Increasing Availability by Implementing Software Redundancy in the CMS Detector Control System

The Detector Control System (DCS) of the Compact Muon Solenoid (CMS) experiment ran with high availability throughout the first physics data-taking period of the Large Hadron Collider (LHC). This was achieved through the consistent improvement of the control software and the provision of a 24-hour expert on-call service. One remaining potential cause of significant downtime was the failure of the computers hosting the DCS software. To minimize the impact of these failures after the restart of the LHC in 2015, it was decided to implement a redundant software layer for the control system where two computers host each DCS application. By customizing and extending the redundancy concept offered by WinCC Open Architecture (WinCC OA), the CMS DCS can now run in a fully redundant software configuration. The implementation involves one host being active, handling all monitoring and control tasks, with the second host running in a minimally functional, passive configuration. Data from the active host is constantly copied to the passive host to enable a rapid switchover as needed. This paper describes details of the implementation and practical experience of redundancy in the CMS DCS.