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 implemented a small subset of ESI, because most of the rest of the ESI specifications facilities are easier and better done with VCL:

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" />
-->

Footnotes about ESI

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

Varnish will peek at the first byte of an object and if it is not a "<" Varnish assumes you didn't really mean to ESI process it. You can disable this check by:

param.set feature +esi_disable_xml_check

Ignoring BOM in ESI objects

If you backend spits out a Unicode Byte-Order-Mark as the first bytes of the reponse, the "<" check will fail unless you set:

param.set feature +esi_remove_bom

ESI on invalid XML

The ESI parser expects the XML to be reasonably well formed, but this may fail if you are ESI including non-XML files. You can make the ESI parser disrecard anything but ESI tags by setting:

param.set feature +esi_ignore_other_elements

ESI includes with HTTPS protocol

If ESI:include tags specify HTTPS protocol, it will be ignored by default, because Varnish has no way to fetch it encryption enabled. If you want to treat HTTPS in ESI:include tags as if it were HTTP, set:

param.set feature +esi_ignore_https

ESI on partial responses (206)

Varnish can pass range requests but it is ESI processing a partial response makes no sense, so the fetch fails if you do ask for that. If you really know what you are doing, you can use this workaround:

sub vcl_backend_response {
    if (beresp.status == 206 && beresp.http.secret == "swordfish") {
        set beresp.do_esi = True;
        set beresp.status = 1206;
    }
}

ESI and return(vcl(...))

If the original client request switched to a different VCL using return(vcl(...)) in vcl_recv, any esi:include-requests will still start out in the same VCL as the original did, not in the one it switched to.