A General Characterization of Indulgence (Invited Paper)

An indulgent algorithm is a distributed algorithm that, besides tolerating process failures, also tolerates arbitrarily long periods of instability, with an unbounded number of timing and scheduling failures. In particular, no process can take any irrevocable action based on the operational status, correct or failed, of other processes. This paper presents an intuitive and general characterization of indulgence. The characterization can be viewed as a simple application of Murphy's law to partial runs of a distributed algorithm, in a computing model that encompasses various communication and resilience schemes. We use our characterization to establish several results about the inherent power and limitations of indulgent algorithms.