Question

How in opencart would I only write/get the cache if the domain is not localhost?

The class responsible for caching is;

class Cache { 
    private $expire = 3600; 

    public function __construct() {
        $files = glob(DIR_CACHE . 'cache.*');

        if ($files) {
            foreach ($files as $file) {
                $time = substr(strrchr($file, '.'), 1);

                if ($time < time()) {
                    if (file_exists($file)) {
                        unlink($file);
                    }
                }
            }
        }
    }

    public function get($key) {
        $files = glob(DIR_CACHE . 'cache.' . preg_replace('/[^A-Z0-9\._-]/i', '', $key) . '.*');

        if ($files) {
            $cache = file_get_contents($files[0]);

            return unserialize($cache);
        }
    }

    public function set($key, $value) {
        $this->delete($key);

        $file = DIR_CACHE . 'cache.' . preg_replace('/[^A-Z0-9\._-]/i', '', $key) . '.' . (time() + $this->expire);

        $handle = fopen($file, 'w');

        fwrite($handle, serialize($value));

        fclose($handle);
    }

    public function delete($key) {
        $files = glob(DIR_CACHE . 'cache.' . preg_replace('/[^A-Z0-9\._-]/i', '', $key) . '.*');

        if ($files) {
            foreach ($files as $file) {
                if (file_exists($file)) {
                    unlink($file);
                }
            }
        }
    }
}

Answer 1

Just a quick patch …

in your get function

public function get($key) {
   if ($_SERVER['REMOTE_ADDR'] == '127.0.0.1') {return;}

in your set function

public function set($key, $value) {
   if ($_SERVER['REMOTE_ADDR'] == '127.0.0.1') {return; }

But remember that when your are about to patch the core of opencart you can use VQmod