The rapidly growing production of digital data, together with their increasing importance and essential demands for their longevity, urgently require systems that provide reliable long-term preservation of digital objects. Most importantly, these systems have to ensure guaranteed availability over a long period of time, as well as integrity and authenticity of the preserved data and their metadata. This means that all kinds of technical problems need to be reliably handled and that the evolution of data formats is supported. At the same time, systems need to scale with the volume of data to be archived. In this paper, we present DISTARNET, a fully distributed system that reliably executes pre-defined workflows for long-term preservation. Moreover, DISTARNET is designed as an open system that allows the curators of digital objects to specify new processes to cope with additional challenges.