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。

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，

输入图片说明

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
    
      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    }

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。

///         /// 获取下一个区块的记账人列表        /// 
        /// 
    
     返回一组公钥，表示下一个区块的记账人列表
            public ECPoint[] GetValidators()        {            lock (_validators)            {                if (_validators.Count == 0)                {                    _validators.AddRange(GetValidators(Enumerable.Empty
    
     ()));                }                return _validators.ToArray();            }        }        public virtual IEnumerable
     
       GetValidators(IEnumerable
      
        others)        {            DataCache
       
         accounts = GetStates
        
         ();            DataCache
         
           validators = GetStates
          
           (); MetaDataCache
           
             validators_count = GetMetaData
            
             (); ///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
             
               sv = new HashSet
              
               (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
               
                 result; if (pubkeys.Length == count) { result = pubkeys; } else { HashSet
                
                  hashSet = new HashSet
                 
                  (pubkeys); for (int i = 0; i < StandbyValidators.Length && hashSet.Count < count; i++) hashSet.Add(StandbyValidators[i]); result = hashSet; } return result.OrderBy(p => p); }

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
    
     ();        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
     
       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。

输入图片说明