NEAR共识机制

Posted 2022-02-22 mutourend

1. 引言

NEAR采用PoS共识机制，使用名为NighShade的分片设计，采用名为DoomSlug的区块生成机制。

2. 相关定义及符号

为了维护共识，交易会打包进区块。存在一个preconfigured block $G$，称为genesis block。除$G$之外，每个block都有a link pointing to the previous block $prev(B)$，其中$B$为block，通过该link最终可连接到$G$（不存在cycles）。

• $A<B$：即意味着$A\ne B$，且$B$可通过previous block link连接到$A$。
• $A\le B$：即意味着$A<B$ 或 $A=B$。
• $A\sim B$：即意味着$A<B$ 或 $A=B$ 或 $A>B$。
• $A\nsim B$：即为非$A\sim B$。

chain $chain(T)$：是指a set of blocks reachable from block $T$，称为tips，即$chain(T)=B|B\le T$。对于任意的区块$A,B$，当且仅当$A\sim B$时，$A,B$之间存在a chain。此时，可成$A,B$在同一链上。

每个区块都有一个整数height $h(B)$。区块高度是单调递增的，即对于任意的区块$B\ne G$，有$h(B)>h(prev(B))$。但不要求区块高度是连续的，如$h(G)$可不为0。每个节点keep track of a valid block with the largest height it knows about, which is called its head。

区块会打包进epoch。在链上，属于同一epoch的一组区块形成连续的范围：若区块$A<B$都属于同一epoch，则每个满足$A<X<B$的区块$X$都属于该epoch。epoch可 以sequential indices来标记：$G$属于index为0的epoch；对于每个block $B$，其所属epoch index要么等于$prev(B)$的epoch index，要么大于。

每个epoch关联 a set of block producers that are validating blocks in that epoch, as well as an assignment of block heights to block producers that are responsible for producing a block at that height。

负责生产高度为$h$区块的block producer称为block proposer at $h$。

epoch index $i\ge 2$ 的validator set和block height assignment信息会在epoch index为$i-2$的最后一个block确定。对于epoch index为0和1的情况，其validator set和block height assignment为preconfigured的。因此，若2链共享某epoch的最后一个区块，则其后续2个epoch具有相同的validator set和相同的block height assignment，但是对于再更后的epoch则无法保证。

固化：若block $B$固化了，则任何未来的固化区块都是仅能基于$B$构建的。因此，在$B$及之前区块的交易将永远不会被reverse。$final(B,T)$，其中$B\le T$，意味着$B$在$chain(T)$上固化了。若$final(B,T)$为真，则对于所有的$T^{\prime }\ge T$，$final(B,T^{\prime })$均为真。

2. Data structures

NEAR区块头中与共识相关的字段有：

struct BlockHeader 
    ...
    prev_hash: BlockHash,
    height: BlockHeight,
    epoch_id: EpochId,
    last_final_block_hash: BlockHash,
    approvals: Vec<Option<Signature>>
    ...

在特定epoch的block producers会交换多种类型的消息，与共识相关的消息有2种：

• Blocks
• Approvals

具体包含了以下字段：

enum ApprovalInner 
    Endorsement(BlockHash),//为the hash of the approved block
    Skip(BlockHeight),//为the height of the approved block 


struct Approval 
    inner: ApprovalInner,
    target_height: BlockHeight,//为the specific height at which the approval can be used (an approval with a particular `target_height` can be only included in the `approvals` of a block that has `height = target_height`)
    signature: Signature,//为对`(inner, target_height)`的签名
    account_id: AccountId //为the account of the block producer who created the approval

3. Approvals Requirements

除genesis block之外的每个区块$B$中，都必须逻辑上包含来自于block producers的approvals，这些block producers的累计stake必须超过该epoch中总stake的2/3。在epoch切换时，要求block producers的累计stake需超过下一epoch总stake的2/3。

approval有2种类型：

• 当且仅当$h(B)=h(prev(B))+1$时，为Endorsement。
• 否则为Skip with the height of $prev(B)$。

存储在区块中的每个approval逻辑上对于每个block producer都是相同的（除了account_id和signature不同），因此， 存储所有approvals是冗余的。事实物理上值存储了signatures of the approvals，具体的存储方式为：

• 获取当前epoch的block producers ordered set。

若某block在epoch边界，也需要包含approvals form the next epoch，因此需要add new accounts from the new epoch：

def get_accounts_for_block_ordered(h, prev_block):
    cur_epoch = get_next_block_epoch(prev_block)
    next_epoch = get_next_block_next_epoch(prev_block)

    account_ids = get_epoch_block_producers_ordered(cur_epoch)
    if next_block_needs_approvals_from_next_epoch(prev_block):
        for account_id in get_epoch_block_producers_ordered(next_epoch):
            if account_id not in account_ids:
                account_ids.append(account_id)

    return account_ids

因此，该边界block中包含了a vector of optional signatures of the same or smaller size than the resulting set of account_ids, with each element being Noneif the approval for such account is absent, or the signature on the approval message if it is present。因此，很容易根据区块中的信息重构出签名approvals的block producers，并验证相应的签名。若the vector of signature短于account_ids的长度，则剩余的signatures均假设为None。

4. Messages

当收到approval消息，参与者仅将其存入the collection of approval messages中：

def on_approval(self, approval):
    self.approvals.append(approval)

当参与者收到一个block，与共识相关的操作有：

• 更新head
• 初始化a timer来开始发送该block的approvals给the block producers at the consecutive target_heights。计时器超时时间取决于俄最后固化区块的高度，该信息也会persisted。

def on_block(self, block):
    header = block.header

    if header.height <= self.head_height:
        return

    last_final_block = store.get_block(header.last_final_block_hash)

    self.head_height = header.height
    self.head_hash = block.hash()
    self.largest_final_height = last_final_block.height

    self.timer_height = self.head_height + 1
    self.timer_started = time.time()

    self.endorsement_pending = True

定时器会定期检查如下逻辑：

def get_delay(n):
    min(MAX_DELAY, MIN_DELAY + DELAY_STEP * (n-2))

def process_timer(self):
    now = time.time()

    skip_delay = get_delay(self.timer_height - self.largest_final_height)

    if self.endorsement_pending and now > self.timer_started + ENDORSEMENT_DELAY:

        if self.head_height >= self.largest_target_height:
            self.largest_target_height = self.head_height + 1
            self.send_approval(head_height + 1)

        self.endorsement_pending = False

    if now > self.timer_started + skip_delay:
        assert not self.endorsement_pending

        self.largest_target_height = max(self.largest_target_height, self.timer_height + 1)
        self.send_approval(self.timer_height + 1)

        self.timer_started = now
        self.timer_height += 1

def send_approval(self, target_height):
    if target_height == self.head_height + 1:
        inner = Endorsement(self.head_hash)
    else:
        inner = Skip(self.head_height)

    approval = Approval(inner, target_height)
    send(approval, to_whom = get_block_proposer(self.head_hash, target_height))

其中get_block_proposer返回的下一block proposer。

同时要求ENDORSEMENT_DELAY < MIN_DELAY，尽管不影响正确性，NEAR中要求ENDORSEMENT_DELAY * 2 <= MIN_DELAY。

5. Block Production

定义了获取特定高度区块中approvals的函数：

def get_approvals(self, target_height):
    return [approval for approval
                     in self.approvals
                     if approval.target_height == target_height and
                        (isinstance(approval.inner, Skip) and approval.prev_height == self.head_height or
                         isinstance(approval.inner, Endorsement) and approval.prev_hash == self.head_hash)]

只要get_approvals返回的approvals中对应的block producers的累计质押量超过了总质押量的2/3，被指定该高度的产块者即可产块。

6. 固化条件

block $B$固化在$chain(T)$，其中$T\ge B$，要么$B=G$，要么block $X\le T$ 使得$B=prev(prev(X共识机制$

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