Why Does SQLAlchemy Seem to Cache Data, and How Can I Fix This MySQL Issue?

SQLAlchemy Caching Problem: Understanding Transaction Isolation

In utilizing SQLAlchemy, caching issues can arise when inserting and updating data in a MySQL database. The discrepancy between the old and updated data retrieved by SQLAlchemy suggests the presence of caching, which can be disabled to resolve this issue.

Underlying Issue

Commonly mistaken for caching, this behavior stems from the concept of transaction isolation in SQLAlchemy. By default, its session operates in a transactional mode, holding data changes until session.commit() is invoked. Other concurrent transactions will not perceive these changes during this period.

Transaction Isolation Twist

However, transaction isolation introduces an additional layer. Not only will these concurrent transactions miss the uncommitted data, but they may continue to display outdated information until their respective transactions are committed or rolled back.

Repeatable Reads

In transactions with average isolation levels, the loaded state persists within the transaction. This means returning the same unchanged data despite the underlying database modifications, a phenomenon known as repeatable reads.

Solution

To rectify this issue and ensure accurate data retrieval, consider the following solutions:

• Disable repeatable reads by setting the isolation_level attribute of the engine.
• Commit the concurrent transactions to release the cached data.
• Use a higher isolation level, such as "SERIALIZABLE", to avoid conflicting reads and writes.

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