Online Monitoring in the CDF II experiment

We describe the online monitoring system of the CDF II experiment designed to check the data quality of all subsystems in real-time. A subset of the events accepted by the highest trigger level is made available to 10 analysis programs which check the data quality on an event-by-event basis and produce diagnostic histograms. The monitoring results are distributed to the user via a client-server scheme. The display clients feature an intuitive GUI which allows to browse and request the available results. The monitoring package is coded in C++ and makes use of the ROOT analysis framework which offers histogramming methods, networking classes and graphics handling. We report about the design and the implementation of the monitoring system and discuss our experiences during the first months of operation.