Solana中的PoH_solana poh是什么

1. 引言

• tick：为a ledger entry that estimates wallclock duration。
• tick height：为the Nth tick in the ledger。
• transactions entry：为可并行执行的a set of transactions。
• entry：为an entry on the ledger，要么是a tick，要么是a transactions entry。

/// Typed entry to distinguish between transaction and tick entries
pub enum EntryType<'a> {
    Transactions(Vec<SanitizedTransaction<'a>>),
    Tick(Hash),
}

		// Entries without transactions are used to track real-time passing in the ledger and
        // cannot be generated by `record()`
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• entry id：为对应the final contents of an entry的hash值。entry id 是 entry的全局唯一标识。
  entry id hash值可用于证明：
  1）该entry being generated after a duration of time。
  2）在该entry中包含特定的transactions。
  3）该entry与ledger中其他entry的相对位置。
• sdk/program/src/clock.rs中，定义了组成ticks, slots等的网络时钟：

// The default tick rate that the cluster attempts to achieve.  Note that the actual tick
// rate at any given time should be expected to drift
pub const DEFAULT_TICKS_PER_SECOND: u64 = 160; // 每秒对应有160个ticks。

// At 160 ticks/s, 64 ticks per slot implies that leader rotation and voting will happen
// every 400 ms. A fast voting cadence ensures faster finality and convergence
pub const DEFAULT_TICKS_PER_SLOT: u64 = 64; //每个slot对应为64个ticks

// GCP n1-standard hardware and also a xeon e5-2520 v4 are about this rate of hashes/s
pub const DEFAULT_HASHES_PER_SECOND: u64 = 2_000_000; // 硬件每秒可做200万次hash运算。

#[cfg(test)]
static_assertions::const_assert_eq!(DEFAULT_HASHES_PER_TICK, 12_500);
pub const DEFAULT_HASHES_PER_TICK: u64 = DEFAULT_HASHES_PER_SECOND / DEFAULT_TICKS_PER_SECOND;
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• entry/src/entry.rs：为fundamental building block of PoH。
  其中num_hashes对应下图PoH sequence中的Index，hash对应Output Hash，transactions为unordered list of transactions。

pub struct PohEntry {
    pub num_hashes: u64,
    pub hash: Hash,
}

/// Each Entry contains three pieces of data. The `num_hashes` field is the number
/// of hashes performed since the previous entry.  The `hash` field is the result
/// of hashing `hash` from the previous entry `num_hashes` times.  The `transactions`
/// field points to Transactions that took place shortly before `hash` was generated.
///
/// If you divide `num_hashes` by the amount of time it takes to generate a new hash, you
/// get a duration estimate since the last Entry. Since processing power increases
/// over time, one should expect the duration `num_hashes` represents to decrease proportionally.
/// An upper bound on Duration can be estimated by assuming each hash was generated by the
/// world's fastest processor at the time the entry was recorded. Or said another way, it
/// is physically not possible for a shorter duration to have occurred if one assumes the
/// hash was computed by the world's fastest processor at that time. The hash chain is both
/// a Verifiable Delay Function (VDF) and a Proof of Work (not to be confused with Proof of
/// Work consensus!)

#[derive(Serialize, Deserialize, Debug, Default, PartialEq, Eq, Clone)]
pub struct Entry {
    /// The number of hashes since the previous Entry ID.
    pub num_hashes: u64,

    /// The SHA-256 hash `num_hashes` after the previous Entry ID.
    pub hash: Hash,

    /// An unordered list of transactions that were observed before the Entry ID was
    /// generated. They may have been observed before a previous Entry ID but were
    /// pushed back into this list to ensure deterministic interpretation of the ledger.
    pub transactions: Vec<Transaction>,
}
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PoH会借助target_poh_time来将tick_number和num_hashes映射到真实的时间（ns级）：【若没达到target_time，会继续等待。】

pub fn target_poh_time(&self, target_ns_per_tick: u64) -> Instant {
        assert!(self.hashes_per_tick > 0);
        let offset_tick_ns = target_ns_per_tick * self.tick_number;
        let offset_ns = target_ns_per_tick * self.num_hashes / self.hashes_per_tick;
        self.slot_start_time + Duration::from_nanos(offset_ns + offset_tick_ns)
    }
// Number of hashes to batch together.
// * If this number is too small, PoH hash rate will suffer.
// * The larger this number is from 1, the speed of recording transactions will suffer due to lock
//   contention with the PoH hashing within `tick_producer()`.
//
// Can use test_poh_service to calibrate this
pub const DEFAULT_HASHES_PER_BATCH: u64 = 64;

pub const DEFAULT_PINNED_CPU_CORE: usize = 0;

const TARGET_SLOT_ADJUSTMENT_NS: u64 = 50_000_000;
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• poh/src/poh_services.rs：实现了a service，用于records the passing of “ticks”，a measure of time in the PoH stream。

struct PohTiming {
    num_ticks: u64,
    num_hashes: u64,
    total_sleep_us: u64,
    total_lock_time_ns: u64,
    total_hash_time_ns: u64,
    total_tick_time_ns: u64,
    last_metric: Instant,
    total_record_time_us: u64,
}
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• Solana中的PoH定义为：

pub struct TransactionRecorder {
    // shared by all users of PohRecorder
    pub record_sender: CrossbeamSender<Record>,
    pub is_exited: Arc<AtomicBool>,
}
// 有：
poh: &TransactionRecorder,
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根据PohRecorderd recoder()函数，可定义TransactionRecorder：

poh: &Arc<Mutex<PohRecorder>>,

    pub fn recorder(&self) -> TransactionRecorder {
        TransactionRecorder::new(self.record_sender.clone(), self.is_exited.clone())
    }

pub struct PohRecorder {
    pub poh: Arc<Mutex<Poh>>,
    tick_height: u64,
    clear_bank_signal: Option<SyncSender<bool>>,
    start_slot: Slot,              // parent slot
    start_tick_height: u64,        // first tick_height this recorder will observe
    tick_cache: Vec<(Entry, u64)>, // cache of entry and its tick_height
    working_bank: Option<WorkingBank>,
    sender: Sender<WorkingBankEntry>,
    leader_first_tick_height: Option<u64>,
    leader_last_tick_height: u64, // zero if none
    grace_ticks: u64,
    id: Pubkey,
    blockstore: Arc<Blockstore>,
    leader_schedule_cache: Arc<LeaderScheduleCache>,
    poh_config: Arc<PohConfig>,
    ticks_per_slot: u64,
    target_ns_per_tick: u64,
    record_lock_contention_us: u64,
    flush_cache_no_tick_us: u64,
    flush_cache_tick_us: u64,
    prepare_send_us: u64,
    send_us: u64,
    tick_lock_contention_us: u64,
    tick_overhead_us: u64,
    total_sleep_us: u64,
    record_us: u64,
    ticks_from_record: u64,
    last_metric: Instant,
    record_sender: CrossbeamSender<Record>,
    pub is_exited: Arc<AtomicBool>,
}

pub struct Poh {
    pub hash: Hash,
    num_hashes: u64,
    hashes_per_tick: u64,
    remaining_hashes: u64,
    ticks_per_slot: u64,
    tick_number: u64,
    slot_start_time: Instant,
}

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2. PoH generator (leader) 选举

具体见 ledger/src/ledger_schedule_utils.rs 和 ledger/src/leader_schedule.rs 中：【随机数种子为epoch编号，leader schedule中权重与stakes质押量有关。且对于相同的种子和质押量，产生的slot_leaders是一模一样的。】

/// Return the leader schedule for the given epoch.
pub fn leader_schedule(epoch: Epoch, bank: &Bank) -> Option<LeaderSchedule> {
    bank.epoch_staked_nodes(epoch).map(|stakes| {
        let mut seed = [0u8; 32];
        seed[0..8].copy_from_slice(&epoch.to_le_bytes());
        let mut stakes: Vec<_> = stakes.into_iter().collect();
        sort_stakes(&mut stakes);
        LeaderSchedule::new(
            &stakes,
            seed,
            bank.get_slots_in_epoch(epoch),
            NUM_CONSECUTIVE_LEADER_SLOTS,
        )
    })
}

/// Stake-weighted leader schedule for one epoch.
#[derive(Debug, Default, PartialEq)]
pub struct LeaderSchedule {
    slot_leaders: Vec<Pubkey>,
    // Inverted index from pubkeys to indices where they are the leader.
    index: HashMap<Pubkey, Arc<Vec<usize>>>,
}

	// Note: passing in zero stakers will cause a panic.
    pub fn new(ids_and_stakes: &[(Pubkey, u64)], seed: [u8; 32], len: u64, repeat: u64) -> Self {
        let (ids, stakes): (Vec<_>, Vec<_>) = ids_and_stakes.iter().cloned().unzip();
        let rng = &mut ChaChaRng::from_seed(seed);
        let weighted_index = WeightedIndex::new(stakes).unwrap();
        let mut current_node = Pubkey::default();
        let slot_leaders = (0..len)
            .map(|i| {
                if i % repeat == 0 {
                    current_node = ids[weighted_index.sample(rng)];
                }
                current_node
            })
            .collect();
        Self::new_from_schedule(slot_leaders)
    }
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3. PohRecorder

poh/src/poh_recorder.rs 中，实现了poh_recorder模块：

• 提供了an object for synchronizing with Proof of History
• 会同步PoH
• 会同步bank’s register_tick
• 会同步ledger

若当前的range of ticks 在指定的WorkingBank range范围内，PohRecoder会发送ticks或entries到该WorkingBank。

PohRecorder会借助PoH同步2种数据结构：

• bank - the LastId’s queue is updated on tick and record events
• sender - the Entry channel that outputs to the ledger

4. 时间和状态分离

所谓的时间和状态分离，体现在：

• transaction_status_sender：为Crossbeam_channel unbounded sender。对应的接收函数为：write_transaction_status_batch()-》write_transaction_status()。Validator在启动时就会一直接收交易的状态信息：

    let transaction_status_service = Some(TransactionStatusService::new(
        transaction_status_receiver,
        max_complete_transaction_status_slot.clone(),
        blockstore.clone(),
        exit,
    ));
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对应的发送为：

				transaction_status_sender.send_transaction_status_batch(
                    bank.clone(),
                    txs,
                    tx_results.execution_results,
                    TransactionBalancesSet::new(pre_balances, post_balances),
                    TransactionTokenBalancesSet::new(pre_token_balances, post_token_balances),
                    inner_instructions,
                    transaction_logs,
                    tx_results.rent_debits,
                );
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TVU中的replay_stage，有：
replay_blockstore_into_bank -》 confirm_slot ：

• verify_ticks：Verify that a segment of entries has the correct number of ticks and hashes
• start_verify：Verifies the hashes and counts of a slice of transactions are all consistent. 即对PoH sequence进行验证。
• verify_and_hash_transactions：验证交易有效性，并hash。
• process_entries_with_callback：批量执行
• finish_verify：

runtime/src/bank_forks.rs：implements BankForks a DAG of checkpointed Banks。【有向无环图】

	/// After setting a new root, prune the banks that are no longer on rooted paths
    ///
    /// Given the following banks and slots...
    ///
    /// ```text
    /// slot 6                   * (G)
    ///                         /
    /// slot 5        (F)  *   /
    ///                    |  /
    /// slot 4    (E) *    | /
    ///               |    |/
    /// slot 3        |    * (D) <-- root, from set_root()
    ///               |    |
    /// slot 2    (C) *    |
    ///                \   |
    /// slot 1          \  * (B)
    ///                  \ |
    /// slot 0             * (A)  <-- highest confirmed root [1]
    /// ```
    ///
    /// ...where (D) is set as root, clean up (C) and (E), since they are not rooted.
    ///
    /// (A) is kept because it is greater-than-or-equal-to the highest confirmed root, and (D) is
    ///     one of its descendants
    /// (B) is kept for the same reason as (A)
    /// (C) is pruned since it is a lower slot than (D), but (D) is _not_ one of its descendants
    /// (D) is kept since it is the root
    /// (E) is pruned since it is not a descendant of (D)
    /// (F) is kept since it is a descendant of (D)
    /// (G) is kept for the same reason as (F)
    ///
    /// and in table form...
    ///
    /// ```text
    ///       |          |  is root a  | is a descendant ||
    ///  slot | is root? | descendant? |    of root?     || keep?
    /// ------+----------+-------------+-----------------++-------
    ///   (A) |     N    |      Y      |        N        ||   Y
    ///   (B) |     N    |      Y      |        N        ||   Y
    ///   (C) |     N    |      N      |        N        ||   N
    ///   (D) |     Y    |      N      |        N        ||   Y
    ///   (E) |     N    |      N      |        N        ||   N
    ///   (F) |     N    |      N      |        Y        ||   Y
    ///   (G) |     N    |      N      |        Y        ||   Y
    /// ```
    ///
    /// [1] RPC has the concept of commitment level, which is based on the highest confirmed root,
    /// i.e. the cluster-confirmed root.  This commitment is stronger than the local node's root.
    /// So (A) and (B) are kept to facilitate RPC at different commitment levels.  Everything below
    /// the highest confirmed root can be pruned.
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     pub const INITIAL_LOCKOUT: usize = 2;
    // The number of slots for which this vote is locked
    pub fn lockout(&self) -> u64 {
        (INITIAL_LOCKOUT as u64).pow(self.confirmation_count)
    }

     // The last slot at which a vote is still locked out. Validators should not
    // vote on a slot in another fork which is less than or equal to this slot
    // to avoid having their stake slashed.
    pub fn last_locked_out_slot(&self) -> Slot {
        self.slot + self.lockout()
    }

pub const VOTE_THRESHOLD_DEPTH: usize = 8;
pub const SWITCH_FORK_THRESHOLD: f64 = 0.38;

pub const VOTE_THRESHOLD_SIZE: f64 = 2f64 / 3f64;
	pub fn calculate_highest_confirmed_slot(&self) -> Slot {
        self.highest_slot_with_confirmation_count(1)
    }
    pub fn get_confirmation_count(&self, slot: Slot) -> Option<usize> {
        self.get_lockout_count(slot, VOTE_THRESHOLD_SIZE)
    }
    fn highest_slot_with_confirmation_count(&self, confirmation_count: usize) -> Slot {
        assert!(confirmation_count > 0 && confirmation_count <= MAX_LOCKOUT_HISTORY);
        for slot in (self.root()..self.slot()).rev() {
            if let Some(count) = self.get_confirmation_count(slot) {
                if count >= confirmation_count {
                    return slot;
                }
            }
        }
        self.commitment_slots.root
    }
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$ solana rent 15000
Rent per byte-year: 0.00000348 SOL
Rent per epoch: 0.000288276 SOL
Rent-exempt minimum: 0.10529088 SOL
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参考资料

[1] Solana Proof of Stake + Proof of History Primer