What I've seen with RabbitMQ is that the memory usage can grow and grow Prior to 2.0, the contents of Rabbit's queues were always in RAM. Even durable queues

What I've seen with RabbitMQ is that the memory usage can grow and grow Prior to 2.0, the contents of Rabbit's queues were always in RAM. Even durable queues with persistent messages always had their contents mirrored in RAM. As of 2.0, Rabbit can store queue contents on disk or in RAM, so as it faces memory pressure, it will send data to disk. If the disk is too slow for it to relieve its pressure, it will block connections that have sent data in the past until it can free up enough memory to continue. Last I checked, Rabbit does by default keep the associative map between queues and message in RAM, so its memory usage isn't technically bounded (it's a around a dozen bytes per message, IIRC). If that's a problem, the toke plugin uses tokyo cabinet to store the associations, which I believe allows Rabbit to have entirely bounded memory use. I also haven't checked on that for a while, so it may be that toke is installed by default now.