public void Subscribe(UserIdentifier user, string notificationName, EntityIdentifier entityIdentifier = null)
        {
            if (IsSubscribed(user, notificationName, entityIdentifier))
            {
                return;
            }

            _store.InsertSubscription(
                new NotificationSubscriptionInfo(
                    _guidGenerator.Create(),
                    user.TenantId,
                    user.UserId,
                    notificationName,
                    entityIdentifier
                    )
                );
        }