 0) {
        set resp.http.X-Cache = "HIT";
    } else {
        set resp.http.X-Cache = "MISS";
    }
    set resp.http.X-Cache-Hits = obj.hits;
}
# ----------------------------------------------

My varnish4 configuration: (only change is the method name vcl_backend_response instead of vcl_fetch )

# ----------------------------------------------
vcl 4.0;

backend default {
    .host = "nginx";
    .port = "80";
}

sub vcl_backend_response {
     set beresp.grace= 5s;
     set beresp.ttl = 1s;
}

sub vcl_deliver {
    if (obj.hits > 0) {
        set resp.http.X-Cache = "HIT";
    } else {
        set resp.http.X-Cache = "MISS";
    }
    set resp.http.X-Cache-Hits = obj.hits;
}
# ----------------------------------------------

The result of a little scenario which retrieves headers of a given resource after some time:

# ----------------------------------------------
Time 14-47-12
VARNISH4:
X-Cache: MISS
X-Cache-Hits: 0
VARNISH3:
X-Cache: MISS
X-Cache-Hits: 0

Time 14-47-13
VARNISH4:
X-Cache: HIT
X-Cache-Hits: 1
VARNISH3:
X-Cache: MISS
X-Cache-Hits: 0

Time 14-47-20
VARNISH4:
X-Cache: MISS
X-Cache-Hits: 0
VARNISH3:
X-Cache: MISS
X-Cache-Hits: 0
# ----------------------------------------------

So as you can see in the scenario, for the first request, both v3 and v4 return a MISS, which is normal. But one second after, the second request returns a MISS for varnish3, which is normal for me, and a HIT for varnish4, that I don't really understand. As I suspected this was related to the grace parameter, I have added a third request in my scenario 7 seconds later ( greeter than 1sec for cache plus 5 seconds for grace), and as expected, both varnish 3 and varnish 4 are MISS.

So if someone can help me to understand / workaround this problem... The objective for me is to get the same result with varnish4 than with varnish3 (while I'm migrating from 3 to 4 ;) ). My current workaround is to set beresp.grace=1ms on varnish4, but I don't like that at all, and I can't do that on every of my configurations :(

Any help would be very appreciated !

Thanks in advance ! :)

Answer 1

Varnish 4 delivers a stale object (see How objects are stored) in the second request.

I suggest that you read Grace mode, and play with Understanding Grace using varnishtest.