11. Chained-BFT共识公共组件

11.1. 概述

超级链共识框架 一文中介绍了超级链底层有一个共识的公共组件叫chained-bft,其是Hotstuff算法的实现。HotStuff是一种简洁而优雅的bft改进算法。它具有以下优点:

  • 它的设计中将liveness和safty解耦开来,使得非常方便与其他的共识进行扩展;
  • 将bft过程拆解成3阶段,每个阶段都是o(n)的通信;
  • 它允许一个节点处于不同的view,并且将view的切换与区块结合起来,使得其能够实现异步共识,进一步提升共识的效率。

这样一个chained-bft可以在给定主集合的场景下确保网络的共识安全性,并且通过与外层共识配合工作实现共识的活性保证。

11.2. 核心数据结构

  1. enum QCState {
  2. NEW_VIEW = 0;
  3. PREPARE = 1;
  4. PRE_COMMIT = 2;
  5. COMMIT = 3;
  6. DECIDE = 4;
  7. }
  8. // QuorumCert is a data type that combines a collection of signatures from replicas.
  9. message QuorumCert {
  10. // The id of block this QC certified.
  11. bytes BlockId = 1;
  12. // The current type of this QC certified.
  13. // the type contains `NEW_VIEW`, `PREPARE`,`PRE_COMMIT`, `COMMIT`, `DECIDE`.
  14. State Type = 2;
  15. // The view number of this QC certified.
  16. int64 ViewNumber = 3;
  17. // SignInfos is the signs of the leader gathered from replicas
  18. // of a specifically certType.
  19. QCSignInfos SignInfos = 4;
  20. }
  21. // QCSignInfos is the signs of the leader gathered from replicas of a specifically certType.
  22. // A slice of signs is used at present.
  23. // TODO zq: It will be change to Threshold-Signatures
  24. // after Crypto lib support Threshold-Signatures.
  25. message QCSignInfos {
  26. // QCSignInfos
  27. map<string, SignInfo> QCSignInfos = 1;
  28. }
  29. // SignInfo is the signature information of the
  30. message SignInfo {
  31. string Address = 1;
  32. string PublicKey = 2;
  33. bytes Sign = 3;
  34. }
  35. // ChainedBftMessage is the message of the protocal
  36. // In hotstuff, there are two kinds of messages, "NEW_VIEW_Message" and "QC_Message".
  37. // In XuperChain, there is only one kind of message, "NEW_VIEW. The "QC_Message" is resuded with "BroadcastBlock" message.
  38. message ChainedBftMessage {
  39. // Message Type
  40. QCState Type = 1;
  41. // Justify is the QC of the leader gathered, send to next leader.
  42. QuorumCert Justify = 2;
  43. }
  44. // ChainedBftMessage is the vote message of
  45. message ChainedBftVoteMessage {
  46. // The id of block this message vote for.
  47. bytes BlockId = 1;
  48. // Replica will sign the QCMessage if the QuorumCert if valid.
  49. SignInfo signature = 2;
  50. }

整个chained-bft中主要包括三部分,分别是状态机 SmrSafetyRulesPacemakerInterface

11.3. Smr

Smr是 chained-bft 的核心实例。他的主要的作用有以下几点:

  1. 维护节点链的chained-bft共识状态机;
  2. 在外层共识的驱动下发起 NewViewNewProposal 等消息并更新本地状态;
  3. 处理其他验证节点的消息并更新本地状态;

../_images/chained-bft.png

11.4. Safety Rule

Safety Rule 是一个验证节点是否要接受一个新的Proposal的安全性规则,主要有三条:

  1. 判断当前Proposal的View值是否大于本地locked Proposal的View值;
  2. 验证当前Proposal中上一个Proposal的投票信息有效性和投票个数是否大于系统矿工数目的2/3;
  3. 验证当前Proposal的ProposalMsg是否有效;

当一个验证节点收到一个新的提案时,如果满足上述 Safety Rule 的认证,则会给这个提案进行投票,否则拒绝这次提案。

11.5. PacemakerInterface

Hotstuff算法的一大特点就是将共识的liveness和safety分开。PacemakerInterface是Hotstuff算法Pacemaker的接口定义,外层共识通过实现这些接口,可以推进内层共识的状态轮转。不同的外层共识可以有不同的实现。目前超级链已经实现了DPoS+Hotstuff,具体的方案如下所示:

../_images/pacemaker.png