Grid Computations without the Computing Element: Interfacing Control Systems for On-Line Computations

Traditionally the computations on the Grid take place on the Computing Element. In the same line, the Instrument Element is meant to Grid-enable instrumentation. In this chapter we introduce a non-classical use of the Instrument Element where it serves as a virtual instrument for performing a computational task. Specifically it has been used as the interface to a Control System that executes a series of High Throughput Computing tasks in an On-Line manner. This had to be done in order to meet the special requirements of an application in the Synchrotron Radiation Facility Elettra. The instrument control in such institutions is often done through Distributed Control Systems. Such a system is TANGO and the Synchrotron Radiation Facility (SRF) Elettra among other synchrotrons is heavily based on it. The application was for a beamline working in medical imaging (SYRMEP) and aimed to be an improvement of an established Computed Tomography workflow. The task was the generation, in parallel, of sinograms of a specific data format based on the acquired X-ray absorption data. The target was the availability of the complete sinogram data set in a Storage Element by the time of the completion of the CT scan. The Grid related latencies, like job submission and queuing, would have been an issue given the near-real-time requirements. Moreover the inclusion of a set of TANGO devices was necessary and a generic gLite WN would not have been as suitable as a dedicated system. Besides the avoidance of certain Grid parts, the Grid Security infrastructure was required to be fully utilised in the final solution. The design followed a bottom-up approach: (a) design and preparation of a dedicated system based on virtualisation, (b) development of a parallel sinogram generator, (c) deployment of suitable TANGO devices for controlling the data acquisition, the generator, and the On-Line progress, (d) a TANGO-to-IE bridge to export the devices as IM, (e) utilising a Grid Web Portal (VCR) in order to serve as the end-user GUI for the application. In this contribution (I) we introduce a novel concept where computation may take place outside the CE, (II) we design an architecture where a Distributed Control System is piloted by an Instrument Manager through the Grid, and (III) we discuss a working implementation of the system.