Failure Detection Protocols in the Application Layer

Failure detection is a fundamental building block to develop fault-tolerant distributed systems. Accurate failure detection in asynchronous systems (Chapter II) is notoriously difficult, as it is impossible to tell whether a process has actually failed or it is just slow. Because of this, several impossibility results have been derived—see for instance the well-known paper (Fischer, Lynch, & Paterson, 1985). As a consequence of these pessimistic results, many researchers have devoted their time and abilities to understanding how to reformulate the concept of system model in a fine-grained alternative way. Their goal was being able to tackle problems such as distributed consensus with the minimal requirements on the system environment. This brought to the theory of unreliable failure detectors for reliable systems, pioneered by the works of Chandra and Toueg (Chandra & Toueg, 1996). This chapter introduces these concepts and the formulation of failure detection protocols in the application layer. In particular a linguistic framework is proposed for the expression of those protocols. As a case study it is described the algorithm for failure detection used in the EFTOS DIR net and in the TIRAN Backbone—that is, the fault-tolerance managers introduced respectively in Chapter III and Chapter VI.