Design and Implementation of the Array Data Handler of the Cherenkov Telescope Array Observatory

The Array Data Handler (ADH) of the Cherenkov Telescope Array Observatory (CTAO) is a central component of the software operations for the telescope arrays. It handles trigger events and raw data from all telescopes, as well as data from environmental and array calibration instruments. Following the initial release of the ADH, which is now capable of handling data from a single telescope, ongoing development is focused on adding support for multiple telescopes and enabling real-time reduction of the raw data volume. These new features are scheduled for release alongside the second release of the Array Control and Data Acquisition (ACADA) system, anticipated in the second half of this year. A single ADH instance, operating on a given set of telescopes, includes one SoftWare Array Trigger (SWAT) instance. The SWAT listens for detection timestamps from events captured by each Cherenkov Camera, sorts these timestamps, identifies temporal coincidences, and makes array trigger decisions that are communicated back to the cameras. Upon an array trigger or during calibration runs, the Cherenkov cameras send their raw, calibrated event data to several Science Data Handler (SDH) components. The SDH performs Data Volume Reduction (DVR), based on the signal strength in each pixel, retaining only a fraction of the original data. The filtered event data then undergoes lossless compression before being written to disk. The orchestration of the ADH components instances is managed by a top-level component known as the ADH Supervisor. The Supervisor instantiates the necessary components, monitors their health and performance, raises alarms and takes corrective action if failures are detected.