# How GZIP, and GZIP+ESI works in Varnish

First of all, everything you read about GZIP here, is controlled by the parameter:

> http_gzip_support

Which defaults to “on” if you do not want Varnish to try to be smart about compression, set it to “off” instead.

## What does http_gzip_support do

A request which is sent into ‘pipe’ or ‘pass’ mode from vcl_recv{} will not experience any difference, this processing only affects cache hit/miss requests.

Unless vcl_recv{} results in “pipe” or “pass”, we determine if the client is capable of receiving gzip’ed content. The test amounts to:

> Is there a Accept-Encoding header that mentions gzip, and if is has a q=# number, is it larger than zero.

Clients which can do gzip, gets their header rewritten to:

> Accept-Encoding: gzip

And clients which do not support gzip gets their Accept-Encoding header removed. This ensures conformity with respect to creating Vary: strings during object creation.

During lookup, we ignore any “Accept-encoding” in objects Vary: strings, to avoid having a gzip and gunzip’ed version of the object, varnish can gunzip on demand. (We implement this bit of magic at lookup time, so that any objects stored in persistent storage can be used with or without gzip support enabled.)

Varnish will not do any other types of compressions than gzip, in particular we will not do deflate, as there are browser bugs in that case.

Before vcl_miss{} is called, the backend requests Accept-Encoding is always set to:

> Accept-Encoding: gzip

Even if this particular client does not support

To always entice the backend into sending us gzip’ed content.

Varnish will not gzip any content on its own (but see below), we trust the backend to know what content can be sensibly gzip’ed (html) and what can not (jpeg)

If in vcl_backend_response{} we find out that we are trying to deliver a gzip’ed object to a client that has not indicated willingness to receive gzip, we will ungzip the object during deliver.

## Tuning, tweaking and frobbing

In vcl_recv{} you have a chance to modify the client’s Accept-Encoding: header before anything else happens.

In vcl_pass{} the clients Accept-Encoding header is copied to the backend request unchanged. Even if the client does not support gzip, you can force the A-C header to “gzip” to save bandwidth between the backend and varnish, varnish will gunzip the object before delivering to the client.

In vcl_miss{} you can remove the “Accept-Encoding: gzip” header, if you do not want the backend to gzip this object.

In vcl_backend_response{} two new variables allow you to modify the gzip-ness of objects during fetch:

> set beresp.do_gunzip = true;

Will make varnish gunzip an already gzip’ed object from the backend during fetch. (I have no idea why/when you would use this…)

> set beresp.do_gzip = true;

Will make varnish gzip the object during fetch from the backend, provided the backend didn’t send us a gzip’ed object.

Remember that a lot of content types cannot sensibly be gziped, most notably compressed image formats like jpeg, png and similar, so a typical use would be:

    sub vcl_backend_response {
            if (bereq.url ~ "html$") {
                    set beresp.do_gzip = true;
            }
    }

## GZIP and ESI

First, note the new syntax for activating ESI:

    sub vcl_backend_response {
            set beresp.do_esi = true;
    }

In theory, and hopefully in practice, all you read above should apply also when you enable ESI, if not it is a bug you should report.

But things are vastly more complicated now. What happens for instance, when the backend sends a gzip’ed object we ESI process it and it includes another object which is not gzip’ed, and we want to send the result gziped to the client ?

Things can get really hairy here, so let me explain it in stages.

Assume we have a ungzipped object we want to ESI process.

The ESI parser will run through the object looking for the various magic strings and produce a byte-stream we call the “VEC” for Varnish ESI Codes.

The VEC contains instructions like “skip 234 bytes”, “deliver 12919 bytes”, “include /foobar”, “deliver 122 bytes” etc and it is stored with the object.

When we deliver an object, and it has a VEC, special esi-delivery code interprets the VEC string and sends the output to the client as ordered.

When the VEC says “include /foobar” we do what amounts to a restart with the new URL and possibly Host: header, and call vcl_recv{} etc. You can tell that you are in an ESI include by examining the ‘req.esi_level’ variable in VCL.

The ESI-parsed object is stored gzip’ed under the same conditions as above: If the backend sends gzip’ed and VCL did not ask for do_gunzip, or if the backend sends ungzip’ed and VCL asked for do_gzip.

Please note that since we need to insert flush and reset points in the gzip file, it will be slightly larger than a normal gzip file of the same object.

When we encounter gzip’ed include objects which should not be, we gunzip them, but when we encounter gunzip’ed objects which should be, we gzip them, but only at compression level zero.

So in order to avoid unnecessary work, and in order to get maximum compression efficiency, you should:

    sub vcl_miss {
            if (object needs ESI processing) {
                    unset req.http.accept-encoding;
            }
    }

    sub vcl_backend_response {
            if (object needs ESI processing) {
                    set beresp.do_esi = true;
                    set beresp.do_gzip = true;
            }
    }

So that the backend sends these objects uncompressed to varnish.

You should also attempt to make sure that all objects which are esi:included are gziped, either by making the backend do it or by making varnish do it.