A Reliable Protocol for Synchronous Rendezvous (note)

In the presence of failure, any protocol for distributed atomic commitment necessarily has a ‘window of vulnerability’ where the crash of one party makes other parties block. This vulnerability turns out not so serious in one special case – that of synchronous rendezvous. Rendezvous is important because it is the basis for process calculi, which themselves underpin several new experimental languages and also web services. We give a simplified three phase commit protocol specially tailored to rendezvous. In the presence of arbitrary message loss and permanent site failure, the protocol is strongly non-blocking for one party – the party can always unblock immediately. This is useful for writing a reliable non-blocking web service. If message loss is fair and site failure is not permanent, then the protocol is also weakly non-blocking for the other party – the chance of it remaining blocked tends to zero as time increases. (This yields a solution to the classic ‘Two Generals’ problem, which is a degenerate case of rendezvous). The proof of non-blocking uses a novel technique involving Markov processes. It is a general technique that applies to any calculus and any implementation with message loss, so long as the two are bisimilar.