Configuration

As seen previously, Symfony2 is smart enough to detect whether it talks to a reverse proxy that understands ESI or not. It works out of the box when you use the Symfony2 reverse proxy, but you need a special configuration to make it work with Varnish. Thankfully, Symfony2 relies on yet another standard written by Akamaï (Edge Architecture), so the configuration tips in this chapter can be useful even if you don't use Symfony2.

First, configure Varnish so that it advertises its ESI support by adding a Surrogate-Capability header to requests forwarded to the backend application:

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sub vcl_recv {
    set req.http.Surrogate-Capability = "abc=ESI/1.0";
}

Then, optimize Varnish so that it only parses the Response contents when there is at least one ESI tag by checking the Surrogate-Control header that Symfony2 adds automatically:

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sub vcl_fetch {
    if (beresp.http.Surrogate-Control ~ "ESI/1.0") {
        unset beresp.http.Surrogate-Control;

        // for Varnish >= 3.0
        set beresp.do_esi = true;
        // for Varnish < 3.0
        // esi;
    }
}

Cache Invalidation

You should never need to invalidate cached data because invalidation is already taken into account natively in the HTTP cache models (see HTTP Cache).

Still, Varnish can be configured to accept a special HTTP PURGE method that will invalidate the cache for a given resource:

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sub vcl_hit {
    if (req.request == "PURGE") {
        set obj.ttl = 0s;
        error 200 "Purged";
    }
}

sub vcl_miss {
    if (req.request == "PURGE") {
        error 404 "Not purged";
    }
}