Background job processing is an important component of most large Rails applications. Two of the most common solutions - Resque and Sidekiq - are largely differentiated by one underlying architectural decision: processes or threads? Is this talk, we'll provide a gentle introduction to threads and processes, and then crack open the Resque and Sidekiq gems to see how they work. Along the way, we'll learn a bit about concurrency at the OS and Ruby layers, and about how some of the tools we rely on every day are written.