How to cache an object on Varnish, but tell the client not the cache it

Question

I am caching the product details page on Varnish, and then I purge the cache from the backend server whenever the product is updated. I want my clients to never cache this page on their side, but always ask from Varnish, so that I can serve them the most recent copy.

Currently, I have the below config for vcl_backend_response:

sub vcl_backend_response {
    unset beresp.http.Set-Cookie;
    #unset beresp.http.Cache-Control;
    #set beresp.http.Cache-Control = "no-cache";

    if (bereq.url ~ "^/products/\d+/details") {
        set beresp.ttl = 1h;
    }
}

But, using this config, client caches the response for 1 hour, and does not ask again, even the cache is purged on Varnish.

If I uncomment the cache-control related lines, this time Varnish does not cache the page and always asks for a fresh copy from the backend server.

Is this achievable in Varnish v6.0?

Answer 1

Yes, it's possible:

• Define the logic for how long things are cached by Varnish inside the vcl_backend_response.
• Define the logic for how long things are cached by browser cache inside the vcl_deliver.

So clients (browsers) can be instructed to cache with a different TTL than Varnish. The following will ensure that browsers will not cache response:

sub vcl_deliver {
    set resp.http.Pragma = "no-cache";
    set resp.http.Expires = "-1";
    set resp.http.Cache-Control = "no-store, no-cache, must-revalidate, max-age=0";
}

Moreover, if you can modify your app, you can resort to a much finer approach outlined in first solution here, that is sending a single Cache-Control header which defines caching TTL for shared caches (Varnish) and private caches (browser) differently:

Cache-Control: s-maxage=31536000, max-age=86400

The header above will instruct a browser to cache resource for 86400 seconds, while Varnish will cache for 31536000. This is because s-maxage only applies to shared caches. Varnish evaluates it, while browsers don’t.