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