Content composition with Edge Side Includes

Varnish can create web pages by assembling different pages, called fragments, together into one page. These fragments can have individual cache policies. If you have a web site with a list showing the five most popular articles on your site, this list can probably be cached as a fragment and included in all the other pages.

Used properly this strategy can dramatically increase your hit rate and reduce the load on your servers.

In Varnish we've only so far implemented a small subset of ESI. As of version 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. At least if you look at the roadmap from a certain angle. During a full moon.

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:

#!/bin/sh

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

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

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

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

sub vcl_backend_response {
    if (bereq.url == "/test.html") {
       set beresp.do_esi = true; // Do ESI processing
       set beresp.ttl = 24 h;    // Sets the TTL on the HTML above
    } elseif (bereq.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 the end ("-->") when the page is processed, while still processing the contents. If the page is not processed, it will remain intact, becoming a 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:

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

Doing ESI on JSON and other non-XML'ish content

Please note that Varnish will peek at the included content. If it doesn't start with a "<" Varnish assumes you didn't really mean to include it and disregard it. You can alter this behaviour by setting the 'esi_syntax' parameter (see ref:ref-varnishd).