Decoding a 513 Error: Unveiling the Solution in My Flask App Deployment

Greetings coding enthusiasts! I recently encountered a puzzling 513 error while deploying my Flask app for image conversion.

A quick glance at the Gunicorn logs using tail -f hinted at a 413 error. Let's dive deeper into each command used during this troubleshooting saga.

The Puzzle:

Imagine this: Ready to upload images, but out of the blue, a 513 error disrupts the flow. What's the deal, right?

Attempt #1: Monitoring Gunicorn Logs in Real-Time

Command: tail -f gunicorn_error.log


Monitors the Gunicorn error log file in real time for live updates and insights.


  • tail: Displays the last part of a file.

  • -f: Follows the content of the file, providing real-time updates.


  • Executed to observe live Gunicorn error log entries.

  • Essential for tracking errors and gaining insights during the troubleshooting process.

Attempt #2: Starting Gunicorn with Specific Configurations


gunicorn -w 4 -b app:app --error-logfile ./gunicorn_error.log --access-logfile ./gunicorn_access.log


Starts the Gunicorn server with specified worker processes and logging configurations.


  • gunicorn: Command to run the Gunicorn server.

  • -w 4: Specifies the number of worker processes.

  • -b Binds Gunicorn to the specified address and port.

  • app:app: Indicates the Flask app to run.

  • --error-logfile ./gunicorn_error.log: Sets the path for the error log file.

  • --access-logfile ./gunicorn_access.log: Sets the path for the access log file.


  • Initiates Gunicorn with 4 worker processes, binding to all available network interfaces on port 8000.

  • Configures error and access logging for monitoring and diagnostics.

Attempt #3: Nginx Configuration for Handling Larger Uploads

Nginx Configuration Block:

client_max_body_size 523M;


Specifies the maximum allowed size of the client request body in Nginx.


  • client_max_body_size: Limits the size of the client request body.

  • 1G: Specifies the maximum size (1 gigabyte).


  • Ensures Nginx can handle larger file uploads by increasing the allowed client request body size.

The "Aha" Moment:

After carefully monitoring Gunicorn logs in real-time, adjusting Gunicorn configurations, and ensuring Nginx is set up to handle larger uploads, the 513 error was resolved.


Deployments can be tricky, and this one had a real-time twist. By employing tail -f to watch Gunicorn logs and meticulously configuring Gunicorn and Nginx, the mystery behind the 513 error was unraveled. If you encounter similar issues, use these commands and configurations for a smoother deployment. Happy coding!