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2022-07-22-transaction-savepoint.md

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Proposal: Session Manager

Abstract

This proposes a design of transaction savepoint.

Background

Some Applications need to use transaction savepoint feature. And in order to be better compatible with some ORM frameworks, such as gorm.

Goal

  • Support transaction savepoint syntax.
  • Support transaction savepoint feature, and compatible with MySQL.

Non-Goals

  • Not for implementing savepoint on the entire database, but savepoint on transaction modifications.

Proposal

Savepoint Implementation

SAVEPOINT is used to set a savepoint of a specified name in the current transaction. If a savepoint with the same name already exists, it will be deleted and a new savepoint with the same name will be set.

SAVEPOINT need to record a snapshot of current transaction, when users execute ROLLBACK TO SAVEPOINT statement, current transaction will roll back to the specified savepoint.

Specifically, SAVEPOINT needs to record the snapshot(checkpoint) of transaction MemDB, and the snapshot of TransactionContext.

The savepoint data struct is:

// SavepointRecord indicates a transaction's savepoint record.
type SavepointRecord struct {
    // name is the name of the savepoint
    Name string
    // MemDBCheckpoint is the transaction's memdb checkpoint.
    MemDBCheckpoint *tikv.MemDBCheckpoint
    // TxnCtxSavepoint is the savepoint of TransactionContext
    TxnCtxSavepoint TxnCtxNeedToRestore
}

// TxnCtxNeedToRestore stores transaction variables which need to be restored when rolling back to a savepoint.
type TxnCtxNeedToRestore struct {
    // TableDeltaMap is used in the schema validator for DDL changes in one table not to block others.
    // It's also used in the statistics updating.
    // Note: for the partitioned table, it stores all the partition IDs.
    TableDeltaMap map[int64]TableDelta
    
    // pessimisticLockCache is the cache for pessimistic locked keys,
    // The value never changes during the transaction.
    pessimisticLockCache map[string][]byte
    
    // CachedTables is not nil if the transaction write on cached table.
    CachedTables map[int64]interface{}
}

// Transaction Savepoints will be stored in TransactionContext.
type TransactionContext struct {
	...
    Savepoints []SavepointRecord
}

The pseudocode about SAVEPOINT implementation is following:

func executeSavepoint(savepointName string){
	// If a savepoint with the same name already exists, it will be deleted.
	deleteSavepoint(savepointName)
	memDBCheckpoint := txn.GetMemDBCheckpoint()
	txnCtxSavepoint := txnCtx.GetCurrentSavepoint()
	txnCtx.Savepoints = append(txnCtx.Savepoints, SavepointRecord
	    Name: savepointName, 
	    MemDBCheckpoint: memDBCheckpoint, 
	    TxnCtxSavepoint: txnCtxSavepoint}
    )
}

Warning Savepoint is not support when TiDB binlog enabled. Since binlog is not recommended after TiDB 4.0.

ROLLBACK TO SAVEPOINT Implementation

ROLLBACK TO SAVEPOINT rolls back a transaction to the savepoint of a specified name and does not terminate the transaction. Data changes made to the table data after the savepoint will be reverted in the rollback, and all savepoints after the savepoint are deleted.

Specifically, ROLLBACK TO SAVEPOINT needs to roll back following information:

  • Transaction MemDB
  • TransactionContext

The pseudocode about ROLLBACK TO SAVEPOINT implementation is following:

func RollbackToSavepoint(savepointName string){
	for idx, sp := range TxnCtx.Savepoints {
		if savepointName == sp.Name {
            TxnCtx.RollbackToSavepoint(sp.TxnCtxSavepoint)
            txn.RollbackMemDBToCheckpoint(sp.MemDBCheckpoint)
			// Delete the later savepoint.
			TxnCtx.Savepoints = sessVars.TxnCtx.Savepoints[:idx+1]
			return nil
		}
	}
	return errSavepointNotExists
}

Also need to roll back the LazyTxn.stagingHandle information. Since LazyTxn.stagingHandle stores the checkpoint of transaction MemDB when start to execute the current SQL statement, so after ROLLBACK TO SAVEPOINT execute, LazyTxn.stagingHandle should store the roll backed MemDB checkpoint handle.

Warning ROLLBACK TO SAVEPOINT statements won't roll back AUTO_INCREMENT and SEQUENCE, which means the assigned AUTO_INCREMENT/SEQUENCE ID won't be reclaimed.

RELEASE SAVEPOINT Implementation

RELEASE SAVEPOINT statement removes the named savepoint and all savepoints after this savepoint from the current transaction,
without committing or rolling back the current transaction. If the savepoint of the specified name does not exist, the following error is returned:

ERROR 1305 (42000): SAVEPOINT identifier does not exist

MySQL Compatibility

When ROLLBACK TO SAVEPOINT is used to roll back a transaction to a specified savepoint, MySQL releases the locks held only after the specified savepoint, while in TiDB pessimistic transaction, TiDB does not immediately release the locks held after the specified savepoint. Instead, TiDB releases all locks when the transaction is committed or rolled back.