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Updated on 2025-04-13

MySQL's data consistency guarantee and delay problem handling

1. Definition of data consistency

In the database field,Data consistencyMainly refers to:

  • Data remains intact, correct and undamaged before and after a transaction.
  • Under the master-slave replication architecture, the data of the master and slave libraries are kept synchronized to prevent data inconsistencies.

1. Strong consistency vs. Final consistency

  • Strong Consistency: The data is always consistent among multiple nodes, and there will be no delay or mismatch when reading data at any time.
  • Eventual Consistency: The data may have short-term inconsistencies, but it will gradually reach a consistent state over time (such as MySQL master-slave replication).

MySQL, as a relational database, follows by defaultStrong consistency, but inMaster-slave copyorDistributed architectureNext, it can only ensure final consistency.

2. MySQL data consistency guarantee

MySQL ensures data consistency through multiple mechanisms, mainly includingTransaction, lock mechanism, replication methodas well asConsistent Read。

1. Transaction

Transactions are the core guarantee of MySQL data consistency, and followACID(atomicity, consistency, isolation, durability) principles.

  • Atomicity: All operations in the transaction will either succeed or fail.
  • Consistency: Before and after the transaction is executed, the state of the database always satisfies the integrity constraints.
  • Isolation: Multiple transactions are independent of each other and will not interfere with each other's data operations.
  • Durability: Once the transaction is committed, the data will be saved permanently.

Example: Use transactions to ensure consistency

START TRANSACTION;
UPDATE accounts SET balance = balance - 100 WHERE user_id = 1;
UPDATE accounts SET balance = balance + 100 WHERE user_id = 2;
COMMIT;

If one of these steps fails, the transaction will roll back (ROLLBACK), ensuring that data inconsistency does not occur.

2. Locks

MySQL adoptsRow-level lock、Table-level lockandOptimistic/Psychological LockMechanism to prevent data inconsistency caused by concurrent updates.

(1) Row-level Lock

Suitable for InnoDB, introduce row locks to reduce concurrent conflicts and improve database throughput.

SELECT * FROM users WHERE id = 1 FOR UPDATE; -- Additional Lock，Prevent other transactions from being modified

(2) Table-level lock (Table-level lock)

Suitable for MyISAM, the locking granularity is large and affects performance, but it is suitable for scenarios with more read-only operations.

LOCK TABLES users WRITE; -- Lock the entire table

3. Consistent Read

The InnoDB storage engine uses MVCC (multi-version concurrent control) to provide consistent reads and avoid read-write conflicts.

  • Snapshot Read: Read the data version at the beginning of the transaction and is not affected by subsequent transactions.
  • Current Read: Read the latest data and lock it to prevent data modification.
SELECT * FROM orders WHERE order_id = 100; -- Snapshot reading，No locking
SELECT * FROM orders WHERE order_id = 100 FOR UPDATE; -- Current reading，Add lock

3. MySQL data delay problem and its handling

existMaster-slave copy、Distributed databaseandHigh concurrency services, data latency is a common problem and may causeThe slave database data lags behind the main database, even causing inconsistencies in reading and writing.

1. Reasons for data delay

  • Master-slave replication delay: MySQL replication adoptsAsynchronousorSemi-SynchronousMethod, may cause data from the database to lag.
  • High concurrent write: When there are too many write requests, the main library is under too much pressure, which may affect the synchronization speed.
  • Network delay: Network problems between master and slave databases can cause replication lag.

2. Solution

(1) Use semi-synchronous replication

MySQL is used by defaultAsynchronous replication, the master library will not wait for the slave library to confirm, so data inconsistency may occur. Can be turned onSemi-synchronous copy, make sure that at least one receives data from the library before committing the transaction.

INSTALL PLUGIN rpl_semi_sync_master SONAME 'semisync_master.so';
INSTALL PLUGIN rpl_semi_sync_slave SONAME 'semisync_slave.so';

SET GLOBAL rpl_semi_sync_master_enabled = 1;
SET GLOBAL rpl_semi_sync_slave_enabled = 1;

(2) Monitoring replication delay

Can be usedSHOW SLAVE STATUSCommand checkSeconds_Behind_Master, determine whether there is a copy delay in the slave library.

SHOW SLAVE STATUS\G;

(3) Avoid reading old data when reading and writing separation

existRead and write separation architecture (ProxySQL / MySQL Router)Next, you canForced to read key data from the main library, avoid reading outdated data.

SELECT * FROM orders WHERE order_id = 100 /* MASTER */;

(4) Copy using GTID

GTID (global transaction identity) can ensure that the execution order of transactions on different servers is consistent, reducing replication latency issues.

SET GLOBAL enforce_gtid_consistency = ON;
SET GLOBAL gtid_mode = ON;

(5) Optimize the performance of the main library

Reducing the pressure on the main library and increasing the transaction submission speed can effectively reduce replication delays. For example:

  • Using partition table: Reduce the data table size and speed up query speed.
  • Optimize index: Use index reasonably to improve data query efficiency.
  • Add buffer pool: Improve InnoDB'sinnodb_buffer_pool_size, reduce disk IO.

4. Summary

Data consistencyandData delayThese are two key issues that cannot be ignored in MySQL design.

  • passTransactions, locking mechanisms, MVCC, consistency reading, MySQL can effectively ensure data consistency.
  • existMaster-slave replication architectureUnder, data delay may lead to inconsistency in reading and writing, which can be passedSemi-synchronous replication, GTID, monitoring replication delayOptimization in other ways.
  • existHigh concurrency servicesIn scenarios, optimizationDatabase performanceIt is the key to reducing data latency.

Rationally utilizing these mechanisms provided by MySQL can effectively improve the reliability and consistency of the database and ensure the accuracy and real-timeness of business data.

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