Working with Edge Side Includes

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Gateway caches are a great way to make your website perform better. But they have one limitation: they can only cache whole pages. If your pages contain dynamic sections, such as the user name or a shopping cart, you are out of luck. Fortunately, Symfony provides a solution for these cases, based on a technology called ESI, or Edge Side Includes. Akamai wrote this specification in 2001 and it allows specific parts of a page to have a different caching strategy than the main page.

The ESI specification describes tags you can embed in your pages to communicate with the gateway cache. Only one tag is implemented in Symfony, include, as this is the only useful one outside of Akamai context:

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<!DOCTYPE html>
<html>
    <body>
        <!-- ... some content -->

        <!-- Embed the content of another page here -->
        <esi:include src="http://..."/>

        <!-- ... more content -->
    </body>
</html>

Note

Notice from the example that each ESI tag requires a fully-qualified URL. An ESI tag represents a page fragment that can be fetched via the given URL.

When a request is handled, the gateway cache fetches the entire page from its cache or requests it from the backend application. If the response contains one or more ESI tags, these are processed in the same way. In other words, the gateway cache either retrieves the included page fragment from its cache or requests the page fragment from the backend application again. When all the ESI tags have been resolved, the gateway cache merges each into the main page and sends the final content to the client.

All of this happens transparently at the gateway cache level (i.e. outside of your application). As you'll see, if you choose to take advantage of ESI tags, Symfony makes the process of including them almost effortless.

Using ESI in Symfony

First, to use ESI, be sure to enable it in your application configuration:

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# app/config/config.yml
framework:
    # ...
    esi: { enabled: true }

Now, suppose you have a page that is relatively static, except for a news ticker at the bottom of the content. With ESI, you can cache the news ticker independently of the rest of the page:

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// src/AppBundle/Controller/DefaultController.php

// ...
class DefaultController extends Controller
{
    public function aboutAction()
    {
        $response = $this->render('static/about.html.twig');
        // sets the shared max age - which also marks the response as public
        $response->setSharedMaxAge(600);

        return $response;
    }
}

In this example, the full-page cache has a lifetime of ten minutes. Next, include the news ticker in the template by embedding an action. This is done via the render helper (see How to Embed Controllers in a Template for more details).

As the embedded content comes from another page (or controller for that matter), Symfony uses the standard render helper to configure ESI tags:

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{# app/Resources/views/static/about.html.twig #}

{# you can use a controller reference #}
{{ render_esi(controller('AppBundle:News:latest', { 'maxPerPage': 5 })) }}

{# ... or a URL #}
{{ render_esi(url('latest_news', { 'maxPerPage': 5 })) }}

By using the esi renderer (via the render_esi() Twig function), you tell Symfony that the action should be rendered as an ESI tag. You might be wondering why you would want to use a helper instead of just writing the ESI tag yourself. That's because using a helper makes your application work even if there is no gateway cache installed.

Tip

As you'll see below, the maxPerPage variable you pass is available as an argument to your controller (i.e. $maxPerPage). The variables passed through render_esi also become part of the cache key so that you have unique caches for each combination of variables and values.

When using the default render() function (or setting the renderer to inline), Symfony merges the included page content into the main one before sending the response to the client. But if you use the esi renderer (i.e. call render_esi()) and if Symfony detects that it's talking to a gateway cache that supports ESI, it generates an ESI include tag. But if there is no gateway cache or if it does not support ESI, Symfony will just merge the included page content within the main one as it would have done if you had used render().

Note

Symfony detects if a gateway cache supports ESI via another Akamai specification that is supported out of the box by the Symfony reverse proxy.

The embedded action can now specify its own caching rules entirely independently of the master page:

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// src/AppBundle/Controller/NewsController.php
namespace AppBundle\Controller;

// ...
class NewsController extends Controller
{
    public function latestAction($maxPerPage)
    {
        // ...
        $response->setSharedMaxAge(60);

        return $response;
    }
}

With ESI, the full page cache will be valid for 600 seconds, but the news component cache will only last for 60 seconds.

When using a controller reference, the ESI tag should reference the embedded action as an accessible URL so the gateway cache can fetch it independently of the rest of the page. Symfony takes care of generating a unique URL for any controller reference and it is able to route them properly thanks to the FragmentListener that must be enabled in your configuration:

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# app/config/config.yml
framework:
    # ...
    fragments: { path: /_fragment }

One great advantage of the ESI renderer is that you can make your application as dynamic as needed and at the same time, hit the application as little as possible.

Caution

The fragment listener only responds to signed requests. Requests are only signed when using the fragment renderer and the render_esi Twig function.

Note

Once you start using ESI, remember to always use the s-maxage directive instead of max-age. As the browser only ever receives the aggregated resource, it is not aware of the sub-components, and so it will obey the max-age directive and cache the entire page. And you don't want that.

The render_esi helper supports two other useful options:

alt
Used as the alt attribute on the ESI tag, which allows you to specify an alternative URL to be used if the src cannot be found.
ignore_errors
If set to true, an onerror attribute will be added to the ESI with a value of continue indicating that, in the event of a failure, the gateway cache will remove the ESI tag silently.
This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.
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