Sidekiq 
uses threads to handle many jobs at the same time in the same process. It does not require Rails but will integrate tightly with Rails to make background processing dead simple.
You need server and client, and to understand queues. A worker watches queues that are defined on startup with the -q flag.
Two important queues for sidekiq are mailers and default which you can get sidekiq to watch by running: ```bash bundle exec sidekiq -q default -q mailers ```