In the coming years, Xenon 1 T, a ten-fold expansion of Xenon 100, will further explore the dark matter WIMP parameter space and must be able to cope with correspondingly higher data rates. With a focus on sustainable software architecture, and a unique experimental scale compared to collider experiments, a high-level trigger system is being designed for the next many years of Xenon 1 T running. As a high level trigger, processing time must be minimized while maintaining efficient event selection. By using Python as a 'glue' language around quicker C routines, the complexity of the system can be minimized and testing facilitated. After discussing the data flow and parallelization, the talk will focus on testing techniques and software sustainability. For example, in addition to unit tests, in situ (i.e., control room) and ex situ (i.e., cloud based) system tests use old data to demonstrate the reliability of and benchmark the system. Strategies appropriate to this experimental scale that ensure sustainability will also be discussed, including the documentation and training policies, testing requirements, style guides, and general project management. In this way, the high level trigger system will be maintainable, robust, and reliable for the foreseen lifetime of the experiment.