Barbara Liskov has just been announced as the recipient of the 2008 Turing Award, which is one of the most important prizes in computer science, and can be thought of as our field’s equivalent to the various Nobel Prizes. Professor Liskov is a worthy recipient of the award, even if judged alone by her citation which lists a number of the important contributions she has made to operating systems, programming languages and distributed systems.
Professor Liskov seems to be particularly well known for the Liskov substitution principle which says that some property of a supertype ought to hold of its subtypes. I’m not in any position to speak as to the importance of this contribution. However, her more recent work has been regarding the tolerance of Byzantine failures in distributed systems, which is much more close to my heart.
The only work of Liskov’s that I am really familiar with is the late 90s work on Practical Byzantine Fault Tolerance with Miguel Castro and is first published in this OSDI ’99 paper. I’m not going to do a full review, but the topic sits so nicely with my recent focus on consensus protocols that it makes sense to briefly discuss its importance.
This is one of my favourite papers from OSDI ’08 (yes, still doing a few reviews, trying to get to five or so before SOSP…). FlightPath is a system developed by some folks mainly at UT Austin for peer-to-peer streaming in dynamic networks. This is a reasonably challenging problem in itself, although one that’s seen a good deal of work before. However, the really cool thing about this paper is that they treat participants in the network as potentially rational agents. Since Lamport’s seminal work on the Byzantine generals problem, it’s been standard practice to assign one of two behaviour modes to members of distributed systems: either you’re alturistic, which means that you do exactly what the protocol tells you to do, no matter what the cost to yourself, or Byzantine, which means that you do whatever you like, again no matter what the cost to yourself.
It was realised recently that this is a false dichotomy: there’s a whole class of behaviour that’s not captured by these two extremes. Rational agents participate in a protocol as long as it is worth their while to do so. At its most simple, this means that rational agents will not incur a cost unless they expect to recoup a benefit that is worth equal to or more than the original cost to them. This gave rise to the Byzantine-Alturistic-Rational (BAR) model, due to the same UTA group, which can be used to more realistically model the performance of peer-to-peer protocols.