Conditions and alignment extensions of the DD4hep detector description toolkit

The detector description is an essential component to analyze data resulting from particle collisions in high energy physics experiments. The interpretation of data from particle collisions typically requires auxiliary data which describe in detail the state of the experiment. These accompanying data include alignment parameters, parameters describing the electronics as well as calibration- and environmental constants. We present a mechanism to manage such data in multiple simultaneous versions depending on their validity. The detector conditions data are made available to the physics algorithms through a number of transient objects grouped to collections. Such a collection represents a coherent slice of all conditions data necessary to process one or several events depending on the valid interval of the slice being the intersection of the individual conditions. A multi-threaded application may hold several such collections in parallel depending on the time-stamps of the events currently processed. Once prepared, these collections are read-only and can easily be shared between threads with minimal requirements for locking and hence minimal overhead. We deliberately restrained ourselves from providing a persistent data solution, which in the past were fields of expertise of the experiments, but rather provided the necessary hooks to populate the conditions cache. We will present the use-cases that have driven the development, the main design choices and details of the implementation.