When I was responsible for the infrastructure at Reddit, the most important thing I maintained was Postgres, but a close second was RabbitMQ, our message broker. It was essential to the operation of reddit — everything went into a distributed queue before it went to a database. For example, if you upvoted a post, that was written to the queue and the cache, and then returned success to the user. Then a queue runner would take that item, and attempt to write it to the database as well as create a...