CODAS for long lasting experiments. The SPIDER experience

The SPIDER experiment is the first of two experiments being held at the ITER Neutral Beam Test Facility in Padova (Italy). SPIDER has been operating since 2018, initially with pulse duration of a few seconds and currently with pulses lasting up to 3000 s. The paper reports the CODAS experience gained after three years of operation. In particular, Data Storage and Data Access adopted strategies will be discussed, that proved to be of high impact in overall system performance and maintainability. Regarding Data Storage strategy, a tradeoff must be defined between the continuous and event driven data acquisition. Continuous data acquisition, i.e. sampling data at a constant frequency, represents the normal operation in short experiments, but can easily lead to an unmanageable amount of data for long lasting experiments. On the other side, Data acquisition at a varying rate, that is increased upon the occurrence of given events leading to an improved signal dynamics, is required for a subset of signals that describe physical phenomena with fast dynamics. Several strategies have been adopted in SPIDER to handle varying rate data acquisition and are discussed here. Considering data access strategy, an important Use Case, especially when the pulse duration is long, is the concurrent access to the pulse file for online analysis and visualization. Concurrent data read and write is supported by MDSplus, the adopted data system, but performance can be affected by the required locks in file access. For this reason it is important to limit as far as possible useless data access. This has been achieved in different ways, such as setting a Region of Interest (ROI) in data access and by the extended usage of on-the-fly resampling in conjunction with the availability in the pulse file of different versions of the same data item, acquired at different sampling speeds.