Add a Varnish service

Add the following to your .symfony/services.yaml file:

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varnish:
    type: varnish:6.0
    relationships:
        application: 'app:http'
    configuration:
        vcl: !include
            type: string
            path: config.vcl

In the relationships block, define a relationship (application) to the application container (app) using the http endpoint. That allows Varnish to talk to the application container.

The configuration block is required, and must reference a VCL file (here config.vcl). The file name is relative to the .symfony directory.

Create a VCL template file

The VCL file you provide has three specific requirements over and above the VCL syntax itself.

1. You MUST NOT define a vcl_init() function. SymfonyCloud will auto-generate that function based on the relationships you define. In particular, it will define a "backend" for each relationship defined in services.yaml, named the same as the relationship.

1. You MUST NOT include the preamble at the beginning of the file, specifying the VCL version. That will be auto-generated as well. You CAN add imports, but not std and directors.
2. You MUST specify the backend to use in vcl_recv(). If you have a single app container/relationship/backend, it's just a single line. If you want to split requests to different relationships/backends based on some rule then the logic for doing so should be incorporated into the vcl_recv() function.

The absolute bare minimum VCL file is:

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sub vcl_recv {
    set req.backend_hint = application.backend();
}

Where `application` is the name of the relationship defined in services.yaml (of the relationship was named differently, use that name instead).

If you have multiple applications behind the same Varnish instance then you will need to include logic to determine to which application a request is forwarded:

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varnish:
    type: varnish:6.0
    relationships:
        blog: 'blog:http'
        main: 'app:http'
    configuration:
        vcl: !include
            type: string
            path: config.vcl

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# config.vcl
sub vcl_recv {
    if (req.url ~ "^/blog/") {
        set req.backend_hint = blog.backend();
    } else {
        set req.backend_hint = main.backend();
    }
}

This configuration will direct all requests to a URL beginning with a `/blog/` path to the application on the relationship blog, and all other requests to the application on the relationship main.

Besides that, the VCL file, including the vcl_recv() function, can be arbitrarily complex to suit the needs of the project. That includes additional include directives if appropriate. See the Varnish documentation for more details on the functionality offered by Varnish.

Route incoming requests to Varnish

To enable Varnish now, edit the .symfony/routes.yaml file to point to the Varnish service you just created. You also need to disable the router cache as it is now entirely redundant with Varnish:

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"https://{default}/":
    type: upstream
    upstream: "varnish:http"
    cache:
        enabled: false

That will map all incoming requests to the Varnish service rather than the application. Varnish will then, based on the VCL file, forward requests to the application as appropriate.