主题： consensus
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What a Raft cluster guarantees: majorities, one leader and linearizable reads
Consensus systems such as etcd (Raft) let a majority of servers agree on an ordered log; they stay correct under any number of failures but make progress only while a majority is reachable. Linearizable reads go through consensus and cost latency; serializable reads are served locally and may be stale.
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Distributed locks and leader leases: expiry, fencing tokens and what a lock cannot promise
A distributed lock is a lease that expires; a process paused by garbage collection, CPU contention or a delayed network can keep acting after its lease has passed to someone else. Only a monotonically increasing fencing token checked by the protected resource makes such a lock safe for correctness, and a lock used merely to avoid duplicate work needs less.
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Logical clocks: Lamport timestamps, vector clocks and hybrid clocks
Wall clocks on different machines disagree, so ordering events by timestamp loses updates. Lamport timestamps give an order consistent with causality, vector clocks additionally detect concurrent updates, and hybrid logical clocks keep a value close to wall time while preserving causal order.
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