In Varnish 3.0 we introduced native support for compression, using gzip encoding. Before 3.0, Varnish would never compress objects.
In Varnish 4.0 compression defaults to “on”, meaning that it tries to be smart and do the sensible thing.
If you don’t want Varnish tampering with the encoding you can disable compression all together by setting the parameter ‘http_gzip_support’ to false. Please see man varnishd for details.
The default behaviour for Varnish is to check if the client supports our compression scheme (gzip) and if it does it will override the ‘Accept-Encoding’ header and set it to “gzip”.
When Varnish then issues a backend request the ‘Accept-Encoding’ will then only consist of “gzip”. If the server responds with gzip’ed content it will be stored in memory in its compressed form. If the backend sends content in clear text it will be stored in clear text.
You can make Varnish compress content before storing it in cache in vcl_backend_response by setting ‘do_gzip’ to true, like this:
sub vcl_backend_response {
if (beresp.http.content-type ~ "text") {
set beresp.do_gzip = true;
}
}
Please make sure that you don’t try to compress content that is uncompressable, like jpgs, gifs and mp3. You’ll only waste CPU cycles. You can also uncompress objects before storing it in memory by setting ‘do_gunzip’ to true but that will usually not be the most sensible thing to do. Generally, Varnish doesn’t use much CPU so it might make more sense to have Varnish spend CPU cycles compressing content than doing it in your web- or application servers, which are more likely to be CPU-bound.
If you are using Edge Side Includes (ESIs) you’ll be happy to note that ESI and GZIP work together really well. Varnish will magically decompress the content to do the ESI-processing, then recompress it for efficient storage and delivery.
If the client does not support gzip the ‘Accept-Encoding’ header is left alone then we’ll end up serving whatever we get from the backend server. Remember that the backend might tell Varnish to Vary on the ‘Accept-Encoding’.
If the client does not support gzip but we’ve already got a compressed version of the page in memory Varnish will automatically decompress the page while delivering it.
Poul-Henning Kamp has written How GZIP, and GZIP+ESI works in Varnish which talks a bit more about how the implementation works.