Muon calibration data extraction and distribution for the ATLAS experiment

In the ATLAS experiment, fast calibration of the detector is vital to feed both prompt data reconstruction with fresh calibration constants. At the same time, online data extraction presents several advantages, since the data rate needed to have data sets taken in homogeneous conditions can be achieved without performing special runs that would require special tuning of parameters and architecture of the ATLAS TDAQ system. The best place to get muon tracks suitable for muon detector calibration is the second level trigger, where the pre-selection of data sitting in a limited region by the first level trigger allows to select all (and only) the hits from a single track and to add some useful information to speed up the calibration process. The MDT (monitored drift tube) calibration is shown as a use case, as it can be generalized to the entire muon system. In this case, the extracted fragment size has been evaluated to be about 800 bytes. Since at low luminosity the achievable data throughput is about 12 kHz, the total data throughput is 9.6 MB/s. The data collection model is based on a two level data concentration: pre-selected hits from muon tracks plus some auxiliary information are sent by any second level trigger machine in a rack to a server; a calibration server collects data from the several rack servers and sends them to one or more calibration farm(s). Different options are being explored to ensure the quality of service needed for data distribution.