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。

十三、参考

源码 :[https://github.com/neo-project/neo/tree/master/neo/Consensus)

官方白皮书:http://docs.neo.org/zh-cn/basic/consensus/consensus.html

DBFT :https://www.jianshu.com/p/2383c7841d41

本文来源：NEOFANS

原文链接：http://neofans.org/

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