Web-based streamed waveform display using MDSplus events and Node.js

Streamed data visualization is a requirement for long lasting discharges, such as in ITER and W7X, and more in general for long lasting related experiments, such as the ITER neutral beam test facility. A prerequisite for live visualization is the ability of the underlying data system to support streaming in data acquisition. On-line visualization is routinely performed during long lasting experiments, but implementing streamed data visualization, such as strip charts, would overload the data system, especially if a large number of charts are being displayed. A different approach for streamed data visualization is proposed here, using MDSplus events, rather than directly accessing stored data. Events are implemented as UDP multicast packets and can bring data. A data acquisition program can therefore, in addition to using MDSplus for streaming data segments, generate MDSplus events bringing the most recent chunk of samples. Data carried by events are made available to Web applications by means of a Node.js server, listening for the UDP packets and updating the connected Web clients using HTML5 Server-Sent Events.