3 ms·

Right, but Paxos is double hard in comparison. I’ve read both papers multiple times, tried to implement and failed, and I still don’t think I understand Paxos.

candiddevmike 2y ago
IMO, Paxos has a lot less edge cases than Raft, mostly because of the complexity of the implementation covers them/forces you to think about how to handle them.
- hinkley 2y ago
  How many people actually understand Paxos? The assertion at the time was that only a few people understood it well enough to make a correct implementation, the others were full of bugs. The problem we have with Lamport is that he’s very good at talking to computers but not so good at talking to humans. I think the world would be a better place today if someone had forced him to learn to speak human. He did a presentation at MS just after he won the Turing Award. He prefaces it with how important writing is to thinking, and how you don’t really know what you think until you write it down. Those are the words and thinking of an introvert. Writing is still the shallow end of understanding. The deep end is teaching. If you understand something and you teach it to others, then you have proven that you understand it, and caused that understanding not to be lost to posterity. If you only write about it, it might work as instructional material, or it may require some very clever people who can teach themselves using your words. But they may also get it wrong, and not have you for feedback. The latter is where seem to be with Leslie’s works. The consensus is that few people actually understand what he’s talking about well enough to implement it correctly. I had a coworker once who was shocked to learn I read the ACM SIGPLAN proceedings. “You can read those??” I knew what he meant and yeah, a lot of those were very unapproachable and I understood two thirds of them and only half of each of the rest. Before I committed to using Raft I gave Lamport’s paper a try. It was a slog and he doesn’t sell the why of each part. He’s just giving you a very, very long recipe without the mental models necessary to reproduce it robustly.
mrkeen 2y ago
I'm on the other side. I think Leslie Lamport asserted that Paxos is minimal, and that "all other consensus algorithms are just Paxos with more steps". I'm inclined to believe him. I've implemented Paxos but I can't get through "Raft for dummies" style blog posts. Regarding Raft [1]: > The consensus problem is divided into three sub-problems: Leader election, Replication and Safety. What is leader election? It's a distributed system coming to consensus on a fact (i.e. who the leader is.) Then once you have the leader, you do additional steps. The entirety of Paxos is a distributed system coming to consensus on a fact. When I read these posts, i see things like "timeout", "heartbeat", and I think: timeout according to whom? I read "once the leader has been elected", um, hangon, according to whom? Has node 1 finally agreed on the leader, just while node 3 has given up and started another election? I don't doubt that Raft is correct, but the writing about it seems simple by glossing over details. Paxos, on the other hand, seems timeless. (And the writing about it doesn't trigger my "distributed system fallacies" reaction) [1] https://www.brianstorti.com/raft/ https://www.brianstorti.com/raft/
- spmurrayzzz 2y ago
  I tend to agree that many explanations of raft dont get into the useful details and handwave some of the hard problems. But the original paper does do a good job of this and is pretty accessible to read IMO. > I read "once the leader has been elected", um, hangon, according to whom? Has node 1 finally agreed on the leader, just while node 3 has given up and started another election? The simple response I think to "according to whom" is "the majority of voting nodes". When the leader assumes its role, it sends heartbeats which are then accepted by the other nodes in the cluster. Even if (in your example) node 3 starts a new election, it will only succeed if it can get a majority of votes. If node 2 has already acknowledged a leader, it won't vote for node 3 in the same term. There's some implicit concessions inherent there around eventual consistency, but I don't think thats novel to Raft compared to other distributed consensus protocols.
  - withinboredom 2y ago
    > The simple response I think to "according to whom" is "the majority of voting nodes". Reminds me of this one time we had a Raft cluster arguing over who was the leader for 20 minutes in production. Raft leader election is non-deterministic, while Paxos is deterministic. It can 'randomly' get into a situation it cannot resolve for quite a long time.
Vervious 2y ago
Raft has a problem where, in the protocol description, sometimes I have no idea why some line is there, but if you take it out the protocol comes to a grinding halt... it's really a mumble jumble. It has good intuition, but the details are really messy, it's edge cases all the way down. Whereas I think each line of pseudocode in Paxos is much more motivated. In other words, if a philosopher had to design a crash-fault protocol from scratch, without having seen any before, I think 80% of the time it would look exactly like Paxos.
jeffbee 2y ago
Students of raft say they understand it but when they have to implement it they make as many mistakes as students of paxos. The simplicity is misleading.

candiddevmike 2y ago

IMO, Paxos has a lot less edge cases than Raft, mostly because of the complexity of the implementation covers them/forces you to think about how to handle them.

hinkley 2y ago
How many people actually understand Paxos? The assertion at the time was that only a few people understood it well enough to make a correct implementation, the others were full of bugs. The problem we have with Lamport is that he’s very good at talking to computers but not so good at talking to humans. I think the world would be a better place today if someone had forced him to learn to speak human. He did a presentation at MS just after he won the Turing Award. He prefaces it with how important writing is to thinking, and how you don’t really know what you think until you write it down. Those are the words and thinking of an introvert. Writing is still the shallow end of understanding. The deep end is teaching. If you understand something and you teach it to others, then you have proven that you understand it, and caused that understanding not to be lost to posterity. If you only write about it, it might work as instructional material, or it may require some very clever people who can teach themselves using your words. But they may also get it wrong, and not have you for feedback. The latter is where seem to be with Leslie’s works. The consensus is that few people actually understand what he’s talking about well enough to implement it correctly. I had a coworker once who was shocked to learn I read the ACM SIGPLAN proceedings. “You can read those??” I knew what he meant and yeah, a lot of those were very unapproachable and I understood two thirds of them and only half of each of the rest. Before I committed to using Raft I gave Lamport’s paper a try. It was a slog and he doesn’t sell the why of each part. He’s just giving you a very, very long recipe without the mental models necessary to reproduce it robustly.

mrkeen 2y ago

I'm on the other side. I think Leslie Lamport asserted that Paxos is minimal, and that "all other consensus algorithms are just Paxos with more steps". I'm inclined to believe him. I've implemented Paxos but I can't get through "Raft for dummies" style blog posts. Regarding Raft [1]: > The consensus problem is divided into three sub-problems: Leader election, Replication and Safety. What is leader election? It's a distributed system coming to consensus on a fact (i.e. who the leader is.) Then once you have the leader, you do additional steps. The entirety of Paxos is a distributed system coming to consensus on a fact. When I read these posts, i see things like "timeout", "heartbeat", and I think: timeout according to whom? I read "once the leader has been elected", um, hangon, according to whom? Has node 1 finally agreed on the leader, just while node 3 has given up and started another election? I don't doubt that Raft is correct, but the writing about it seems simple by glossing over details. Paxos, on the other hand, seems timeless. (And the writing about it doesn't trigger my "distributed system fallacies" reaction) [1] https://www.brianstorti.com/raft/ https://www.brianstorti.com/raft/

spmurrayzzz 2y ago
I tend to agree that many explanations of raft dont get into the useful details and handwave some of the hard problems. But the original paper does do a good job of this and is pretty accessible to read IMO. > I read "once the leader has been elected", um, hangon, according to whom? Has node 1 finally agreed on the leader, just while node 3 has given up and started another election? The simple response I think to "according to whom" is "the majority of voting nodes". When the leader assumes its role, it sends heartbeats which are then accepted by the other nodes in the cluster. Even if (in your example) node 3 starts a new election, it will only succeed if it can get a majority of votes. If node 2 has already acknowledged a leader, it won't vote for node 3 in the same term. There's some implicit concessions inherent there around eventual consistency, but I don't think thats novel to Raft compared to other distributed consensus protocols.
- withinboredom 2y ago
  > The simple response I think to "according to whom" is "the majority of voting nodes". Reminds me of this one time we had a Raft cluster arguing over who was the leader for 20 minutes in production. Raft leader election is non-deterministic, while Paxos is deterministic. It can 'randomly' get into a situation it cannot resolve for quite a long time.

Vervious 2y ago

Raft has a problem where, in the protocol description, sometimes I have no idea why some line is there, but if you take it out the protocol comes to a grinding halt... it's really a mumble jumble. It has good intuition, but the details are really messy, it's edge cases all the way down. Whereas I think each line of pseudocode in Paxos is much more motivated. In other words, if a philosopher had to design a crash-fault protocol from scratch, without having seen any before, I think 80% of the time it would look exactly like Paxos.

jeffbee 2y ago

Students of raft say they understand it but when they have to implement it they make as many mistakes as students of paxos. The simplicity is misleading.