More on Fischer, Lynch, Patterson and the parrot theorem.

I’m thinking about distributed consensus algorithms, timestamping, and databases and if you read that literature you will see many references to the Fischer, Lynch, Paterson “theorem”. Here is the problem statement

The problem is for all the data manager processes that have participated in the processing of a particular transaction to agree on whether to install the transaction’s results in the database or to discard them. The latter action might be necessary, for example, if some data managers were, for any reason, unable to carry out the required transaction processing. Whatever decision is made, all data managers must make the same decision in order to preserve the consistency of the database.

A set of data manager processes  must come to a consensus about whether to commit or to discard. The problem statement requires that ALL of the processes must agree either “yes” or “no” and, presumably a single “no” vote must persuade the others. An implicit but key property for the desired consensus is that if a process fails the others can ignore its opinion. That is, a dead process does not count in the consensus. And that’s the core problem here. The processes consult with each other possibly all agree on “yes” except for one process that does not answer. Is it a slow process that will say “yes” or is it a slow dissenter that will say “no” or has it crashed so its opinion can be ignored? This is a real and interesting problem – consider what happens if a router crashes and comes up 20 minutes later, after 99 processes agreed to commit a transaction and suddenly the 100th process is back on line objecting. But FLP then ask a peculiar question: if there is no upper bound on how long it can take for that possible dissenter to get around to telling us “no”, is there any way for the other processes to distinguish between “delayed” and crashed? The answer is obvious, making all the laborious formalism that follows unfortunate, but it’s also the wrong question.

In this paper, we show the surprising result that no completely asynchronous consensus protocol can tolerate even a single unannounced process death. We do not consider Byzantine failures, and we assume that the message system is reliable: it delivers all messages correctly and exactly once. Nevertheless, even with these assumptions, the stopping of a single process at an inopportune time can cause any distributed commit protocol to fail to reach agreement.

I cannot imagine what was ever surprising about this result. The problem statement says: you cannot distinguish between Crashed and Delayed. And “surprising result” is that – you cannot distinguish between Crashed and Delayed. Surely there is something more here?

We also assume that processes do not have access to synchronized clocks, so algorithms based on time-outs, for example, cannot be used. (In particular, the solutions in [6] are not applicable.) Finally, we do not postulate the ability to detect the death of a process, so it is impossible for one process to tell whether another has died (stopped entirely) or is just running very slowly.

Here’s the key phrase: “it is impossible for one process to tell whether another has died (stopped entirely) or is just running very slowly.”  Maybe someone can enlighten me on what I missed but this problem statement looks tautological.

From a systems engineering perspective, we don’t care whether a process has crashed or not if it won’t or can’t participate in the consensus process.  The obvious solution is to have something like a heartbeat message so that when the slow process rejoins the network it knows it has probably been ruled out of the consensus group and can participate in some sort of catch up protocol.  This is actually a pretty simple and durable test: a process that has not been able to participate in the protocol for some period of time should consider itself suspect until it can rejoin the protocol and waiting past that period without interaction is all the evidence that the remaining processes need to conclude this process is faulty (see also the CAP principle also sometimes unfortunately called a theorem.) There are also reasons to look for semantic solutions in addition to time based solutions. FLP do not consider semantics and rule out timing. What remains is, as they note, an environment where there are unsolvable reliability problems. The big mysteries that remain however are why this result is considered so surprising and why, having noticed the problem, researchers spent so much effort attempting to produce completely asynchronous methods.

(edited Sept 3 2016)