What is the principle behind InnoDB transaction implementation?

The implementation principle of InnoDB transactions ensures consistency and isolation by using Multi-Version Concurrency Control (MVCC) and undo logs.

1. Multi-Version Concurrency Control (MVCC): Each transaction is assigned a unique transaction ID at the beginning. For each data row that is modified, InnoDB will save a row version and associate it with the transaction ID. This way, when other transactions need to read that data row, they can determine its visibility based on the transaction ID and version number. The version of a committed transaction is visible to other transactions, while the version of an uncommitted transaction is not visible to other transactions.
2. Undo log: When a transaction modifies a data row, InnoDB saves the old data before the modification in an undo log, known as a rollback log. This allows for transactions to be rolled back or for other transactions to read old versions of the data by using the undo log. The undo log also ensures the atomicity of transactions, meaning that when a transaction is rolled back, the undo log can restore the modified data row to its state before the transaction began.

InnoDB can achieve the following features through the use of MVCC and undo logs:

1. Atomicity: Achieve transaction rollback and recovery through the use of undo logs.
2. Consistency: Use MVCC to ensure isolation between transactions, where each transaction can only see data that has been committed by other transactions.
3. Isolation: Achieve isolation when multiple transactions are executed concurrently through MVCC, where each transaction can only see the data state prior to its own start.
4. Persistence: Achieving the durability of transactions by writing the modifications to disk ensures the reliability of data.

In conclusion, the implementation principle of InnoDB transactions is to use MVCC and undo logs to achieve transaction consistency, isolation, and durability.