Edge Side Includes

Edge Side Includes is a language to include fragments of web pages in other web pages. Think of it as HTML include statement that works over HTTP.

On most web sites a lot of content is shared between pages. Regenerating this content for every page view is wasteful and ESI tries to address that letting you decide the cache policy for each fragment individually.

In Varnish we've only implemented a small subset of ESI. As of 2.1 we have three ESI statements:

  • esi:include
  • esi:remove
  • <!--esi ...-->

Content substitution based on variables and cookies is not implemented but is on the roadmap.

Varnish will not process ESI instructions in HTML comments.

Example: esi:include

Lets see an example how this could be used. This simple cgi script outputs the date:


echo 'Content-type: text/html'
echo ''
date "+%Y-%m-%d %H:%M"

Now, lets have an HTML file that has an ESI include statement:

The time is: <esi:include src="/cgi-bin/date.cgi"/>
at this very moment.

For ESI to work you need to activate ESI processing in VCL, like this:

sub vcl_fetch {
    if (req.url == "/test.html") {
       set beresp.do_esi = true; /* Do ESI processing               */
       set beresp.ttl = 24 h;    /* Sets the TTL on the HTML above  */
    } elseif (req.url == "/cgi-bin/date.cgi") {
       set beresp.ttl = 1m;      /* Sets a one minute TTL on        */
                                 /*  the included object            */

Example: esi:remove and <!--esi ... -->

The <esi:remove> and <!--esi ... --> constructs can be used to present appropriate content whether or not ESI is available, for example you can include content when ESI is available or link to it when it is not. ESI processors will remove the start ("<!--esi") and end ("-->") when the page is processed, while still processing the contents. If the page is not processed, it will remain, becoming an HTML/XML comment tag. ESI processors will remove <esi:remove> tags and all content contained in them, allowing you to only render the content when the page is not being ESI-processed. For example:

  <a href="http://www.example.com/LICENSE">The license</a>
<p>The full text of the license:</p>
<esi:include src="http://example.com/LICENSE" />

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