Versioning of the DD4hep-based LHCb Detector Description

The LHCb Detector software project is home to the detector description of the LHCb experiment, utilized across all data processing applications, from simulation to reconstruction. It is based on the DD4hep package relying on the combination of XML files and C++ code. A suite of custom developments were introduced to support different versions of the detector layout in different data taking periods. The detector descriptions are organized into self-contained, versioned component folders, each containing C++ code and DD4hep CompactXML files. The description of the overall LHCb detector combines specific versions of the components for the period being described. When a sub-detector is identical in two LHCb layouts it can be loaded by both, avoiding code duplication and facilitating future upgrade simulation studies. A convention for naming the versions consistent between the components and the whole detector has been set up. Dependency and convention checks as well as geometry monitoring are enforced by using GitLab’s CI testing. Critically, they include verifications that changes introduced do not affect existing versions.