The new inter process communication middle-ware for the ATLAS Trigger and Data Acquisition system

The ATLAS Trigger & Data Acquisition (TDAQ) project was started almost twenty years ago with the aim of providing scalable distributed data collection system for the experiment. While the software dealing with physics data flow was implemented by directly using the low-level communication protocols, like TCP and UDP, the control and monitoring infrastructure services for the TDAQ system were implemented on top of the CORBA communication middle-ware. CORBA provides a high-level object oriented abstraction for the inter process communication, hiding communication complexity from the developers. This approach speeds up and simplifies development of communication services but incurs some extra cost in terms of performance and resources overhead. Our experience of using CORBA for control and monitoring data exchange in the distributed TDAQ system was very successful, mostly due to the outstanding quality of the CORBA brokers, which have been used in the project: omniORB for C++ and JacORB for Java. However, due to a number of shortcomings and technical issues the CORBA standard has being gradually losing its initial popularity in the last decade and the long term support for the open source implementations of CORBA becomes questionable. Taking into account the time scale of the ATLAS experiment, which goes beyond the next two decades, the TDAQ infrastructure team reviewed the requirements for the inter process communication middle-ware and performed the survey of the communication software market in order to access the modern technologies which raised in the past years. Based on the result of that survey several technologies were evaluated for estimating the long-term benefits and drawbacks of using them as a possible replacement for CORBA during the next long LHC shutdown, which is scheduled in 2 years from now. The evaluation concluded recently with the recommendation of using communication library called ZeroMQ in place of CORBA. The article presents the methodology and the results of the evaluation as well as the plans of organizing the migration from CORBA to ZeroMQ.