fastcgi caching

[Grant]

I'm using the following config to cache only /piwik/piwik.php:

fastcgi_cache_path /var/cache/php-fpm levels=1:2 keys_zone=piwik:10m;

fastcgi_cache_key "$scheme$request_method$host$request_uri";

location /piwik/piwik.php {
    fastcgi_cache piwik;
    add_header X-Cache $upstream_cache_status;
    fastcgi_pass unix:/run/php-fpm.socket;
    include fastcgi.conf;
}

I'm getting "X-Cache: HIT".  I tried to set up a minimal config, but
am I missing anything essential?  Is setting up a manual purge
required or will this manage itself?

- Grant