The only issue I have with raft (besides bugs in the various implementations I have to deal with at work) is that it is not yet proven to truly be simpler than Paxos. In other words, is it a genuinely simpler algorithm that provides the the same guarantees as Paxos, or is it simpler because it leaves edge cases unspecified, forcing implementors to grind through the edge cases with heuristics? If it's the second, it's not actually simpler, it's just worse.
I'm not saying that it isn't genuinely simpler. Just that in discussions about raft, I see most people blindly assuming that it is. But I don't believe that it has been proven yet. I think this is the nuclear power plant part of the bikeshed discussion. [1]
If you scrub to the 50 minute mark of this talk[2] by Leslie Lamport (creator of Paxos), my coworker David Varvel asks him this question and he gives more or less the same answer.
Is there a good open-source Paxos implementation? There are many open-source Raft implementations, some of which are good. goraft powers etcd, for example.
I believe most developers just want to consume a working library. It seems likely that the plethora of Raft libraries and dearth of Paxos libraries is because it is easier to write a Raft library, but I also don't think it matters _why_, it only matters that there are good Raft libraries.
CoreOs (etcd team) is currently rewriting their raft implementation because it had a lot of problems. I don't believe there is a "good" implementation out there yet, because it's all very new. And should an implantation be "good", AKA implement the spec without introducing incidental errors, will it still have edge cases because the spec has edge cases? This is all currently unknown. It's precisely these assumptions that I'm pointing out.
A quick google search brings up libpaxos and other open paxos implementations, so it appears that they exist.
The new etcd raft implementation is working well and has a solid set of deterministic tests. We recently released an alpha of etcd that utilizes this improved raft implementation: https://github.com/coreos/etcd/releases.
The core raft state machine comes in under 600 LOC last time I checked and the docs are here: https://godoc.org/github.com/coreos/etcd/raft. There are a few non-etcd projects that have started working with it, most notably the cockroachdb folks have started to use it to implement the "multi-raft" that they need for their DB. You can find the discussion and work on github.com/cockroachdb.
It turns out getting a raft implementation that is easy to audit with a copy of the paper in hand and easy to test in a fast and consistent way requires a significant engineering effort. I hope that based on the lessons learned from goraft that this implementation is something that a number of Go projects can use and leverage. Particularly since we carefully separated the WAL, RPC and state machine from each other.
Interesting - thanks for the libpaxos link. Is it any good?
I'm curious: what are these edge cases you're talking about? I've worked on a Raft implementation, and I don't remember any particular edge cases (though it's been a while).
There's definitely some interesting choices for when to replace nodes in the face of failure, but I don't think that's a Raft algorithm shortcoming.
I believe most developers just want to consume a working library.
Right. Maybe they'd even prefer not to have to bother, they just want the availability thing solved for parallel service instances. Perhaps the more interesting question is, "how do you neatly fit raft/paxos like functionality at deployment time in to the average SOA service development process?" (subtext: without making devs learn either algorithm)
I think the answer there is an improved devops process that discourages any form of assumption around infrastructure type and can easily run failure tests at various levels of the deployment topology.
Ensemble module, the way I understand it, provides ability to have "consistent" CP (in CAP theorem sense) updates in the otherwise default AP database setup in Riak.
Disclaimer: I am not affiliated in any way with Basho, just follow riak_core module development because I think it is a piece of very cool engineering.
To me it's clearly simpler. Paxos involces special client numbers (to guarantee ordering), but you still have to fall back to randomized delays to avoid livelock; raft just starts with randomized delays up front.
If you can read the raft paper and come away also knowing how group membership changes (wow) and log compaction works, in additional to the "basic" log replication consensus, that's huge.
edit: I'm sure it's still 'hard' to implement, like bringing a brand new replacement machine up from scratch (copying the full data set over, to warm him up, and then starting replication efficiently), i.e. there's still a lot of coding to do
I doubt any consensus algorithm can be simpler than single decree paxos. But what people usually care about is deciding on a sequence of operations. Paxos made live, and others have engineered their own solutions to make multi-paxos efficient at this. Raft solves much of this for you, right from the start.
> In other words, is it a genuinely simpler algorithm that provides the the same guarantees as Paxos, or is it simpler because it leaves edge cases unspecified, forcing implementors to grind through the edge cases with heuristics?
Isn't it formally proven correct? And "simpler" would be subjective right? So what do you mean by "proven to truly be simpler than Paxos"? Thanks, and also that talk looks really great.
I'm not saying that it isn't genuinely simpler. Just that in discussions about raft, I see most people blindly assuming that it is. But I don't believe that it has been proven yet. I think this is the nuclear power plant part of the bikeshed discussion. [1]
If you scrub to the 50 minute mark of this talk[2] by Leslie Lamport (creator of Paxos), my coworker David Varvel asks him this question and he gives more or less the same answer.
[1] http://en.wikipedia.org/wiki/Parkinson's_law_of_triviality [2] http://channel9.msdn.com/Events/Build/2014/3-642