Dynamic Evolution In A Survivable Application Infrastructure

We describe a highly scalable algorithm for secure system evolution in an infrastructure for widely distributed Byzantine fault-tolerant applications. To maintain high availability, the system and its applications evolve on-line, providing uninterrupted service during installation of upgrades. Installations are made to appear atomic with respect to other installations and application execution steps. Our algorithm guarantees safe installation despite Byzantine faulty replicas and replica groups. An initial phase prepares replica groups for an upgrade, while a second phase triggers the installation of the upgrade by gossip among groups. A simple but novel scheme using secret sharing and Byzantine quorums prevents faulty replicas and replica groups from disrupting or maliciously exploiting installations. Installation message complexity and computational complexity grow linearly with the number of replicas.