How Can I Safely Share Data Between Requests in a Flask Application?

Thread Safety of Global Variables in Flask and Data Sharing Between Requests

Problem Description

In Flask applications, storing application state in global variables may seem convenient, but it raises concerns about thread safety. Global variables are shared across all threads and processes, potentially leading to data corruption when concurrent requests modify the same data simultaneously.

Thread Safety and Concurrent Access

The issue with global variables is that they lack synchronization mechanisms to protect against concurrent access. Multiple threads or processes executing concurrently can access and modify global variables without any control, leading to unexpected and incorrect results.

In the example provided:

global_obj = SomeObj(0)

Each thread or process accessing global_obj will have its own local reference to the object. If multiple clients perform queries simultaneously, it's possible that the param attribute won't be incremented in the expected order. This can result in skipped numbers or incorrect responses.

Alternatives to Global Variables

There are several alternative approaches to storing data that is shared between requests in a thread-safe manner:

• External Data Sources: Store global data in an external data source such as a database, memcached, or Redis. This ensures that data is persisted and accessible to multiple processes.
• Session Data: Use Flask's session object to store data on a per-user basis. This isolates data ownership and prevents interference between requests from different users.
• multiprocessing.Manager: Use the multiprocessing.Manager class in Python to create shared data structures that are thread-safe and accessible from multiple processes.
• Request-Specific Data: Store data that is specific to the current request in Flask's g object. This approach provides a clean and isolated environment for storing data within a single request.

Considerations for Asynchronous Servers

While the development server may run in single thread and process mode, enabling threads or processes (e.g., app.run(threaded=True)) can expose the thread safety issues. Similarly, WSGI servers that support asynchronous workers may require additional mechanisms to ensure thread safety of global variables.

Conclusion

Global variables should not be used to store data that is shared between requests in Flask applications due to the potential for data corruption and incorrect behavior. Instead, consider using alternative approaches described above to ensure thread safety and maintain data integrity in a multithreaded or multiprocess environment.

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