# 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](http://www.mikeperham.com/2015/10/14/optimizing-sidekiq/).

To upgrade cleanly:

* Upgrade to the latest Sidekiq 3.x.
```ruby
gem 'sidekiq', '< 4'
```
* Fix any deprecation warnings you see.
* Upgrade to 4.0.x.
```ruby
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](http://www.mikeperham.com/2015/09/24/storing-data-with-redis/)), you'll need to add the gem to your Gemfile:
```ruby
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.