Primary-backup replication: from a time-free protocol to a time-based implementation

Fault-tolerant control systems can be built by replicating critical components. However replication raises the issue of inconsistency. Multiple protocols for ensuring consistency have been described in the literature. PADRE (Protocol for Asymmetric Duplex REdundancy) is such a protocol, and an interesting case study of a complex and sensitive problem: the management of replicated traffic controllers in a railway system. However, the low level at which the protocol has been developed embodies system details, namely timeliness assumptions, that make it difficult to understand and may narrow its applicability. We argue that, when designing a protocol, it is preferable to consider first a general solution that does not include any timeliness assumptions; then, by taking into account an additional hypothesis, one can easily design a time-based solution tailored to a specific environment. This paper illustrates the benefit of a top-down protocol design approach and shows that PADRE can be seen as an instance of a standard primary-backup replication protocol based on view-synchronous communication (VSC).