Why Does My Python Web App Get Stale Session Data from MySQL Despite Deletion and Insertion?

Caching Issue in MySQL Database: Stale Data Retrieval After Deletion and Insertion

In Python web applications, managing user sessions is a crucial task. However, certain implementations using MySQL connections encounter an issue where, despite deleting old sessions and creating new ones, some connections retrieve stale data during subsequent database selects. This leads to session validation failures and the need for users to log in again.

Root Cause

The root cause of this issue lies in MySQL's default isolation level, "REPEATABLE READ." With this setting, connections within a transaction are isolated from changes made by other connections after the transaction started, regardless of whether those changes have been committed.

Solution

To resolve this issue, there are two main approaches:

1. End the Transaction:

   • Issue a COMMIT or ROLLBACK statement in the affected sessions. This will terminate the "REPEATABLE READ" transaction and allow the connections to access the updated data.

2. Change the Isolation Level:

   • Modify the isolation level for the affected sessions to "READ COMMITTED." This will ensure that the sessions read data that has already been committed, resolving the stale data retrieval issue.
   • Alternatively, consider setting the default isolation level to "READ COMMITTED" globally to prevent future occurrences. Consult the MySQL documentation for more details on this approach.

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