Patron, from here: https://github.com/toland/patron. We use it in combination with our (open source) FastSend gem to keep the streamed data away from the Ruby garbage collector, since it gets a bit emotional if it has to clear out hundreds of MBs every second. If you're interested why we chose this architecture instead of just redirecting to S3, our lead engineer gave a talk about the reasoning behind this structure. You can find the slides at https://speakerdeck.com/julik/streaming-large-files-with-rub....