How to Fix error code 503 and 504 in Azure hosted applications.

Question

Frontend application is deployed in StreamLit which has backend as FLASK framework container in Azure Environment.
The Code is running perfectly fine in local, but not when deployed in Azure Environments.

When ever the code is is executed it is giving 503 service unavailable error or 504 DNS/Gateway timeout error.

For debugging, we have removed all the logic and kept simple one liner logic in the code but still give the same error code as above.

Could anyone help me figure out what could be the issue here and possible way to debug and fix them in a go..

Answer 1

Hi Prasant Kumar Das
Welcome to the Microsoft Q&A Platform!
The 503 and 504 errors in Azure-hosted applications generally indicate problems with connectivity, configuration, or service dependencies.
Possible ways:

• Backend Flask app not running or crashing in Azure.
• Health checks for the container fail.
• Incorrect environment variables (e.g., FLASK_APP).
• Misconfigured Azure App Service or networking settings.
• Azure Networking (VNet, NSG) or Load Balancer misconfiguration.
• Incorrect routing between Streamlit frontend and Flask backend.
• Insufficient scaling or resource limits (CPU, memory).
• Free-tier service plan limitations.
• Frontend request timeout is too short.
• Backend processing delays.

Steps to Debug:

• View container logs
  az container logs --name <container-name> --resource-group <resource-group>
• Enable and review Application Logs in Azure Portal.
• Add a health check route to Flask.

    @app.route('/')
    def health_check():
        return "Running", 200
  

• Test using curl or Postman.
• Ensure the backend is accessible from the frontend. curl http://<backend-url>
• Review App Service plan quotas (CPU, memory) az appservice plan show --name <plan-name> --resource-group <resource-group>
• Enable Application Insights for detailed error analysis.

Fix the issue

• Deploy a basic Flask app to confirm the environment works.

    @app.route('/')
    def hello():
        return "Hello, Azure!", 200
  

• Set the FLASK_APP variable properly in Azure App Service settings.
• Update timeout settings in the Azure portal.
• Configuration > General Settings > Request Timeout > Set to 120 seconds.
• Upgrade to a higher App Service plan .
• Remove and redeploy the app to rule out deployment issues. ref:https://learn.microsoft.com/en-us/azure/app-service/troubleshoot-diagnostic-logs https://learn.microsoft.com/en-us/azure/app-service/troubleshoot-http-502-http-503 https://learn.microsoft.com/en-us/azure/application-gateway/application-gateway-troubleshooting-502
  If the answer is helpful, please click Accept Answer and kindly upvote it so that other people who faces similar issue may get benefitted from it.

Answer 2

Thanks for the reply.