The Offline Software of the Pierre Auger Observatory: Lessons Learned

The Offline software framework for data analysis of the Pierre Auger Observatory is a set of computational tools developed to cater to the needs of a large and geographically dispersed collaboration established to measure the spectrum, arrival directions, and composition of ultra-high energy cosmic rays over a period of 20 years. One of its design goals was to facilitate the collaborative effort by allowing collaborators to progressively contribute small portions of code. The observatory has grown over time and it has undergone improvements and additions that have tested the flexibility of the framework. The framework was originally thought to accommodate a hybrid view of cosmic ray detection, made of a surface and a fluorescence detector. Since then, the framework has been extended to include a radio antenna array and both under-ground and above-ground scintillator arrays. Different tools from the framework have been used by other collaborations, notably NA61/Shine and HAWC. All these experiences accumulated over the years allow us to draw conclusions in terms of the successes and failures of the original design.