Varnish Enterprise



The image vmod provides a tool for image modification and optimization.


Convert to WebP

This example assumes that you want to convert all JPEG and PNG images to WebP. If this is not the case, see the comment in vcl_backend_response below to adjust to your needs. The example also includes error handling, which makes it longer than most other examples.

import image;
import headerplus;

sub vcl_recv {
	if (req.http.Accept ~ "image/webp") {
		set req.http.X-Varnish-Webp = "enabled";
	} else {
		unset req.http.X-Varnish-Webp;
	unset req.http.X-Varnish-Webp-Result;

sub vcl_backend_response {
	# The following "if" test controls which assets should be
	# converted to WebP. If you want a different set than all
	# JPEG and PNG images, you need to updates the contents
	# of this "if" test accordingly.
	if (beresp.http.Content-Type == "image/jpeg" ||
		beresp.http.Content-Type == "image/png")
		if (bereq.http.X-Varnish-Webp && bereq.retries == 0) {
			set bereq.http.X-Varnish-Webp-Result = "active";
		headerplus.attr_set("Vary", "X-Varnish-Webp");

sub vcl_backend_error {
	if (bereq.retries == 0
		&& bereq.http.X-Varnish-Webp-Result == "active") {
		set bereq.http.X-Varnish-Webp-Result = "failed";
		# Getting here is usually because conversion
		# failed. On the next go, we will not try to
		# convert, as bereq.retries will be 1
		return (retry);

The above uses Vary on a self-maintained header (X-Varnish-Webp) to facilitate serving both the original image and the webp version as different variants, and makes sure that Vary is updated for exactly the assets where we might attempt WebP conversion.

In addition to this, the header X-Varnish-Webp-Result is used to keep track of what has happened. The header will make it easier to debug as you can see the header updates in the log, and even filter requests by this header value. The X-Varnish-Webp-Result can also be used by the backend to understand what Varnish is doing.

Note that if we get bereq.http.X-Varnish-Webp-Result = "failed", then this means that the following has happened, in order:

  • Varnish has gotten a full set of backend response headers
  • VCL processing of the backend response headers has happened in sub vcl_backend_response where conversion to WebP was turned on.
  • The image VMOD turned off streaming (equivalent to set beresp.do_stream = false; in VCL), tried to fetch the entire response body and convert it.
  • An error occurred, either when fetching the body or trying to convert the image.
  • As a result of the error happening when streaming was disabled, sub vcl_backend_error was called.
  • In sub vcl_backend_error, a retry of the backend fetch was initiated.

In the scenario above, the second backend fetch will be happen, and if the fetch is successful, an object will be inserted. Subsequent fetches from clients accepting WebP will then get the original - no additional conversion will happen until the inserted objects has left the cache.



VOID webp([INT quality = 75], [INT resize_width = 0], [INT resize_height = 0], [BOOL aspect_lock = 0], [INT crop_x = 0], [INT crop_y = 0], [INT crop_width = 0], [INT crop_height = 0], [BOOL lossless = 0], [INT lossless_level = 6])

Convert an image to the WebP format and store it in cache. Optionally scale, resize, crop, or change the quality of conversion. Scaling is done by using only one of resize_width or resize_height. quality and lossless are mutually exclusive options. The supported content types to convert are image/png and image/jpeg. When called this function disables streaming. It should not be re-enabled for proper functionality.

Note: The area to crop is defined by X,Y coordinates from the top left corner and then the desired width and height.

Note: When using this function to both crop and rescale an image, the crop will take place prior to rescaling.

Note: quality is a number between 0 and 100, a lower number will result in a smaller size but lower quality. mode is a number between 0 and 9, a lower number is faster but produces a larger file size.

Note: aspect_lock will ensure that the resulting image obeys the aspect ratio of the original image. If the desired resize values do not obey this ratio then the correct dimensions will be calculated using the smallest of the two input values, with resize_width being prioritized if both values are identical.


  • quality accepts type INT with a default value of 75 optional

  • resize_width accepts type INT with a default value of 0 optional

  • resize_height accepts type INT with a default value of 0 optional

  • aspect_lock accepts type BOOL with a default value of 0 optional

  • crop_x accepts type INT with a default value of 0 optional

  • crop_y accepts type INT with a default value of 0 optional

  • crop_width accepts type INT with a default value of 0 optional

  • crop_height accepts type INT with a default value of 0 optional

  • lossless accepts type BOOL with a default value of 0 optional

  • lossless_level accepts type INT with a default value of 6 optional

Type: Function

Returns: None

Restricted to: vcl_backend_response


The image VMOD is available in the varnish-plus-vmods-extra package.