public static function pull($config = null, $clientId = null)
    {
        // Return if not enabled
        if (!self::isEnabled($config)) {
            return null;
        }

        $tokens = null;
        $cachePath = self::cachePath($config);
        if (file_exists($cachePath)) {
            // Read from the file
            $cachedToken = file_get_contents($cachePath);
            if ($cachedToken && JsonValidator::validate($cachedToken, true)) {
                $tokens = json_decode($cachedToken, true);
                if ($clientId && is_array($tokens) && array_key_exists($clientId, $tokens)) {
                    // If client Id is found, just send in that data only
                    return $tokens[$clientId];
                } else if ($clientId) {
                    // If client Id is provided, but no key in persisted data found matching it.
                    return null;
                }
            }
        }
        return $tokens;
    }