Prompt Processing of LHC Collision Data with the ATLAS Reconstruction Software.

The LHC started operating in November 2009. The ATLAS experiment has successfully recorded, reconstructed and analyzed many million collision events delivered by LHC taking advantage of the experience collected with cosmic ray and single beam data during the commissioning period. Data coming out of the detector is reconstructed on an on-site computing farm with little delay. This prompt reconstruction of the data taken is essential to provide quick feedback on the data quality and allow analysis of the data just taken. It runs in two stages, allowing for calibration updates between the first stage and the second stage. The first stage processes a subset of the data taken and finishes within about one hour after the data is taken. It is used for Data Quality assessment and to derive calibration contants. The second stage starts after a fixed delay and proces ses the complete dataset. The reconstruction software used is regularly updated to incorporate new features and improvements. A sophisticated testing infrastructure is in place to ensure continuous software quality. This talk will report about the software infrastructure used for prompt reconstruction and the experience with the ATLAS offline software framework in this context.