public async Task <Subscription> SaveSubscriptionAsync(Guid userId, EventId eventId)
        {
            // enqueue
            var message = new SaveSubscriptionMessage {
                UserId = userId, EventId = eventId
            };
            await _queue.AddMessageAsync(message, TimeSpan.Zero);

            // persist
            return(await _subscriptionStore.SaveSubscriptionAsync(userId, eventId));
        }