Resolving SCRIPT_NAME Issues for AJAX Requests in Flask Applications Behind Nginx

Learn how to properly prefix URLs in AJAX requests in Flask applications when using Gunicorn and Nginx by utilizing the `url_for` function for effective routing.
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This video is based on the question https://stackoverflow.com/q/69137037/ asked by the user 'Vitto' ( https://stackoverflow.com/u/3146304/ ) and on the answer https://stackoverflow.com/a/69243575/ provided by the user 'Vitto' ( https://stackoverflow.com/u/3146304/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Problem with AJAX Requests in Flask

When running a Flask application that's hosted behind Nginx, you may encounter unexpected behavior with AJAX requests. This is especially true when dealing with multiple applications that are resolved through a reverse proxy. In this scenario, requests to your application must include a specific prefix, known as SCRIPT_NAME, to ensure they route correctly.

The Scenario

Imagine you have multiple Flask applications running on separate ports on a single server. Nginx acts as a middleman, redirecting requests based on the URL prefixes. For example:

Requests to my_ip/app1/... should redirect to my_ip:port1.

In your Flask app, you correctly set the SCRIPT_NAME to app1, allowing relative links (like CSS and JS files) to automatically incorporate this prefix. However, there's a problem with AJAX requests made using jQuery. If these requests do not include the correct prefix, they will fail to route properly, causing errors in your application.

Solution: Properly Generating URLs with url_for

To address the issue with AJAX requests not recognizing the SCRIPT_NAME, you need to generate the URLs dynamically using Flask’s url_for function correctly. Here’s how you can resolve this problem:

Using url_for Effectively

Identify Your Route:
Ensure that you know the name of the function associated with the route you want to call with AJAX. For example, if your route is defined as:

[[See Video to Reveal this Text or Code Snippet]]

The name of the function is get_distribution.

Generate the URL:
Instead of hardcoding paths or using simple relative links, use:

[[See Video to Reveal this Text or Code Snippet]]

In this case, main represents the blueprint where your route is defined, allowing Flask to compute the full URL, including the SCRIPT_NAME prefix.

Update Your AJAX Calls:
When making AJAX requests, ensure that you replace your previous relative paths with the dynamically generated URLs. Here’s an example of how to set up your AJAX call:

[[See Video to Reveal this Text or Code Snippet]]

Advantages of This Approach

Portability: By using url_for, your code remains portable and doesn't require hardcoding of prefixes which can lead to maintenance challenges.

Dynamic Resolvers: Flask calculates the required URL dynamically based on the current configuration, ensuring that your application will always work as expected regardless of deployment settings.

Conclusion

Routing in Flask applications, especially when behind a reverse proxy like Nginx, can be tricky. By ensuring that all AJAX requests properly reference their corresponding Flask routes through url_for, you can avoid issues related to missing prefixes like SCRIPT_NAME. This not only makes your application more robust but also simplifies the process of managing multiple applications on the same server.

With these changes implemented, your AJAX requests should now correctly resolve the full paths, maintaining the functionality of your application without compromising on portability.