..
Copyright (c) 2012-2020 Varnish Software AS
SPDX-License-Identifier: BSD-2-Clause
See LICENSE file for full text of license
.. _users-guide-esi:
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.
.. XXX:What other pages? benc
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
Content substitution based on variables and cookies is not implemented.
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::
The time is:
at this very moment.
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
}
}
Note that ``set beresp.do_esi = true;`` is not required, and should
be avoided, for the included fragments, unless they also contains
```` instructions.
Example: esi:remove and
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The `` and `` 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 ("") 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 `` tags and all content contained
in them, allowing you to only render the content when the page is not
being ESI-processed.
For example::
The license
What happens when it fails ?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
By default, the fragments must have ``resp.status`` 200 or 204 or
their delivery will be considered failed.
Likewise, if the fragment is a streaming fetch, and that fetch
fails, the fragment delivery is considered failed.
If you include synthetic fragments, that is fragments created in
``vcl_backend_error{}`` or ``vcl_synth{}``, you must set
``(be)resp.status`` to 200 before ``return(deliver);``, for example
with a ``return (synth(200))`` or ``return (error(200))`` transition.
Failure to properly deliver an ESI fragment has no effect on its
parent request delivery by default. The parent request can include
the ESI fragment with an ``onerror`` attribute::
This attribute is ignored by default. In fact, Varnish will treat
failures to deliver ESI fragments as if there was the attribute
``onerror="continue"``. In the absence of this attribute with this
specific value, Varnish should normally abort the delivery of the
parent request.
We say "abort" rather than "fail", because by the time Varnish
starts inserting the fragments, the HTTP response header has long
since been sent, and it is no longer possible to change the parent
requests's ``resp.status`` to a 5xx, so the only way to signal that
something is amiss, is to close the connection in the HTTP/1 case or
reset the stream for h2 sessions.
However, it is possible to allow individual ``