Clusters support persistence, by providing a directory service, which associates string identifiers with the persistence roots. The GC is aware of such persistent roots, and uses them to implement the PBR paradigm. Once an application program retrieves a root object from a cluster, it can navigate the store by dereferencing the pointers contained in the object itself. An object may contain a reference to some other object, allocated in another cluster, thereby creating a distributed network of objects. Stubs keep track of outgoing references from the cluster's objects, and scions of incoming references.
Scions represent additional, but temporary, roots of persistence for the cluster.
Clusters implement a simple protection scheme. Similarly to Unix files, they have user ownership and protection attributes. A typical PerDiS system involves a number of sites, each site being a modern interactive workstation. Some sites may be more powerful file servers dedicated to storage and reliable backup. The ULL is linked to the application code to form an application process.