ESI Plugin

This plugin implements the ESI specification.


Supported ESI tags:

<!--esi ... -->

Extended ESI tags: esi:special-include

Supported variables:

$(HTTP_COOKIE{name}) or $(HTTP_COOKIE{name;subkey})

Note: the name is the key name such as “username”, “id” etc. For cookie support sub-name or sub-key, the format is: name;subkey, such as “l;u”, “l;t” etc. e.g. such cookie string: l=u=test&t=1350952328, the value of $(HTTP_COOKIE{“l;u”}) is test and the value of $(HTTP_COOKIE{“l;t”}) is 1350952328

Compilation and Installation

This plugin is considered stable and is included with Traffic Server by default. There are no special steps necessary for it to be built and installed.

Enabling ESI

  1. First, enable the ESI plugin by adding an entry for it in plugin.config. Here is an example of such an entry without passing any optional arguments to ESI:
  1. There are four optional arguments that can be passed to the above entry:

  • --private-response will add private cache control and expires headers to the processed ESI document.

  • --packed-node-support will enable the support for using the packed node feature, which will improve the performance of parsing cached ESI document. As mentioned below, this option is not extensively tested and is therefore not recommended for production environments

  • --disable-gzip-output will disable gzipped output for output which would not already be gzipeed anyway.

  • --first-byte-flush will enable the first byte flush feature, which will flush content to users as soon as the entire ESI document is received and parsed without all ESI includes fetched. The flushing will stop at the ESI include markup till that include is fetched.

  1. HTTP_COOKIE variable support is turned off by default. It can be turned on with -f <handler_config> or -handler <handler_config>. For example: -f handler.conf

The handler.conf file then contains the list of allowed cookie names. For example, to allow the A and LOGIN cookies, the file will look like the following:

allowlistCookie A
allowlistCookie LOGIN

You can also allow all cookies for HTTP_COOKIE variable by using a wildcard character. For example:

allowlistCookie *
  1. An entry in remap.config will be needed to map to the orginer server providing the ESI response. Assume that the ATS proxy is, your origin server is, and the URI containing ESI markup is In this case, the following line in remap.config will be needed:

  1. Your response should contain ESI markup and a response header of X-Esi: 1. Here is a PHP example:

<?php   header('X-Esi: 1'); ?>
Hello, <esi:include src=""/>
  1. You will also need a mapping for the resource in the ESI include ( in this case) in remap.config if it is not already present:


Or if both your ESI response and the ESI include comes from the same origin server, your remap.config entry can have the following single generic rule for all resources instead of separate rules for date.php and esi.php:

  1. Here is sample PHP content for date.php:

header ("Cache-control: no-cache");
echo date('l jS \of F Y h:i:s A');

Useful Notes

  1. With proper cache control headers for each, the ESI response and the ESI include responses can be cached separately. This is extremely useful for rendering a page with multiple modules. The page layout can be an ESI response with multiple ESI includes, each for a different module. Thus Traffic Server can have a single cached entry for the page layout ESI response while each individual ESI included responses can also be cached separately, each with a different duration per their cache-control headers.

  2. We do not recommend running the plugin with “packed node support” because it is not fully tested.

Differences from Spec -

  1. <esi:include> does not support “alt” and “onerror” attributes.

  2. <esi:inline> is not supported.

  3. You cannot have <esi:try> inside another <esi:try>.

  4. HTTP_USER_AGENT variable is not supported.

  5. HTTP_COOKIE supports fetching for sub-key.

  6. HTTP_HEADER supports accessing request headers as variables except “Cookie”.