最近有点懒散，竟然有一周没有读源码了。想来惭愧，今天重拾bitcoin源码，来看看比特币的共识机制。
我们都知道比特币采用的共识机制是工作量证明(POW),将区块链的记账的权利通过一个数学问题的解决来决定。前面，在用python搭建简单的区块链框架中，我们简单地通过一个简单的🌰模拟了POW机制的挖矿原理。
- 源码路径:.../src/consensus/params.h
namespace Consensus {
enum DeploymentPos
{
DEPLOYMENT_TESTDUMMY,
DEPLOYMENT_CSV, // Deployment of BIP68, BIP112, and BIP113.
DEPLOYMENT_SEGWIT, // Deployment of BIP141, BIP143, and BIP147.
// NOTE: Also add new deployments to VersionBitsDeploymentInfo in versionbits.cpp
MAX_VERSION_BITS_DEPLOYMENTS
};
/**
* Struct for each individual consensus rule change using BIP9.
*
**使用bip9改变共识规则的结构体
*/
struct BIP9Deployment {
/** Bit position to select the particular bit in nVersion. */
/**用来标志nVersion中特定位的bit*/
int bit;
/** Start MedianTime for version bits miner confirmation. Can be a date in the past */
/** 矿工确认版本位的平均开始时间。这个时间可以设置在过去*/
int64_t nStartTime;
/** Timeout/expiry MedianTime for the deployment attempt. */
/** 尝试部署的平均超时时间*/
int64_t nTimeout;
/** Constant for nTimeout very far in the future. */
static constexpr int64_t NO_TIMEOUT = std::numeric_limits<int64_t>::max();
/** Special value for nStartTime indicating that the deployment is always active.
* This is useful for testing, as it means tests don't need to deal with the activation
* process (which takes at least 3 BIP9 intervals). Only tests that specifically test the
* behaviour during activation cannot use this. */
static constexpr int64_t ALWAYS_ACTIVE = -1;
};
/**
* Parameters that influence chain consensus.
*
**影响链条共识的参数
*/
struct Params {
uint256 hashGenesisBlock; //创世区块哈希
int nSubsidyHalvingInterval; //区块奖励减半的时间间隔
/** Block height at which BIP16 becomes active */
int BIP16Height; //区块高度
/** Block height and hash at which BIP34 becomes active */
int BIP34Height;
uint256 BIP34Hash; //区块重量
/** Block height at which BIP65 becomes active */
int BIP65Height;
/** Block height at which BIP66 becomes active */
int BIP66Height;
/**
* Minimum blocks including miner confirmation of the total of 2016 blocks in a retargeting period,
* (nPowTargetTimespan / nPowTargetSpacing) which is also used for BIP9 deployments.
* Examples: 1916 for 95%, 1512 for testchains.
*
**在2016个区块中至少要有多少个区块被矿工认可，规则改变才生效
* 在BIP9部署时还是使用(nPowTargetTimespan / nPowTargetSpacing)
* eg:1916 for 95%, 1512 for 测试链
*/
uint32_t nRuleChangeActivationThreshold;
uint32_t nMinerConfirmationWindow;
BIP9Deployment vDeployments[MAX_VERSION_BITS_DEPLOYMENTS];
/** Proof of work parameters */
/**POW参数*/
uint256 powLimit; //难度
bool fPowAllowMinDifficultyBlocks; //是否允许最低难度
bool fPowNoRetargeting; //不调整难度
int64_t nPowTargetSpacing; //区块产生平均时间
int64_t nPowTargetTimespan; //难度调整时间 10
//难度调整的比例
int64_t DifficultyAdjustmentInterval() const { return nPowTargetTimespan / nPowTargetSpacing; }
uint256 nMinimumChainWork; //当前难度调整最小值
uint256 defaultAssumeValid; //再次区块之前的区块都认为是有效的
};
} // namespace Consensus
- 源码路径:.../src/pow.cpp
//获取下一次需要的工作量量
unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHeader *pblock, const Consensus::Params& params)
{
assert(pindexLast != nullptr);
//工作量证明的限制值
unsigned int nProofOfWorkLimit = UintToArith256(params.powLimit).GetCompact();
// Only change once per difficulty adjustment interval
// 难度发生调整时才有所改变
if ((pindexLast->nHeight+1) % params.DifficultyAdjustmentInterval() != 0)
{
// 判断是否允许最小难度值
if (params.fPowAllowMinDifficultyBlocks)
{
// Special difficulty rule for testnet:
// If the new block's timestamp is more than 2* 10 minutes
// then allow mining of a min-difficulty block.
/**测试网络的特殊难度规则：
* 如果新区块的时间戳超过两个区块的产生时间20min,那么返回限制挖矿难度
*/
if (pblock->GetBlockTime() > pindexLast->GetBlockTime() + params.nPowTargetSpacing*2)
return nProofOfWorkLimit;
else
{
// Return the last non-special-min-difficulty-rules-block
const CBlockIndex* pindex = pindexLast;
while (pindex->pprev && pindex->nHeight % params.DifficultyAdjustmentInterval() != 0 && pindex->nBits == nProofOfWorkLimit)
pindex = pindex->pprev;
return pindex->nBits;
}
}
//返回新区快的nBits值
return pindexLast->nBits;
}
// Go back by what we want to be 14 days worth of blocks
int nHeightFirst = pindexLast->nHeight - (params.DifficultyAdjustmentInterval()-1);
assert(nHeightFirst >= 0);
const CBlockIndex* pindexFirst = pindexLast->GetAncestor(nHeightFirst);
assert(pindexFirst);
//计算工作量证明
return CalculateNextWorkRequired(pindexLast, pindexFirst->GetBlockTime(), params);
}
//计算工作量证明
unsigned int CalculateNextWorkRequired(const CBlockIndex* pindexLast, int64_t nFirstBlockTime, const Consensus::Params& params)
{
//如果不调整难度值
if (params.fPowNoRetargeting)
return pindexLast->nBits;
// Limit adjustment step
// 难度调整的限制
int64_t nActualTimespan = pindexLast->GetBlockTime() - nFirstBlockTime;
if (nActualTimespan < params.nPowTargetTimespan/4)
nActualTimespan = params.nPowTargetTimespan/4;
if (nActualTimespan > params.nPowTargetTimespan*4)
nActualTimespan = params.nPowTargetTimespan*4;
// Retarget
//计算调整后的难度值
const arith_uint256 bnPowLimit = UintToArith256(params.powLimit);
arith_uint256 bnNew;
bnNew.SetCompact(pindexLast->nBits);
bnNew *= nActualTimespan;
bnNew /= params.nPowTargetTimespan;
if (bnNew > bnPowLimit)
bnNew = bnPowLimit;
return bnNew.GetCompact();
}
//检验工作量证明
bool CheckProofOfWork(uint256 hash, unsigned int nBits, const Consensus::Params& params)
{
bool fNegative;
bool fOverflow;
arith_uint256 bnTarget;
bnTarget.SetCompact(nBits, &fNegative, &fOverflow);
// Check range
if (fNegative || bnTarget == 0 || fOverflow || bnTarget > UintToArith256(params.powLimit))
return false;
// Check proof of work matches claimed amount
if (UintToArith256(hash) > bnTarget)
return false;
return true;
}
每一个链上的区块在加入区块链之前都做了大量的计算，因此要想修改某一块的某些交易数据的成本是极其大的。这也是比特币不可篡改特性的一个重要技术依据。
工作量证明本质是在计算一个符合系统设定条件的哈希值，这个哈希值是通过对区块头信息进行哈希得到的。为了得到这个符合条件的哈希值，必须创建一个不超过特定值的区块块头的哈希。例如，如果最大可能的散列值是2 256 - 1，则可以通过产生小于2 255的散列值来证明您尝试了两种组合。
如果新的区块的哈希值符合共识协议期望的目标难度值条件，那么只会将新区块添加到区块链中。每2016 个块网络使用存储在每个区块头中的时间戳 来计算在生成最后2016 个区块的第一个和最后一个之间所经过的秒数。理想值是1209600秒（两周）。
-
如果生成2016 个区块的时间少于两个星期，则预期难度值会按比例增加（最多达300％），以便下一个2016 [个块]应该花费两周时间，以便以相同速率检查哈希值。
-
如果花费两个多星期来生成这些区块，出于同样的原因，预期的 难度值会成比例地下降（最高达75％）。