APSIM Next Generation: overcoming challenges in modernising a farming systems model

From 1990, the Agricultural Production Systems sIMulator (APSIM) has grown from a paddock-focused farming systems framework used by a small number of people, into a large collection of models used by thousands of modellers internationally. The software grew to consist of several hundred thousand lines of code in multiple programming languages. This has led to a large, complex software ecosystem that is difficult to maintain. In addition, systems modellers increasingly require software systems that integrate multiple disciplines, can represent evermore complex farming systems, can run on multiple operating systems (desktop, web, mobile), can operate at or be scaled to multiple scales (field, farm, region, continent, global) and run faster for larger simulation analyses. This is difficult to achieve in an aging framework. Traditionally, the software evolves to meet new demands. This is possible when there are sufficient software development resources and time but becomes more difficult when teams are small. Even when teams are sufficiently large, the new demands on the software may be too great to consider an evolutionary approach. For these reasons, the APSIM Initiative is building the next generation of APSIM that is a completely new framework, capturing the science from the legacy code base and redesigning it to meet new problem domains and IT approaches. In addition, the Initiative have adopted new software processes that improve the way APSIM models are designed, developed, tested and documented. We expect APSIM Next Generation to provide a better user experience across operating systems, improvements in runtime speed, better model construction tools, increased flexibility to represent on-farm management practices, improvements in documentation and better software development processes.