Interview question: How does MySQL implement transactions, and what should we pay attention to in daily use

In database development,TransactionsIt is one of the most important concepts, especially in scenarios with high concurrency and strict data consistency requirements. As a popular relational database, MySQL provides strong support for transactions, but many developers are not familiar with its implementation principles, design logic, and precautions in daily use. This article will deeply analyze the transaction implementation mechanism of MySQL, explain the four major characteristics of transactions and the problems they solve, analyze why MySQL is designed in this way, and put forward points that need to be paid attention to during the development process to help you use transactions better.

1. What is a database transaction?

TransactionsA transaction is a group of database operations that are considered as a single logical unit. These operations either succeed or fail, and no partial success occurs. The main purpose of a transaction is to ensure the integrity of the database.consistencyandCompleteness, especially in the case of concurrent operations and system anomalies.

2. Four major characteristics of MySQL transactions (ACID)

The implementation of the transaction followsACID The four major characteristics are: Atomicity, Consistency, Isolation, and Durability. The following will explain each characteristic, how they are implemented, and the problems they solve.

1. Atomicity

AtomicityEnsure that all operations in a transaction either succeed or fail. If any operation fails, all executed operations will berollback, to restore the database to the state before the transaction started.

Implementation:
MySQL usage undo log To achieve atomicity, when a step in a transaction fails, the InnoDB engine will roll back the executed operation according to the undo log.
Problems solved:
This avoids data inconsistency problems caused by partial successful execution.
Design reasons:
In complex businesses, a transaction often contains multiple SQL statements. Atomicity ensures that developers do not have to worry about the situation where some statements are executed successfully while others fail.

2. Consistency

consistencyIt means that the data in the database should be kept consistent before and after the transaction is executed, that is, allConstraints(such as foreign keys, unique constraints).

Implementation:
MySQL usageForeign key constraints, check constraintsAnd the transaction rollback mechanism to ensure consistency. If a piece of data violates the constraints, the transaction will fail immediately and roll back.
Problems solved:
Ensure that data complies with the database integrity rules in any state.
Design reasons:
Data consistency is one of the core goals of a database, ensuring that data will not be corrupted no matter what happens.

3. Isolation

IsolationEnsures that multiple concurrent transactions do not interfere with each other. MySQL supports multipleIsolation LevelTo balance data consistency and concurrency performance.

Implementation:
MySQL provides the following isolation levels:
1. Read Uncommitted(Read Uncommitted)
2. Read Committed（Read Committed）
3. Repeatable Read(Repeatable Read, default isolation level)
4. Serializable（Serializable）
Isolation throughLock mechanism (shared lock, exclusive lock)andMVCC (Multi-version Concurrency Control)Implementation to avoid problems such as dirty reads, non-repeatable reads, and phantom reads.
Problems solved:
Ensure that transactions do not read dirty data that is not committed by other transactions, thereby ensuring data consistency.
Design reasons:
In high-concurrency scenarios, MySQL needs to provide multiple isolation levels to meet the needs of different businesses, ensuring performance while minimizing data conflicts.

4. Durability

PersistenceIt means that once a transaction is committed, its results will be permanently saved and will not be lost even if the system crashes.

Implementation:
MySQL usageredo logRecords the operations of committed transactions. When the database crashes, InnoDB will use the redo log to recover the data that has been committed but not written to disk.
Problems solved:
Avoid data loss due to system crashes or downtime.
Design reasons:
Persistence ensures that the results of every user operation will not be lost, thereby improving the reliability of the system.

3. How are transactions implemented in MySQL?

In order to gain a deeper understanding of the implementation of transactions in MySQL, we need to analyze its underlying storage engine, InnoDB, because it is the core component that supports transactions. The following is a detailed explanation of MySQL's transaction implementation, including the log mechanism, lock mechanism, and multi-version concurrency control (MVCC). These technologies work together to ensure the ACID characteristics of transactions.

1. Log system: Undo Log and Redo Log

The implementation of MySQL transactions is highly dependent on the log system, especially the Undo Log and Redo Log, which solve the atomicity and durability problems respectively.

1.1 Undo Log

effect: Record the reverse operation of each modification operation during the transaction execution so that the database can be restored to its initial state when the transaction fails or rolls back.
Trigger scene: When a step in the transaction fails, or the user actively executes the ROLLBACK statement.
Implementation process:

Before each UPDATE, DELETE, or INSERT operation, InnoDB records a snapshot of the original data in the Undo Log.
If the transaction needs to be rolled back, MySQL will read the Undo Log and perform the operations in reverse.

Data Structure: Undo Log is usually stored in the form of a linked list, which makes it easy to roll back all operations in sequence.

Use with MVCC: Undo Log is also used to support multi-version concurrency control (MVCC), helping MySQL implement snapshot reads and avoid lock contention.

1.2 Redo Log

effect: Ensure that modifications of committed transactions will not be lost even after a system crash, ensuring data persistence.
Implementation process:

When data pages are modified during transaction execution, these changes are first written to the Redo Log instead of directly to disk.
After the transaction is committed, InnoDB will persist the Redo Log to disk to ensure that even if the database crashes, the data can be recovered through the Redo Log.

WAL (Write-Ahead Logging) mechanism:
MySQL uses the WAL mechanism, which means "write to log first, then write to disk." This can improve write performance while ensuring data security.
Brushing strategy:
- Asynchronous disk flushing: Reduce I/O overhead and improve system throughput.
- Synchronous disk brushing: When users need strict persistence guarantees, they can configure forced synchronous disk flushing.

2. Locking mechanism: row-level lock and intention lock

The lock mechanism is the key to achieving transaction isolation. InnoDB uses row locks and intent locks, combined with a deadlock detection mechanism to prevent conflicts between transactions.

2.1 Row Lock

type:

Shared lock (S lock): Allows multiple transactions to read the same row concurrently, but not to modify it.
Exclusive lock (X lock): Prevents other transactions from reading or modifying the row until the lock is released.

Implementation process:
- When a transaction reads or modifies data, InnoDB locks the corresponding row to ensure that other transactions cannot operate on the row before the lock is released.
advantage: The row-level lock has a small granularity and can support high concurrency.
shortcoming: Requires a more complex lock management mechanism, which may lead to deadlock.

2.2 Intent Lock

effect: In order to avoid conflicts between full table locks and row locks, InnoDB introduced intention locks.
accomplish:
- When a transaction wants to add a row-level lock to a row in a table, it will first add an intention lock to the table, indicating that some row data will be locked.
- This avoids conflicts when other transactions lock the entire table.

2.3 Deadlock Detection and Handling

Deadlock: Deadlock occurs when multiple transactions wait for each other to release locks.
MySQL’s solution:
- Deadlock Detection: InnoDB detects the dependency graph between transactions and actively rolls back one of the transactions if a deadlock is detected.
- Waiting timeout mechanism: Set the lock waiting timeout, after which the transaction will automatically roll back.

3. Multi-version concurrency control (MVCC)

MVCC (Multi-Version Concurrency Control) It is the key mechanism for MySQL to ensure read consistency while supporting high concurrency. Through MVCC, different transactions can read the same data at the same time.Different versions, thus avoiding lock contention.

3.1 MVCC Implementation

Snapshot Read：What is read is the historical version of the data, not the latest version. For example, at the beginning of a transaction, aConsistent Read View, even if other transactions commit new data, it can only see the data version at the beginning of the transaction.
Current Reading: Some SQL statements (such as SELECT ... FOR UPDATE, UPDATE) need to read the latest version of data, so they will be locked to ensure that other transactions cannot modify it at the same time.

3.2 The role of Undo Log in MVCC

Maintenance of historical versions: InnoDB will store the modified old version of data in the Undo Log to support snapshot reads in MVCC.
Version number management: Each piece of data contains a transaction ID. By comparing the transaction ID, InnoDB can determine whether the data is visible to the current transaction.

3.3 Advantages of MVCC

Avoid lock contention: Snapshot reading does not require locking, which greatly improves concurrency performance.
Improve query efficiency: Read operations do not need to wait for the write lock to be released, reducing lock waits.

4. Automatic commit and transaction control

MySQL is in defaultAutocommit mode, that is, each SQL statement will be automatically submitted as a separate transaction. Developers can manage transactions in the following ways:

Explicitly start a transaction:

START TRANSACTION; # SQL operation COMMIT;

Rollback Transaction:
```
ROLLBACK;
```
Turn off autocommit mode:
```
SET autocommit = 0;
```
When autocommit mode is turned off, all operations are included in the same transaction until they are manually committed or rolled back.

4. Why does MySQL need these designs?

Data integrity and consistency:
MySQL's transaction design ensures that data will not be lost or corrupted even in the event of high concurrency or unexpected downtime.
Balance between performance and isolation:
Multiple isolation levels provide flexible choices for performance and consistency, allowing developers to make trade-offs based on actual business needs.

5. How to use transactions correctly in actual development?

1. Notes and best practices

Choose the appropriate isolation level:
Select the isolation level based on business needs. For example, in scenarios where there are more reads than writes, you can useRepeatable Read, and scenarios with strong consistency requirements need to useSerializable, use the default in most scenariosRepeatable ReadThat way, performance loss can be avoided.
Controlling the granularity of transactions:
Try to control transactions within a reasonable range to avoid lock waiting and deadlock problems caused by long transactions. Try to shorten the execution time of transactions to avoid occupying lock resources for a long time.
Avoiding Deadlock:
When writing SQL, ensure that the execution order of multiple transactions is consistent to reduce the probability of deadlock. Use InnoDB's deadlock detection mechanism to promptly detect and optimize problematic SQL.
Using Batch Submission:
For large-scale write operations, transactions can be submitted in batches to reduce lock holding time and improve system performance.
Monitor and optimize transaction performance:
use SHOW ENGINE INNODB STATUS Or use slow query logs to monitor transaction execution and optimize in a timely manner.
Avoid full table locks:
Give priority to using row-level locks to improve concurrency performance.

6. Conclusion

MySQL's transaction mechanism is implemented through ACID The four major features ensure the consistency and integrity of data, while providing flexible isolation levels to meet the needs of different business scenarios. During the development process, correct use of transactions, selection of appropriate isolation levels, and control of transaction granularity can significantly improve system performance and reliability. MySQL implements comprehensive support for transactions through the log system (Undo Log and Redo Log), locking mechanism, and MVCC. The collaborative relationship between them is as follows:

Undo Log Responsible for transaction rollback and snapshot reading;
Redo Log Ensure data persistence and recover data even if it crashes;
Lock mechanism Ensure concurrency security and prevent multiple transactions from interfering with each other;
MVCC Improved read performance and avoided lock contention.
These technologies jointly guarantee the ACID characteristics of MySQL transactions and provide developers with reliable and efficient transaction support.