NEO PDFT共识算法

dbft算法，通过多次网络请求确认，最终获得多数共识。缺点是网络开销大，如果网络有问题或者记账人性能不够会拖慢系统速度，如果记账人过多也会导致网络通信膨胀，难以快速达成一致。不适合在公链使用。而NEO定位是私有链或联盟链。记账人节点有限，而且机器，网络环境可以控制，因此适用于这种算法。既能避免较大的算力开销也能保证一致性。

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

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，

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。

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
    }

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;
    }

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);
            }
        }
    }

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();
            }
        }
    }
    }

议员广播信任信息

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;
    }

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;
        }
    }

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);
    }

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。

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

转自：（魂祭心） https://my.oschina.net/hunjixin/blog/1801892

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