Why Does My MySQL Application See Stale Data After Delete-Insert Operations?

Resolving Inconsistencies in MySQL Retrievals after Delete-Insert Operations

Encountering stale data after a delete and insert operation within MySQL is a common issue for multithreaded applications. To understand this behavior, let's delve into the underlying principles.

MySQL Isolation Level: The Culprit

By default, MySQL operates with the "REPEATABLE READ" isolation level. This setting ensures that a transaction will not be affected by any subsequent alterations to the database, regardless of whether the changes were made within or beyond the transaction.

In the context of your issue, after you delete old sessions and create new ones, the open connections serving other threads may still see the stale sessions because their transactions initiated prior to these modifications. Even if other connections commit or rollback their transactions, they might not observe the updated data due to the "REPEATABLE READ" isolation level.

Solution: Modifying Isolation Level or Committing Transactions

To address this caching issue, you have two primary options:

1. Change the Isolation Level: Adjust the isolation level for the connections that experience stale results to "READ COMMITTED." This allows the connections to see changes made by other transactions after their own transactions start.
2. Commit Transactions: Forcefully ending the transactions of the problematic connections by issuing COMMIT or ROLLBACK commands will also eliminate the caching issue. This ensures that the connections see the most up-to-date data after their transactions end.

Additional Caching Considerations

Apart from MySQL's isolation level, other factors could contribute to caching behaviors:

• Database Proxy or Load Balancer: An external layer between your application and MySQL may introduce caching mechanisms. Check if these components have caching enabled and configure them appropriately.
• Application Caching Layer: Review your application code for any caching mechanisms that might interfere with the expected data retrieval behavior.
• Operating System Caching: In some cases, file system caching can impact data access. Ensure that your operating system's caching configurations are optimized for the expected workload.

By addressing the MySQL isolation level and considering other caching factors, you can resolve inconsistencies in data retrieval and ensure the accurate handling of database updates within your application.

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