HELICS (Hierarchical Engine for Large-scale Infrastructure Co-Simulation) is an open-source, general-purpose, modular, highly-scalable co-simulation framework that runs cross-platform (Linux, Windows, and Mac OS X). It is not a modeling tool by itself, but rather an integration tool that enables multiple existing simulation tools (and/or multiple instances of the same tool), known as "federates," to exchange data during runtime and stay synchronized in time such that together they act as one large simulation, or "federation". This enables bringing together established (or new/emerging) off-the-shelf tools from multiple domains to form a complex software-simulation without having to change the individual tools (known as "black-box" modeling). All that is required is for someone to write a thin interface layer for each tool that interfaces with existing simulation time control and data value updating, such as through an existing scripting interface. Moreover, the HELICS community has a growing ecosystem of established interfaces for popular tools, such that many users can simply mix and match existing tools with their own data and run complex co-simulations with minimal coding.
