We have a tiny ruby process, based on event machine, that subscribes to various queues (we happen to use RabbitMQ). When a message arrives, the process makes a request to the passenger instance passing along the message data and waits for a response. The process limits the number of requests it makes to prevent background requests from blocking out front-end requests (for example, 20% of passenger_max_pool_size). We're also simulating priority by using different prefetch values for different queues (for example, 10 messages for high queue and 5 messages for low queue).