We show the advantages of modular and hierarchical
design in obtaining fault-tolerant software. Modularity enables
the identification of faulty software units simplifying key
operations, like software removal and replacement. We describe
three approaches to repair faulty software based on replication,
namely, Passive Replication, N-Version Replication, and Active Replication,
based on modular components. We show that the key construct to
represent these tactics is the ability to make ad hoc
changes in software topologies. We consider hierarchical mobility
as a useful operation to introduce new software units for
replacing faulty ones. For illustration purposes, we use
connecton, a hierarchical, modular, and self-modifying software
specification formalism, and its implementation in the Desmos
framework
