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NEXT社区小课堂 | 第五课：NEO-共识算法dBFT源码解析

日期：2019-04-19点击：437收藏

NEXT社区 | 小课堂

由于近期NEXT社区加入很多新的小伙伴，有在校大学生，有对区块链感兴趣的传统企业从业者。为了更方便、更系统的让NEXT社区的伙伴们了解NEO的技术知识，因此我们开设了小课堂，每周3节，向大家普及NEO相关的知识要点！ 公号：NEONEXT

NEXT社区小课堂 | 第五课

NEO-共识机制dBFT源码解析

一、NEO DBFT共识算法

dbft改进自算法pbft算法，pbft算法通过多次网络请求确认，最终获得多数共识。其缺点在于随着随着节点的增加，网络开销呈指数级增长，网络通信膨胀，难以快速达成一致。neo的解决方案是通过投票选取出一定数量的节点作为记账人，由此减少网络消耗又可以兼顾交易速度，这也是dbft中d的由来。

二、代码结构说明

├── Consensus │ ├── ChangeView.cs //viewchange 消息 │ ├── ConsensusContext.cs //共识上下文 │ ├── ConsensusMessage.cs //共识消息 │ ├── ConsensusMessageType.cs //共识消息类型 ChangeView/PrepareRequest/PrepareResponse │ ├── ConsensusService.cs //共识核心代码 │ ├── ConsensusState.cs //节点共识状态 │ ├── PrepareRequest.cs //请求消息 │ └── PrepareResponse.cs //签名返回消息

三、共识状态变化流程

1、开启共识的节点分为两大类，非记账人和记账人节点，非记账人的不参与共识，记账人参与共识流程

2、选择议长，Neo议长产生机制是根据当前块高度和记账人数量做MOD运算得到，议长实际上按顺序当选

3、节点初始化，议长为primary节点，议员为backup节点。

4、满足出块条件后议长发送PrepareRequest

5、议员收到请求后，验证通过签名发送PrepareResponse

6、记账节点接收到PrepareResponse后，节点保存对方的签名信息，检查如果超过三分之二则发送 block

7、节点接收到block，PersistCompleted事件触发后整体重新初始化

四、共识上下文核心成员

public const uint Version = 0; public ConsensusState State; //节点当前共识状态 public UInt256 PrevHash; public uint BlockIndex; //块高度 public byte ViewNumber; //试图状态 public ECPoint[] Validators; //记账人 public int MyIndex; //当前记账人次序 public uint PrimaryIndex; //当前记账的记账人 public uint Timestamp; public ulong Nonce; public UInt160 NextConsensus; //共识标识 public UInt256[] TransactionHashes; public Dictionary<UInt256, Transaction> Transactions; public byte[][] Signatures; //记账人签名 public byte[] ExpectedView; //记账人试图 public KeyPair KeyPair; public int M => Validators.Length - (Validators.Length - 1) / 3; //三分之二数量

ExpectedView 维护视图状态中，用于在议长无法正常工作时重新发起新一轮共识。

Signatures 用于维护共识过程中的确认状态。

四、节点共识状态

[Flags] internal enum ConsensusState : byte { Initial = 0x00, // 0 Primary = 0x01, // 1 Backup = 0x02, // 10 RequestSent = 0x04, // 100 RequestReceived = 0x08, // 1000 SignatureSent = 0x10, // 10000 BlockSent = 0x20, // 100000 ViewChanging = 0x40, //1000000 }

五、代理节点选择

neo的dbft算法的delegate部分主要体现在一个函数上面,每次产生区块后会重新排序资产，选择前记账人数量的节点出来参与下一轮共识。

/// <summary> /// 获取下一个区块的记账人列表 /// </summary> /// <returns>返回一组公钥，表示下一个区块的记账人列表</returns> public ECPoint[] GetValidators() { lock (_validators) { if (_validators.Count == 0) { _validators.AddRange(GetValidators(Enumerable.Empty<Transaction>())); } return _validators.ToArray(); } } public virtual IEnumerable<ECPoint> GetValidators(IEnumerable<Transaction> others) { DataCache<UInt160, AccountState> accounts = GetStates<UInt160, AccountState>(); DataCache<ECPoint, ValidatorState> validators = GetStates<ECPoint, ValidatorState>(); MetaDataCache<ValidatorsCountState> validators_count = GetMetaData<ValidatorsCountState>(); ///更新账号资产情况 foreach (Transaction tx in others) { foreach (TransactionOutput output in tx.Outputs) { AccountState account = accounts.GetAndChange(output.ScriptHash, () => new AccountState(output.ScriptHash)); if (account.Balances.ContainsKey(output.AssetId)) account.Balances[output.AssetId] += output.Value; else account.Balances[output.AssetId] = output.Value; if (output.AssetId.Equals(GoverningToken.Hash) && account.Votes.Length > 0) { foreach (ECPoint pubkey in account.Votes) validators.GetAndChange(pubkey, () => new ValidatorState(pubkey)).Votes += output.Value; validators_count.GetAndChange().Votes[account.Votes.Length - 1] += output.Value; } } foreach (var group in tx.Inputs.GroupBy(p => p.PrevHash)) { Transaction tx_prev = GetTransaction(group.Key, out int height); foreach (CoinReference input in group) { TransactionOutput out_prev = tx_prev.Outputs[input.PrevIndex]; AccountState account = accounts.GetAndChange(out_prev.ScriptHash); if (out_prev.AssetId.Equals(GoverningToken.Hash)) { if (account.Votes.Length > 0) { foreach (ECPoint pubkey in account.Votes) { ValidatorState validator = validators.GetAndChange(pubkey); validator.Votes -= out_prev.Value; if (!validator.Registered && validator.Votes.Equals(Fixed8.Zero)) validators.Delete(pubkey); } validators_count.GetAndChange().Votes[account.Votes.Length - 1] -= out_prev.Value; } } account.Balances[out_prev.AssetId] -= out_prev.Value; } } switch (tx) { #pragma warning disable CS0612 case EnrollmentTransaction tx_enrollment: validators.GetAndChange(tx_enrollment.PublicKey, () => new ValidatorState(tx_enrollment.PublicKey)).Registered = true; break; #pragma warning restore CS0612 case StateTransaction tx_state: foreach (StateDescriptor descriptor in tx_state.Descriptors) switch (descriptor.Type) { case StateType.Account: ProcessAccountStateDescriptor(descriptor, accounts, validators, validators_count); break; case StateType.Validator: ProcessValidatorStateDescriptor(descriptor, validators); break; } break; } } //排序 int count = (int)validators_count.Get().Votes.Select((p, i) => new { Count = i, Votes = p }).Where(p => p.Votes > Fixed8.Zero).ToArray().WeightedFilter(0.25, 0.75, p => p.Votes.GetData(), (p, w) => new { p.Count, Weight = w }).WeightedAverage(p => p.Count, p => p.Weight); count = Math.Max(count, StandbyValidators.Length); HashSet<ECPoint> sv = new HashSet<ECPoint>(StandbyValidators); ECPoint[] pubkeys = validators.Find().Select(p => p.Value).Where(p => (p.Registered && p.Votes > Fixed8.Zero) || sv.Contains(p.PublicKey)).OrderByDescending(p => p.Votes).ThenBy(p => p.PublicKey).Select(p => p.PublicKey).Take(count).ToArray(); IEnumerable<ECPoint> result; if (pubkeys.Length == count) { result = pubkeys; } else { HashSet<ECPoint> hashSet = new HashSet<ECPoint>(pubkeys); for (int i = 0; i < StandbyValidators.Length && hashSet.Count < count; i++) hashSet.Add(StandbyValidators[i]); result = hashSet; } return result.OrderBy(p => p); }

六、议长选择

在初始化共识状态的时候会设置PrimaryIndex，获知当前议长。原理就是简单的MOD运算。这里有分为两种情况，如果节点正常则直接块高度和记账人数量mod运算即可，如果存在一场情况，则需要根据view_number进行调整。

//file /Consensus/ConsensusService.cs InitializeConsensus方法 if (view_number == 0) context.Reset(wallet); else context.ChangeView(view_number);

//file /Consensus/ConsensusContext.cs public void ChangeView(byte view_number) { int p = ((int)BlockIndex - view_number) % Validators.Length; State &= ConsensusState.SignatureSent; ViewNumber = view_number; PrimaryIndex = p >= 0 ? (uint)p : (uint)(p + Validators.Length);//当前记账人 if (State == ConsensusState.Initial) { TransactionHashes = null; Signatures = new byte[Validators.Length][]; } ExpectedView[MyIndex] = view_number; _header = null; }

//file /Consensus/ConsensusContext.cs public void Reset(Wallet wallet) { State = ConsensusState.Initial; PrevHash = Blockchain.Default.CurrentBlockHash; BlockIndex = Blockchain.Default.Height + 1; ViewNumber = 0; Validators = Blockchain.Default.GetValidators(); MyIndex = -1; PrimaryIndex = BlockIndex % (uint)Validators.Length; //当前记账人 TransactionHashes = null; Signatures = new byte[Validators.Length][]; ExpectedView = new byte[Validators.Length]; KeyPair = null; for (int i = 0; i < Validators.Length; i++) { WalletAccount account = wallet.GetAccount(Validators[i]); if (account?.HasKey == true) { MyIndex = i; KeyPair = account.GetKey(); break; } } _header = null; }

七、状态初始化

如果是议长则状态标记为ConsensusState.Primary，同时改变定时器触发事件，再上次出块15s后触发。议员则设置状态为 ConsensusState.Backup，时间调整为30s后触发，如果议长不能正常工作，则这个触发器会开始起作用（具体后边再详细分析）。

//file /Consensus/ConsensusContext.cs private void InitializeConsensus(byte view_number) { lock (context) { if (view_number == 0) context.Reset(wallet); else context.ChangeView(view_number); if (context.MyIndex < 0) return; Log($"initialize: height={context.BlockIndex} view={view_number} index={context.MyIndex} role={(context.MyIndex == context.PrimaryIndex ? ConsensusState.Primary : ConsensusState.Backup)}"); if (context.MyIndex == context.PrimaryIndex) { context.State |= ConsensusState.Primary; if (!context.State.HasFlag(ConsensusState.SignatureSent)) { FillContext(); //生成mine区块 } if (context.TransactionHashes.Length > 1) { //广播自身的交易 InvPayload invPayload = InvPayload.Create(InventoryType.TX, context.TransactionHashes.Skip(1).ToArray()); foreach (RemoteNode node in localNode.GetRemoteNodes()) node.EnqueueMessage("inv", invPayload); } timer_height = context.BlockIndex; timer_view = view_number; TimeSpan span = DateTime.Now - block_received_time; if (span >= Blockchain.TimePerBlock) timer.Change(0, Timeout.Infinite); else timer.Change(Blockchain.TimePerBlock - span, Timeout.InfiniteTimeSpan); } else { context.State = ConsensusState.Backup; timer_height = context.BlockIndex; timer_view = view_number; timer.Change(TimeSpan.FromSeconds(Blockchain.SecondsPerBlock << (view_number + 1)), Timeout.InfiniteTimeSpan); } } }

八、议长发起请求

议长到了该记账的时间后，执行下面方法，发送MakePrepareRequest请求，自身状态转变为RequestSent，也设置了30s后重复触发定时器（同样也是再议长工作异常时起效）。

//file /Consensus/ConsensusContext.cs private void OnTimeout(object state) { lock (context) { if (timer_height != context.BlockIndex || timer_view != context.ViewNumber) return; Log($"timeout: height={timer_height} view={timer_view} state={context.State}"); if (context.State.HasFlag(ConsensusState.Primary) && !context.State.HasFlag(ConsensusState.RequestSent)) { Log($"send perpare request: height={timer_height} view={timer_view}"); context.State |= ConsensusState.RequestSent; if (!context.State.HasFlag(ConsensusState.SignatureSent)) { context.Timestamp = Math.Max(DateTime.Now.ToTimestamp(), Blockchain.Default.GetHeader(context.PrevHash).Timestamp + 1); context.Signatures[context.MyIndex] = context.MakeHeader().Sign(context.KeyPair); } SignAndRelay(context.MakePrepareRequest()); timer.Change(TimeSpan.FromSeconds(Blockchain.SecondsPerBlock << (timer_view + 1)), Timeout.InfiniteTimeSpan); } else if ((context.State.HasFlag(ConsensusState.Primary) && context.State.HasFlag(ConsensusState.RequestSent)) || context.State.HasFlag(ConsensusState.Backup)) { RequestChangeView(); } } }

九、议员广播签名信息

议员接收到PrepareRequest，会对节点信息进行验证，自身不存在的交易会去同步交易。交易同步完成后，验证通过会进行发送PrepareResponse ，包含自己的签名信息，表示自己已经通过了节点验证。状态转变为ConsensusState.SignatureSent。

//file /Consensus/ConsensusContext.cs private void OnPrepareRequestReceived(ConsensusPayload payload, PrepareRequest message) { Log($"{nameof(OnPrepareRequestReceived)}: height={payload.BlockIndex} view={message.ViewNumber} index={payload.ValidatorIndex} tx={message.TransactionHashes.Length}"); if (!context.State.HasFlag(ConsensusState.Backup) || context.State.HasFlag(ConsensusState.RequestReceived)) return; if (payload.ValidatorIndex != context.PrimaryIndex) return; if (payload.Timestamp <= Blockchain.Default.GetHeader(context.PrevHash).Timestamp || payload.Timestamp > DateTime.Now.AddMinutes(10).ToTimestamp()) { Log($"Timestamp incorrect: {payload.Timestamp}"); return; } context.State |= ConsensusState.RequestReceived; context.Timestamp = payload.Timestamp; context.Nonce = message.Nonce; context.NextConsensus = message.NextConsensus; context.TransactionHashes = message.TransactionHashes; context.Transactions = new Dictionary<UInt256, Transaction>(); if (!Crypto.Default.VerifySignature(context.MakeHeader().GetHashData(), message.Signature, context.Validators[payload.ValidatorIndex].EncodePoint(false))) return; context.Signatures = new byte[context.Validators.Length][]; context.Signatures[payload.ValidatorIndex] = message.Signature; Dictionary<UInt256, Transaction> mempool = LocalNode.GetMemoryPool().ToDictionary(p => p.Hash); foreach (UInt256 hash in context.TransactionHashes.Skip(1)) { if (mempool.TryGetValue(hash, out Transaction tx)) if (!AddTransaction(tx, false)) return; } if (!AddTransaction(message.MinerTransaction, true)) return; if (context.Transactions.Count < context.TransactionHashes.Length) { UInt256[] hashes = context.TransactionHashes.Where(i => !context.Transactions.ContainsKey(i)).ToArray(); LocalNode.AllowHashes(hashes); InvPayload msg = InvPayload.Create(InventoryType.TX, hashes); foreach (RemoteNode node in localNode.GetRemoteNodes()) node.EnqueueMessage("getdata", msg); } }

//file /Consensus/ConsensusContext.cs private bool AddTransaction(Transaction tx, bool verify) { if (Blockchain.Default.ContainsTransaction(tx.Hash) || (verify && !tx.Verify(context.Transactions.Values)) || !CheckPolicy(tx)) { Log($"reject tx: {tx.Hash}{Environment.NewLine}{tx.ToArray().ToHexString()}"); RequestChangeView(); return false; } context.Transactions[tx.Hash] = tx; if (context.TransactionHashes.Length == context.Transactions.Count) { if (Blockchain.GetConsensusAddress(Blockchain.Default.GetValidators(context.Transactions.Values).ToArray()).Equals(context.NextConsensus)) { Log($"send perpare response"); context.State |= ConsensusState.SignatureSent; context.Signatures[context.MyIndex] = context.MakeHeader().Sign(context.KeyPair); SignAndRelay(context.MakePrepareResponse(context.Signatures[context.MyIndex])); CheckSignatures(); } else { RequestChangeView(); return false; } } return true; }

十、共识达成后广播区块

其他节点接收到PrepareResponse后，在自己的签名列表中记录对方的签名信息，再检查自己的签名列表是否有超过三分之二的签名了，有则判断共识达成开始广播生成的区块。状态状变为ConsensusState.BlockSent。

private void CheckSignatures() { if (context.Signatures.Count(p => p != null) >= context.M && context.TransactionHashes.All(p => context.Transactions.ContainsKey(p))) { Contract contract = Contract.CreateMultiSigContract(context.M, context.Validators); Block block = context.MakeHeader(); ContractParametersContext sc = new ContractParametersContext(block); for (int i = 0, j = 0; i < context.Validators.Length && j < context.M; i++) if (context.Signatures[i] != null) { sc.AddSignature(contract, context.Validators[i], context.Signatures[i]); j++; } sc.Verifiable.Scripts = sc.GetScripts(); block.Transactions = context.TransactionHashes.Select(p => context.Transactions[p]).ToArray(); Log($"relay block: {block.Hash}"); if (!localNode.Relay(block)) Log($"reject block: {block.Hash}"); context.State |= ConsensusState.BlockSent; } }

十一、Blockchain_PersistCompleted后恢复状态

区块广播后，节点接收到这个信息，会保存区块，保存完成后触发PersistCompleted事件，最终回到初始状态，重新开始新一轮的共识。

private void Blockchain_PersistCompleted(object sender, Block block) { Log($"persist block: {block.Hash}"); block_received_time = DateTime.Now; InitializeConsensus(0); }

十二、错误分析

如果议长无法完成记账任务

这种情况下议长在超出时间之后依然无法达成共识，此时议员会增长自身的ExpectedView值，并且广播出去，如果检查到三分之二的成员viewnumber都加了1,则在这些相同viewnumber的节点之间开始新一轮共识，重新选择议长，发起共识。如果此时原来的议长恢复正常，time到时间后会增长自身的viewnumber，慢慢追上当前的视图状态。