77 lines
1.4 KiB
Markdown
77 lines
1.4 KiB
Markdown
# 事务与 MVCC
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MVCC（多版本并发控制），提供快照隔离和死锁检测。
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## 使用
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```nim
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import barabadb/core/mvcc
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var tm = newTxnManager()
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let txn = tm.beginTxn()
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# 写操作
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discard tm.write(txn, "key1", cast[seq[byte]]("value1"))
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discard tm.write(txn, "key2", cast[seq[byte]]("value2"))
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# 保存点
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tm.savepoint(txn)
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discard tm.write(txn, "key3", cast[seq[byte]]("value3"))
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discard tm.rollbackToSavepoint(txn)
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# 提交
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discard tm.commit(txn)
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```
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## 事务隔离
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BaraDB 使用**快照隔离**：
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- 读取者不阻塞写入者
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- 写入者不阻塞读取者
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- 每个事务看到一致的快照
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## 死锁检测
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```nim
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import barabadb/core/deadlock
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var detector = newDeadlockDetector()
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if detector.detectCycle(txn1, txn2):
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echo "检测到死锁！"
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```
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## 预写日志
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```nim
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import barabadb/storage/wal
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var wal = newWAL("./wal")
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wal.append(txnId, "SET key value")
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wal.flush()
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```
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## 保存点
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嵌套事务保存点：
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```nim
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tm.savepoint(txn, "sp1")
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tm.rollbackToSavepoint(txn, "sp1")
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```
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## 形式化验证
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MVCC/快照隔离协议在 TLA+ 中形式化规范：
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- **规范：** `formal-verification/mvcc.tla`
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- **验证的属性：**
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- `NoDirtyReads` — 事务从不读取未提交的数据
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- `ReadOwnWrites` — 事务总是看到自己的写入
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- `WriteWriteConflict` — 先提交者胜出
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本地运行 TLC：
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```bash
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cd formal-verification
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java -cp tla2tools.jar tlc2.TLC -config models/mvcc.cfg mvcc.tla
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``` |