pub(crate) fn register_subscriber(

        &self,

        session_id: Option<SessionId>,

        database: Database,

    ) -> Subscriber {

        let subscriber = self.relay().create_subscriber();

        let mut data = self.data.subscribers.write();

        let receiver = Receiver::new_stripping_prefixes(subscriber.receiver().clone());

        let id = loop {

            data.last_id = data.last_id.wrapping_add(1);

            let id = data.last_id;

            let entry = data.subscribers.entry(id);

            if matches!(entry, Entry::Vacant(_)) {

                entry.or_insert(SessionSubscriber {

                    session_id,

                    subscriber: subscriber.clone(),

                });

                break id;

        Subscriber {

            id,

            database,

            subscriber,

            receiver,

        }

    }

    pub(crate) fn unregister_subscriber(&self, subscriber: &Subscriber) {

        let mut data = self.data.subscribers.write();

        data.unregister(subscriber.id);

    }