Upgrading to Sidekiq 4.0

Sidekiq 4.0 contains a completely redesigned core with much higher performance. See my blog for an overview of Sidekiq 4's higher performance.

To upgrade cleanly:

gem 'sidekiq', '< 4'

gem 'sidekiq', '< 5'

What's New

Sidekiq no longer uses Celluloid. If your application code uses Celluloid, you will need to pull it in yourself.
redis-namespace has been removed from Sidekiq's gem dependencies. If you want to use namespacing (and I strongly urge you not to), you'll need to add the gem to your Gemfile:

gem 'redis-namespace'

Redis 2.8.0 or greater is required. Redis 2.8 was released two years ago and contains many useful features which Sidekiq couldn't leverage until now. Redis 3.0.3 or greater is recommended for large scale use.
Sidekiq requires more Redis connections per process due to a change in how jobs are fetched from Redis. You must have a minimum of concurrency + 2 connections in your pool or Sidekiq will exit. When in doubt, let Sidekiq size the connection pool for you.