An introduction to asynchronous programming in Ruby/Rails. For example, if in an action method I want to contact two separate APIs and then use the results of both of those to serve up a response, it would save time to be able to have both the APIs send out requests at the same time without waiting for each other. Or, if I wanted to log information about requests without a response waiting for the log to finish first.

I've encountered a multitude of libraries for asynchronous Ruby programming, so some guidance on which ones you recommend for certain use cases would help too.