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This Symfony bundle enables Edgestash support for Varnish Enterprise to Twig.
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ThijsFerynEdgestashTwigBundle

The ThijsFerynEdgestashTwigBundle is a Symfony bundle that adds Edgestash support to Twig.

Edgestash is a Varnish Enterprise module that processes Mustache templates on the edge.

Edgestash allows for JSON data to be composed into a response using Mustache syntax, ex: {{variable}}. Each response can be assigned its own JSON object for truly dynamic responses. JSON can either be fetched from a backend or generated via VCL and any number of JSONs can be assigned to a template.

This bundle offers a couple of custom Twig functions and filters that facilitate the use of Edgestash in an unobtrusive way, allowing you to efficiently cache personalized data.

Installing ThijsFerynEdgestashTwigBundle

ThijsFerynEdgestashTwigBundle is available on Packagist and can be installed using Composer with the following command:

composer require thijsferyn/edgestash-twig-bundle

How does Edgestash work?

The VCL example on the Edgestash documentation page shows how the Mustache template and the JSON data are cached separately.

VCL stands for Varnish Configuration Language and is the domain-specific programming language that controls the behavior of Varnish.

The edgestash.parse_response() method will process the template and identify the placeholders that require parsing.

Edgestash offers 3 different ways to ingest JSON data used to replace the template placeholders:

Once the JSON is injected, the edgestash.execute() method is executed, the turn the JSON and the template into personalized output.

How do you trigger Edgestash from Twig templates?

Edgestash function

The custom edgestash Twig function will emit an unparsed Mustache variable, will perform the content negotiation with Varnish and will automatically provide the JSON endpoint.

Here's an example:

<div>{{ edgestash('username','/session') }}</div>

This example will emit a {{ username }} variable. Although Twig has similar syntax to Mustache, the variable will not be parsed by Twig.

The /session endpoint containing JSON data will be sent to Varnish. Varnish will process that JSON data, look for the username property and parse it into the {{ username }} variable.

Edgestash filter

The edgestash function is also available as a custom Twig filter. This means you can pipe data to it.

Here's an example:

<div>{{ username | edgestash('username','/session') }}</div>

In this example, Twig will parse the {{ username }} variable if the Symfony bundle didn't detect an Edgestash-supported Varnish server.

However, if this bundle did detect an Edgestash-supported Varnish server, it will output {{ username }} and and send the /session endpoint to Varnish.

The isEdgestash function

Although the edgestash filter is an ideal way to introduce Edgestash in a non-obtrusive way, unfortunately sometimes Edgestash integration requires some custom template code.

Fortunately, the isEdgestash() Twig function allows you to detect whether or not the application sits behind and Edgestash-supported Varnish server. This is the same logic that both the edgestash filter and function use under the hood for detection.

Here's an example:

<div>
    {% if isEdgestash() %}
        //Edgestash-supported logic
    {% else %}
        //Regular Twig logic
    {% endif %}
</div>

Conditions and loops

Edgestash supports the full Mustache syntax and can handle conditions and loops.

Here's an example of an if-else-statement using Edgestash:

{{ edgestash('#username', '/session') }}
    Welcome {{ edgestash('username') }}
{{ edgestash('/username') }}
{{ edgestash('^username') }}
    Welcome guest
{{ edgestash('/username') }}

So if the username property is detected in the JSON data, the username will be printed. If not, we'll refer to the user as a guest.

Please not that a JSON endpoint should only be registered once. Subsequently, the url argument can be left blank. However, if the same endpoint is registered multiple times, it will cause no harm since duplicates are removed.

Here's an example of a loop using Edgestash:

<ul>
    {{ edgestash('#.', '/users') }}
    <li>{{ edgestash('username') }}</li>
    {{ edgestash('/.') }}
</ul>

In this example, we loop over a collection of user objects and list the usernames. The output of the /users endpoint could look like this:

[
    {"username":"user 1"},
    {"username":"user 2"},
    {"username":"user 3"}
]

The final output would then be:

<ul>
    <li>user 1</li>
    <li>user 2</li>
    <li>user 3</li>
</ul>

Multiple JSON endpoints

This Edgestash integration allows for multiple JSON endpoints. Duplicate values are removed and the output of all endpoints is merged into a single JSON object.

Here's an example:

<div>{{ username | edgestash('username','/session') }}</div>
<div>{{ items_in_cart | edgestash('items_in_cart','/cart') }}</div>

How does the ThijsFerynEdgestashTwigBundle interact with Varnish?

As previously mentioned, the edgestash filter works in a non-obtrusive way. This means it can detect whether or not it sits behind an Edgestash-supported Varnish server.

Edgestash detection

In order to announce Edgestash support, Varnish will send the following request header to your origin server containing the application:

Surrogate-Capability: edgestash="EDGESTASH/2.1"

This Symfony bundle looks for this header and if it finds it, Edgestash is supported.

On the way back, Varnish needs to know when it should consider the output to be a Mustache template. The ThijsFerynEdgestashTwigBundle bundle does this by sending the following response header back to Varnish:

Surrogate-Control: edgestash="EDGESTASH/2.1"

When Varnish detects this response header, it executes the edgestash.parse_response() method internally.

Detection happens automatically and the header negotiation is done in EdgestashTwigSubscriber.php

Processing JSON endpoints

When the edgestash function or filter is invoked with a valid URL argument, the URL is added as an internal HTTP request argument in TwigExtension.php.

The argument is processed by EdgestashTwigSubscriber.php and the URL (or URLs) are exposed using a Link header.

Here's an example of such a Link header:

Link: </session>; rel=edgestash

The link header has a rel=edgestash property, making it easy to find when the Link header contains other non-related data.

This is what it would look like when multiple JSON endpoints are registered:

Link: </session>; rel=edgestash, </cart>; rel=edgestash

When Edgestash output is detected by Varnish and the Link header is parsed, Varnish will trigger the edgestash.add_json_url_csv method to retrieve the JSON data through the endpoints. And finally the edgestash.execute() method is executed that parses the JSON and adds it to the cached template's placeholders.

Varnish requirements

Edgestash is a proprietary VMOD that is developed and maintained by Varnish Software. This module is automatically packaged when you sign-up for Varnish Enterprise.

If you do not have a Varnish Enterprise license, you can also try out Varnish Enterprise and Edgestash using one of our Cloud images on AWS, Azure or Google Cloud.

The minimal VCL required to use the ThijsFerynEdgestashTwigBundle is stored in Resources/Config/default.vcl

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