How to Use Varnish to Speed up my Website

Make Symfony Trust the Reverse Proxy

Varnish automatically forwards the IP as X-Forwarded-For and leaves the X-Forwarded-Proto header in the request. If you do not configure Varnish as trusted proxy, Symfony will see all requests as coming through insecure HTTP connections from the Varnish host instead of the real client.

Remember to configure framework.trusted_proxies in the Symfony configuration so that Varnish is seen as a trusted proxy and the X-Forwarded headers are used.

Varnish, in its default configuration, sends the X-Forwarded-For header but does not filter out the Forwarded header. If you have access to the Varnish configuration file, you can configure Varnish to remove the Forwarded header:

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sub vcl_recv {
    unset req.http.Forwarded;
}

If you do not have access to your Varnish configuration, you can instead configure Symfony to distrust the Forwarded header as detailed in How to Configure Symfony to Work behind a Load Balancer or a Reverse Proxy.

Routing and X-FORWARDED Headers

To ensure that the Symfony Router generates URLs correctly with Varnish, an X-Forwarded-Port header must be present for Symfony to use the correct port number.

This port number corresponds to the port your setup is using to receive external connections (80 is the default value for HTTP connections). If the application also accepts HTTPS connections, there could be another proxy (as Varnish does not do HTTPS itself) on the default HTTPS port 443 that handles the SSL termination and forwards the requests as HTTP requests to Varnish with an X-Forwarded-Proto header. In this case, you need to add the following configuration snippet:

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sub vcl_recv {
    if (req.http.X-Forwarded-Proto == "https" ) {
        set req.http.X-Forwarded-Port = "443";
    } else {
        set req.http.X-Forwarded-Port = "80";
    }
}

Cookies and Caching

By default, a sane caching proxy does not cache anything when a request is sent with cookies or a basic authentication header. This is because the content of the page is supposed to depend on the cookie value or authentication header.

If you know for sure that the backend never uses sessions or basic authentication, have Varnish remove the corresponding header from requests to prevent clients from bypassing the cache. In practice, you will need sessions at least for some parts of the site, e.g. when using forms with CSRF Protection. In this situation, make sure to only start a session when actually needed and clear the session when it is no longer needed. Alternatively, you can look into Caching Pages that Contain CSRF Protected Forms.

Cookies created in JavaScript and used only in the frontend, e.g. when using Google Analytics, are nonetheless sent to the server. These cookies are not relevant for the backend and should not affect the caching decision. Configure your Varnish cache to clean the cookies header. You want to keep the session cookie, if there is one, and get rid of all other cookies so that pages are cached if there is no active session. Unless you changed the default configuration of PHP, your session cookie has the name PHPSESSID:

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sub vcl_recv {
    // Remove all cookies except the session ID.
    if (req.http.Cookie) {
        set req.http.Cookie = ";" + req.http.Cookie;
        set req.http.Cookie = regsuball(req.http.Cookie, "; +", ";");
        set req.http.Cookie = regsuball(req.http.Cookie, ";(PHPSESSID)=", "; \1=");
        set req.http.Cookie = regsuball(req.http.Cookie, ";[^ ][^;]*", "");
        set req.http.Cookie = regsuball(req.http.Cookie, "^[; ]+|[; ]+$", "");

        if (req.http.Cookie == "") {
            // If there are no more cookies, remove the header to get page cached.
            unset req.http.Cookie;
        }
    }
}

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sub vcl_recv {
    // Remove all cookies except the session ID.
    if (req.http.Cookie) {
        set req.http.Cookie = ";" + req.http.Cookie;
        set req.http.Cookie = regsuball(req.http.Cookie, "; +", ";");
        set req.http.Cookie = regsuball(req.http.Cookie, ";(PHPSESSID)=", "; \1=");
        set req.http.Cookie = regsuball(req.http.Cookie, ";[^ ][^;]*", "");
        set req.http.Cookie = regsuball(req.http.Cookie, "^[; ]+|[; ]+$", "");

        if (req.http.Cookie == "") {
            // If there are no more cookies, remove the header to get page cached.
            remove req.http.Cookie;
        }
    }
}

Ensure Consistent Caching Behavior

Varnish uses the cache headers sent by your application to determine how to cache content. However, versions prior to Varnish 4 did not respect Cache-Control: no-cache, no-store and private. To ensure consistent behavior, use the following configuration if you are still using Varnish 3:

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sub vcl_fetch {
    /* By default, Varnish3 ignores Cache-Control: no-cache and private
       https://www.varnish-cache.org/docs/3.0/tutorial/increasing_your_hitrate.html#cache-control
     */
    if (beresp.http.Cache-Control ~ "private" ||
        beresp.http.Cache-Control ~ "no-cache" ||
        beresp.http.Cache-Control ~ "no-store"
    ) {
        return (hit_for_pass);
    }
}

Enable Edge Side Includes (ESI)

As explained in the Edge Side Includes article, Symfony detects whether it talks to a reverse proxy that understands ESI or not. When you use the Symfony reverse proxy, you don't need to do anything. But to make Varnish instead of Symfony resolve the ESI tags, you need some configuration in Varnish. Symfony uses the Surrogate-Capability header from the Edge Architecture described by Akamai.

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 {
    // Add a Surrogate-Capability header to announce ESI support.
    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 Symfony adds automatically:

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sub vcl_backend_response {
    // Check for ESI acknowledgement and remove Surrogate-Control header
    if (beresp.http.Surrogate-Control ~ "ESI/1.0") {
        unset beresp.http.Surrogate-Control;
        set beresp.do_esi = true;
    }
}

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sub vcl_fetch {
    // Check for ESI acknowledgement and remove Surrogate-Control header
    if (beresp.http.Surrogate-Control ~ "ESI/1.0") {
        unset beresp.http.Surrogate-Control;
        set beresp.do_esi = true;
    }
}

Cache Invalidation

If you want to cache content that changes frequently and still serve the most recent version to users, you need to invalidate that content. While cache invalidation allows you to purge content from your proxy before it has expired, it adds complexity to your caching setup.