Cactus is an open source problem solving environment designed for scientists and engineers. Its modular structure easily enables parallel computation across different architectures and collaborative code development between different groups. Cactus originated in the academic research community, where it was developed and used over many years by a large international collaboration of physicists and computational scientists. The name Cactus comes from the design of a central core ("flesh") which connects to application modules ("thorns") through an extensible interface. Thorns can implement c