Are Flask's Global Variables Thread-Safe, and What Are the Alternatives for Sharing Data Between Requests?

Are Global Variables Thread-Safe in Flask? Sharing Data Between Requests

When using global variables to store shared data between requests in a Flask application, it's crucial to consider thread safety. In multithreaded or multiprocess environments, it becomes essential to ensure data integrity.

Potential Thread Safety Issues

Consider the example provided:

global_obj = SomeObj(0)

@app.route('/')
def home():
    return global_obj.query()

While this approach works on a single-threaded server, it can lead to data corruption in multithreaded environments. Concurrent requests from multiple clients can increment the self.param of global_obj simultaneously, resulting in skipped numbers or incorrect results.

Alternatives to Global Variables

To ensure data integrity in multithreaded or multiprocess environments, consider the following alternatives to global variables:

• Database: Store shared data in a database outside of Flask.
• Memcached or Redis: Utilize external caches to hold global data.
• Multiprocessing.Manager: For Python data that requires shared access across processes.
• Flask's 'g' Object: Store temporary data during a request that is unique to each request.
• Singleton Objects: Manage a single instance of a class with carefully controlled access to its state.

Additional Considerations

• Enable threading or processes in the development server to observe the thread safety issues.
• Using async workers doesn't entirely eliminate the risk of data corruption, as there can still be race conditions.
• When storing global data during a request, Flask's g object provides a thread-local and transient storage.

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