What kind of consensus used in Hyperledger Fabric?

Question

I don't know if this question make sense, I know Raft is consensus algorithm and use etcd to distributed the data, and i know etcd in Raft Ordering Service have a similar job with zookeeper in Kafka Ordering Service, but what I don't understand is, what kind of consensus used in Kafka ordering service?

Right now ordering service can use Raft or Kafka (deprecated), but Raft is a consensus algorithm yet Kafka is not. Or actually both of them just part of the consensus ordering phase? then does that mean now Fabric uses consensus algorithm to be part of consensus??? then what kind of consensus used in Fabric? I've read somewhere Fabric is not PBFT yet.

Answer 1

Let's talk about it as ordering and consensus and bring in Kafka and Raft.

In a distributed system, where messages are going to multiple nodes, the said nodes need a way to know which message came first, which was second, etc. Think of it as transactions on your bank account. If you have $20 in your account and someone pays you $30 so your account goes to $50, and you pay me $50 and your account goes to $0, its a valid sequence. But if your bank messes the order and you start with $20 and the transfer to me for $50 comes next, that check is going to bounce.

So that sequence (also known as order) is important, and in Fabric this is done by The Order Node.

For redundancy, to mitigate malicious intent, for decentralization and other reasons, you may not want just one node providing order. But, if you have n ordering nodes, how do you make sure they come up with one order of messages and not n variations of that order? You get a consensus among those nodes on the order of those messages. As one of the responders posted - you can achieve that consensus with RAFT or Kafka. Both are Crash Fault Tolerant (CFT) consensus algorithms, which means theoretically as long as majority of the ordering nodes are good, (2 out of 3, or 3 out of 5, etc) you are in good shape.

You are correct and RAFT does use etcd, but I think that's an implementation detail and not tied to the consensus conceptually. Etcd is an open source key-value store used to hold and manage information that distributed systems need to keep running. Its used by RAFT in Fabric, but it's also used by other projects like I think kubernetes uses it to manage all the configuration and metadata, etc

I am not aware of a Byzantine Fault tolerant library (where 2/3rd or fewer ordering nodes can be faulty I think and the system would still function) being available for Hyperledger Fabric yet, although there have been and continue to be discussions on it and the Fabric documentation states that RAFT CFT is a stepping stone to a BFT consensus library for Fabric in the future.

I would also reiterate reviewing the link to The Ordering Service Docs that was posted by another poster as good material to review for more information.

I also really like this introduction to RAFT video, it's not related to Fabric, but does an excellent job of explaining RAFT in general, if you are interested.