How to Safely Share Data Between Requests in a Flask Application?

Sharing Data Between Requests in Flask: Avoiding Thread-Safety Issues

When constructing web applications with multiple threads or processes, handling shared data effectively becomes crucial. One common concern is the potential risk associated with using global variables, particularly in the context of thread safety.

Understanding Thread Safety in Flask

Global variables are not thread-safe in Flask, meaning that their values can be modified concurrently by multiple threads or processes, leading to unpredictable results. This can arise when handling requests across concurrent threads or processes, as in the scenario described in the question.

Alternatives for Sharing Data

To avoid the pitfalls of global variables, consider exploring alternative mechanisms for sharing data between requests:

• External Data Sources: Utilize external data storage like databases, memcached, or Redis for persistent data storage.
• Multiprocessing.Manager: Leverage this Python module for sharing data between processes, providing a thread-safe option.
• Session Object: Utilize the session object to store user-specific data within a request session.

Potential Impact on Development Server

It's worth noting that the development server in Flask may operate with a single thread and process by default. This may mask the thread safety issues described earlier. However, enabling threading or processes (e.g., using app.run(threaded=True) or app.run(processes=10)) will expose these potential problems.

Additional Considerations for Async Servers

Some WSGI servers support asynchronous workers like gevent. While this improves performance, it doesn't eliminate the need for thread safety. Scenarios can occur where multiple workers access shared data, resulting in potential race conditions.

Exception for Request-Specific Data

One exception to the rule of avoiding global variables is for data that is specific to a single request. Flask's g object provides a convenient way to store request-scoped data, as it is unique to each request and ensures proper management of its lifecycle.

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