Safety of setting "beresp.do_gzip" in vcl_backend_response

Dridi Boukelmoune dridi at
Thu Apr 13 11:27:49 CEST 2017

On Thu, Apr 13, 2017 at 8:44 AM, Guillaume Quintard
<guillaume at> wrote:
> You are right, subsequent requests will just be passed to the backend, so no
> gzip manipulation/processing will occur.

I had no idea [1] so I wrote a test case [2] to clear up my doubts:

    varnishtest "uncacheable gzip"

    server s1 {
        txresp -bodylen 100
    } -start

    varnish v1 -vcl+backend {
        sub vcl_backend_response {
            set beresp.do_gzip = true;
            set beresp.uncacheable = true;
            return (deliver);
    } -start

    client c1 {
    } -run

    varnish v1 -expect n_gzip == 1
    varnish v1 -expect n_gunzip == 1

Despite the fact that the response is not cached, it is actually
gzipped, because in all cases backend responses are buffered through
storage (in this case Transient). It means that for clients that don't
advertise gzip support like in this example, on passed transactions
you will effectively waste cycles on doing both on-the-fly gzip and
gunzip for a single client transaction.

That being said, it might be worth it if you have a high rate of
non-cacheable contents, but suitable for compression: less transient
storage consumption. I'd say it's a trade off between CPU and memory,
depending on what you wish to preserve you can decide how to go about

You can even do on-the-fly gzip on passed transactions only if the
client supports it and the backend doesn't, so that you save storage
and bandwidth, at the expense of CPU time you'd have consumed on the
client side if you wanted to save bandwidth anyway.

The only caveat I see is the handling of the built-in VCL:

> I am wondering if it is safe to do this even on responses that may
> subsequently get set as uncacheable by later code?

If you let your VCL flow through the built-in rules, then you have no
way to cancel the do_gzip if the response is marked as uncacheable.


[1] well I had an idea that turned out to be correct, but wasn't sure
[2] tested only with 5.0, but I'm convinced it is stable behavior for 4.0+

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