---
prelude: >
  This is the first 4.x release of Zuul.  It requires some deployment
  changes by operators which have been carefully planned in order to
  facilitate work on Zuul version 5, which will be the first version
  of Zuul where every component is fault tolerant and able to scale.

  If you read the release notes for the last 3.x release, you may have
  already made all of the required changes.  If not, please do so
  before upgrading to version 4.  Every required change in version 4
  is optionally supported in 3.19, so it is safe to make these changes
  and then upgrade.

  Please read all of the notes below, especially in the "Upgrading"
  section for details.  The primary additional requirements are:

    * TLS ZooKeeper connections
    * Network connectivity from all components to ZooKeeper
    * An SQL database

  With these changes in place, it is anticipated that further upgrades
  to Zuul made in support of the scale-out-scheduler work will be done
  with minimal disruption in the course of normal releases between
  version 4 and 5.