分布式事务终极对决：Raft 状态机复制与 Spanner TrueTime 强一致性深度剖析

#01 1. Mapping Consistency Models: From Causal to External Linearizability

In a database deployed across continents, time is the least trustworthy physical quantity there is. Relativistic effects, crystal-oscillator drift and network jitter can leave the local physical clocks (NTP) on different machines hundreds of milliseconds apart.

If transaction T₂ starts in the physical world only after transaction T₁ has committed, the system must guarantee that every observer sees T₁ happen before T₂. This property is called external consistency (strict serializability).

#02 2. The Math of Google TrueTime: Commit Wait Over the TT.now() Uncertainty Interval

Google Spanner's TrueTime API returns an interval [t_{<i>earliest</i>}, t_{<i>latest</i>}], where:

t <i>latest</i> - t <i>earliest</i> = 2ε

and ε is the absolute error bound (usually ≤ 7ms).

When a transaction is ready to commit it picks s = TT.now().latest as its commit timestamp. It is not allowed to report success to the client until TT.now().earliest > s holds. That deliberate pause, the commit wait, guarantees that any transaction starting later is handed a timestamp strictly greater than s.

package consensus import ( "sync" "time" ) // HLC 混合逻辑时钟：兼顾物理时钟与 Lamport 逻辑因果 type HybridLogicalClock struct { mu sync.Mutex l int64 // 物理时间最高位 (毫秒) c int32 // 逻辑计数器 } func (h *HybridLogicalClock) Now() (physical int64, logical int32) { h.mu.Lock() defer h.mu.Unlock() pt := time.Now().UnixMilli() if pt > h.l { h.l = pt h.c = 0 } else { h.c++ } return h.l, h.c } func (h *HybridLogicalClock) Update(msgPhysical int64, msgLogical int32) { h.mu.Lock() defer h.mu.Unlock() pt := time.Now().UnixMilli() maxL := max(h.l, msgPhysical, pt) if maxL == h.l && maxL == msgPhysical { h.c = max(h.c, msgLogical) + 1 } else if maxL == h.l { h.c++ } else if maxL == msgPhysical { h.c = msgLogical + 1 } else { h.c = 0 } h.l = maxL }

分布式事务终极对决：Raft 状态机复制与 Spanner TrueTime 强一致性深度剖析

原子钟与 GPS

TrueTime API 守护进程

Commit Wait 协议

Paxos 状态机

#01 1. Mapping Consistency Models: From Causal to External Linearizability

#02 2. The Math of Google TrueTime: Commit Wait Over the TT.now() Uncertainty Interval