Upgrade of Online Storage and Express-Reconstruction System for the Belle II Experiment

The Belle II experiment started to take data of electron–positron collisions provided by the SuperKEKB accelerator, with all subdetectors from March 2019. The collected data are selected by a software-based high-level trigger (HLT) system and stored on online storage. After storing the data, the online storage sends the data to an express-reconstruction system for semireal-time data quality monitoring (DQM) and event display. The HLT system is built on the ZeroMQ networking library, but the online storage and express-reconstruction system are built on the ring buffer and transmission control protocol/internet protocol (TCP/IP) socket-based framework. To improve stability and future maintainability, we develop a new online storage and express-reconstruction system using the ZeroMQ library. The new systems have a few additional improvements. First, the online storage supports the ROOT output file format with compression. This reduces the online file size, necessary network bandwidth for the online-to-offline file transfer, and offline computing resource usage. Second, two types of express-reconstruction systems are prepared based on the HLT results: for randomly sampled data and for physics-flagged data by the HLT system. The separate express-reconstruction system can process a larger number of statistics of physics-flagged events.