Persistent object versions can be totally or partially ordered. For totally ordered object versions, each version has a unique predecessor object version and can have a unique successor object version, and thus form a directed chain of object versions for a specific object. For partially ordered object versions, each version can have multiple predecessor or successor object versions, and thus form a directed acyclic graph (dag) of object versions; we limit partially ordered object versions to unique predecessor object versions, so that such versions form a directed tree of object versions for a specific object.