Why it matters

WAL is durability foundation. Understanding shapes reliable storage.

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The architecture

Client: change.

Log: append + fsync.

Apply: after log durable.

WAL flowClient changecommit requestWAL append + fsyncdurableApply to statein-memory or on-diskRecovery: replay WAL from last checkpoint after crash
WAL semantics.
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How it works end to end

fsync: forces to durable media before ack.

Checkpointing: periodic save of state to reduce replay.

Replication: stream WAL to replicas.

Compaction: prune old log entries.