interface RequestPolicyInterface

interface RequestPolicyInterface {
    
    /**
     * Allow delivery of cached pages.
     */
    const ALLOW = 'allow';
    
    /**
     * Deny delivery of cached pages.
     */
    const DENY = 'deny';
    
    /**
     * Determines whether delivery of a cached page should be attempted.
     *
     * Note that the request-policy check runs very early. In particular it is
     * not possible to determine the logged in user. Also the current route match
     * is not yet present when the check runs. Therefore, request-policy checks
     * need to be designed in a way such that they do not depend on any other
     * service and only take in account the information present on the incoming
     * request.
     *
     * When matching against the request path, special attention is needed to
     * support path prefixes which are often used on multilingual sites.
     *
     * @param \Symfony\Component\HttpFoundation\Request $request
     *   The incoming request object.
     *
     * @return string|null
     *   One of static::ALLOW, static::DENY or NULL. Calling code may attempt to
     *   deliver a cached page if static::ALLOW is returned. Returns NULL if the
     *   policy is not specified for the given request.
     */
    public function check(Request $request);

}