DDS as middleware of the Southern African Large Telescope control system

摘要

The Southern African Large Telescope (SALT) software control system is realised as a distributed control system, implemented predominantly in National Instruments' Lab VIEW. The telescope control subsystems communicate using cyclic, state-based messages. Currently, transmitting a message is accomplished by performing an HTTP PUT request to a WebDAV directory on a centralised Apache web server, while receiving is based on polling the web server for new messages. While the method works, it presents a number of drawbacks; a scalable distributed communication solution with minimal overhead is a better fit for control systems. This paper describes our exploration of the Data Distribution Service (DDS). DDS is a formal standard specification, defined by the Object Management Group (OMG), that presents a data-centric publish-subscribe model for distributed application communication and integration. It provides an infrastructure for platform-independent many-to-many communication. A number of vendors provide implementations of the DDS standard; RTI, in particular, provides a DDS toolkit for Lab VIEW. This toolkit has been evaluated against the needs of SALT, and a few deficiencies have been identified. We have developed our own implementation that interfaces Lab VIEW to DDS in order to address our specific needs. Our Lab VIEW DDS interface implementation is built against the RTI DDS Core component, provided by RTI under their Open Community Source licence. Our needs dictate that the interface implementation be platform independent. Since we have access to the RTI DDS Core source code, we are able to build the RTI DDS libraries for any of the platforms on which we require support. The communications functionality is based on UDP multicasting. Multicasting is an efficient communications mechanism with low overheads which avoids duplicated point-to-point transmission of data on a network where there are multiple recipients of the data. In the paper we present a performance evaluation of DDS against the current HTTP-based implementation as well as the historical DataSocket implementation. We conclude with a summary and describe future work.