Analyzing MySQL Sleep Connections: Troubleshooting Uncommitted Transactions and Metadata Locks in MySQL 5.7 and 8.0

MySQL is a powerful relational database management system, but it can sometimes exhibit performance issues due to uncommitted transactions, sleep connections, and metadata locks. These issues can lead to blocked DDL (Data Definition Language) operations, causing delays and frustration for database administrators. In this blog, we’ll dive deep into how to analyze MySQL sleep connections, identify uncommitted transactions, and troubleshoot metadata locks. We’ll also explore the differences between MySQL 5.7 and 8.0 in handling these issues.

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Understanding MySQL Sleep Connections What Are Sleep Connections?

In MySQL, a "sleep" connection is a client connection that is idle and waiting for the next query. These connections are typically in the Sleep state, as shown in the processlist. While sleep connections are normal, an excessive number of them can indicate inefficient application behavior or connection pooling issues.

Why Do Sleep Connections Matter?

• Resource Consumption: Each connection consumes memory and other resources.

• Blocking Issues: Sleep connections may hold locks or be part of uncommitted transactions, blocking other operations.

• Performance Degradation: Too many sleep connections can lead to performance bottlenecks.

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Identifying Uncommitted Transactions

Uncommitted transactions are a common cause of blocking issues in MySQL. They can hold locks, prevent DDL operations, and lead to metadata locks.

How to Find Uncommitted Transactions Using INFORMATION_SCHEMA

You can query the INFORMATION_SCHEMA.INNODB_TRX table to identify uncommitted transactions:

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SELECT * FROM INFORMATION_SCHEMA.INNODB_TRX;

This query returns details about active transactions, including:

• trx_id: The transaction ID.

• trx_state: The state of the transaction (e.g., RUNNING, LOCK WAIT).

• trx_started: When the transaction started.

• trx_query: The last query executed in the transaction.

Using SHOW ENGINE INNODB STATUS

This command provides detailed information about InnoDB transactions, including locks and deadlocks:

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SHOW ENGINE INNODB STATUS;

Look for the TRANSACTIONS section to find uncommitted transactions.

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Metadata Locks and Their Impact on DDL Operations

What Are Metadata Locks?

Metadata locks (MDL) are used by MySQL to manage concurrent access to database objects. They are acquired when performing DDL operations (e.g., ALTER TABLE, DROP TABLE) or when a transaction accesses a table.

How Metadata Locks Block DDL Operations

• If a transaction holds an MDL on a table, any DDL operation on that table will be blocked until the transaction is committed or rolled back.

• Sleep connections with uncommitted transactions can hold MDLs, causing DDL operations to wait indefinitely.

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Troubleshooting Metadata Locks Identifying Metadata Locks

You can query the performance_schema.metadata_locks table (available in MySQL 5.7 and 8.0) to identify active metadata locks:

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SELECT * FROM performance_schema.metadata_locks;

This query returns details such as:

• OBJECT_TYPE: The type of object (e.g., TABLE).

• OBJECT_SCHEMA: The database name.

• OBJECT_NAME: The table name.

• LOCK_TYPE: The type of lock (e.g., SHARED_READ, EXCLUSIVE).

• LOCK_STATUS: The status of the lock (e.g., GRANTED, PENDING).

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Differences Between MySQL 5.7 and 8.0

1. Performance Schema Enhancements

• MySQL 5.7: The performance_schema is available but lacks some detailed metadata lock information.

• MySQL 8.0: The performance_schema is more robust and provides detailed insights into metadata locks and transaction states.

2. Metadata Lock Handling

• MySQL 5.7: Metadata locks are less visible, making troubleshooting more challenging.

• MySQL 8.0: Improved visibility into metadata locks, making it easier to identify blocking transactions.

3. Atomic DDL Support

• MySQL 5.7: DDL operations are not atomic, meaning they can leave the database in an inconsistent state if interrupted.

• MySQL 8.0: Introduces atomic DDL, ensuring that DDL operations are either fully completed or rolled back, reducing the risk of metadata lock issues.

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How to Troubleshoot Sleep Connections and Metadata Locks

Step 1: Identify Problematic Connections

Use the SHOW PROCESSLIST command to view active connections:

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SHOW PROCESSLIST;

Look for connections in the Sleep state with long durations.

Step 2: Check for Uncommitted Transactions

Query the INFORMATION_SCHEMA.INNODB_TRX table to find active transactions:

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SELECT * FROM INFORMATION_SCHEMA.INNODB_TRX;

Step 3: Analyze Metadata Locks

In MySQL 8.0, use the performance_schema.metadata_locks table to identify blocking locks:

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SELECT * FROM performance_schema.metadata_locks WHERE LOCK_STATUS = 'PENDING';

Step 4: Kill Problematic Connections

If a connection is holding locks or causing issues, you can terminate it using the KILL command:

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KILL <connection_id>;

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Best Practices to Prevent Issues

1. Optimize Application Logic: Ensure transactions are committed or rolled back promptly.

2. Use Connection Pooling: Limit the number of idle connections.

3. Monitor Long-Running Queries: Use tools like pt-query-digest or MySQL’s slow query log.

4. Upgrade to MySQL 8.0: Take advantage of improved metadata lock handling and atomic DDL.

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Conclusion

Analyzing MySQL sleep connections, uncommitted transactions, and metadata locks is crucial for maintaining database performance and availability. While MySQL 5.7 provides basic tools for troubleshooting, MySQL 8.0 offers significant enhancements, making it easier to identify and resolve issues. By following the steps outlined in this blog, you can effectively troubleshoot and prevent these common problems, ensuring a smooth and efficient database environment.