While reading the Raft paper today and remembering the Paxos implementation in Ceph, I was amazed that it looked so similar. Thanks to your explanation I now understand why ;-) On 14/09/2013 18:48, Gregory Farnum wrote: > On Fri, Sep 13, 2013 at 11:39 PM, Loic Dachary <loic@xxxxxxxxxxx> wrote: >> Hi, >> >> Ceph ( http://ceph.com/ ) relies on a custom implementation of Paxos to provide exabyte scale distributed storage. Like most people recently exposed to Paxos, I struggle to understand it ... but will keep studying until I get it :-) When a friend mentionned Raft ( http://en.wikipedia.org/wiki/Raft_%28computer_science%29 ), it looked like an easy way out. But it's very recent and I would very much appreciate your opinion. Do you think it is a viable alternative to Paxos ? > > Raft *is* the Paxos people use for all intents and purposes. The > original Paxos paper and the follow-on "Paxos Made Simple" are very > much mathematical algorithm papers which describe the necessary > constraints on a system with Paxos' properties, then define a very > general system which solves them, then describe a somewhat more > practical leader-based system. Every implementation I've seen in the > wild takes that leader system and then applies some of the > simplifications/enhancements which Lamport suggests in the end of his > original paper and that Raft has more precisely specified: you elect a > single leader (using what you might consider to be the full paxos > system, with very low commit rates!) who is the only one able to > propose values, then that leader proposes a stream of values which are > accepted by followers and applied to a shared state (eg, our leveldb > instance), and recovery happens by electing a new leader who gathers > the log off of all the other nodes in order to learn what's been > committed and what can be committed. > The reason people are enjoying Raft is that it's targeted at system > implementors instead of theoreticians, so the logical components are > called out a little more clearly and the phases are separated the way > you would split them when implementing the algorithms. That said, I'm > not sure it's *actually* more understandable (even their own test > results don't really support that assertion); I think you should just > read both papers and then use whichever one is more understandable as > the basis for further discussion until you really grok these > consistency algorithms. > > On Sat, Sep 14, 2013 at 8:16 AM, Noah Watkins <noah.watkins@xxxxxxxxxxx> wrote: >> I'm curious about what exactly the consensus requirement and >> assumptions are for the monitors. For instance, in the discussion >> between Loic and Joao, this statement: >> >> Joao: : the recovery logic in our implementation tries to aleviate >> the burden of recovering multiple versions at the same time. We >> propose a version, let the peons accept it, then move on to the next >> version. On ceph, we only provide one value at a time. >> >> seems to indicate that the leader is proposing changes sequentially. >> However, that makes Ceph's use of paxos sound a lot like the reason >> for the development of the Zab protocol used in Zookeeper: >> >> https://cwiki.apache.org/confluence/display/ZOOKEEPER/Zab+vs.+Paxos > > Yes. Our throughput expectations/requirements are significantly lower > than Zookeeper's. We could extend them to create a pipeline if we > really wanted to; the one-at-a-time isn't fundamental to the > algorithms we're using that I can recall. (I am somewhat irked by the > claim that Zab is a significantly different algorithm from Paxos. It > certainly fits into the Paxos family of algorithms, although it might > not be explicitly called out as a variation implementers could use in > the original paper like most others are.) > -Greg > Software Engineer #42 @ http://inktank.com | http://ceph.com > -- > To unsubscribe from this list: send the line "unsubscribe ceph-devel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- Loïc Dachary, Artisan Logiciel Libre All that is necessary for the triumph of evil is that good people do nothing.
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