Compression

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.

Default behaviour

The default behaviour is active when the http_gzip_support parameter is set to “on” and neither beresp.do_gzip nor beresp.do_gunzip are used in VCL.

Unless returning from vcl_recv with pipe or pass, Varnish modifies req.http.Accept-Encoding: if the client supports gzip req.http.Accept-Encoding is set to “gzip”, otherwise the header is removed.

Unless the request is a pass, Varnish sets bereq.http.Accept-Encoding to “gzip” before vcl_backend_fetch runs, so the header can be changed in VCL.

If the server responds with gzip’ed content it will be stored in memory in its compressed form and Accept-Encoding will be added to the Vary header.

To clients supporting gzip, compressed content is delivered unmodified.

For clients not supporting gzip, compressed content gets decompressed on the fly while delivering it. The Content-Encoding response header gets removed and any Etag gets weakened (by prepending “W/”).

For Vary Lookups, Accept-Encoding is ignored.

Compressing content if backends don’t

You can tell Varnish to compress content before storing it in cache in vcl_backend_response by setting beresp.do_gzip to “true”, like this:

sub vcl_backend_response {
    if (beresp.http.content-type ~ "text") {
        set beresp.do_gzip = true;
    }
}

With beresp.do_gzip set to “true”, Varnish will make the following changes to the headers of the resulting object before inserting it in the cache:

• set obj.http.Content-Encoding to “gzip”
• add “Accept-Encoding” to obj.http.Vary, unless already present
• weaken any Etag (by prepending “W/”)

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.

Please make sure that you don’t try to compress content that is uncompressable, like JPG, GIF and MP3 files. You’ll only waste CPU cycles.

Uncompressing content before entering the cache

You can also uncompress content before storing it in cache by setting beresp.do_gunzip to “true”. One use case for this feature is to work around badly configured backends uselessly compressing already compressed content like JPG images (but fixing the misbehaving backend is always the better option).

With beresp.do_gunzip set to “true”, Varnish will make the following changes to the headers of the resulting object before inserting it in the cache:

• remove obj.http.Content-Encoding
• weaken any Etag (by prepending “W/”)

GZIP and ESI

If you are using Edge Side Includes (ESI) 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.

Turning off gzip support

When the http_gzip_support parameter is set to “off”, Varnish does not do any of the header alterations documented above, handles Vary: Accept-Encoding like it would for any other Vary value and ignores beresp.do_gzip and beresp.do_gunzip.

A random outburst

Poul-Henning Kamp has written How GZIP, and GZIP+ESI works in Varnish which talks a bit more about how the implementation works.