All our APIs on Azure function_apps + API Management are responding with internal server error 500

Hi Iain Wadie,

Sorry to hear about the sudden 500 Internal Server Errors across your Azure Function Apps integrated with API Management—it sounds frustrating, especially since everything was fine on Friday. This could stem from various causes like backend connectivity issues, policy misconfigurations, authentication failures, or even a transient Azure service hiccup. I'll outline key troubleshooting steps based on common scenarios to help pinpoint and resolve it quickly.

Initial Quick Checks

• Azure Service Status: Visit the Azure Status page (status.azure.com) and check for any ongoing incidents in your region related to API Management, Function Apps, or networking. Outages can cause widespread 500 errors.

Recent Changes: Review if there were any updates over the weekend, such as deployments, policy edits, scaling, or certificate renewals. Even minor changes can trigger issues.

Step-by-Step Troubleshooting

Enable and Review Traces in APIM:

In the Azure Portal, go to your APIM instance > APIs > [Your API] > Test tab, and enable tracing. Run a test request and look for clues like "Backend service URL is not defined," "Authentication failed," or connection timeouts in the trace JSON.

  Common culprits: Misconfigured backend URL in APIM (ensure it's set correctly under API settings) or forward-request policy errors.
  
  **Check Function App Logs and Health**:
  
     Navigate to your Function App > Monitor > Logs or Application Insights. Filter for errors around the time of failures—look for exceptions like invalid keys, timeouts, or resource exhaustion.
     
        Test the Function App directly (bypass APIM) via its URL to isolate if the issue is in the Function or APIM. If it works directly but fails through APIM, focus on APIM policies or authentication.
        
        **Validate Authentication and Managed Identity**:
        
           If using managed identities or keys, verify they're active and have correct permissions. A 500 can occur from token validation failures—reconfigure if needed and test.
           
              In APIM, check subscription keys, OAuth, or JWT policies for expiry or mismatches.
              
              **Inspect Network and Connectivity**:
              
                 Review APIM's backend settings for timeouts (default is 2 minutes; adjust if your Functions take longer).
                 
                    Check for SNAT port exhaustion or firewall blocks—use Azure Monitor to view metrics like failed requests or backend latency. If VNet-integrated, ensure peering and NSGs allow traffic.
                    
                    **Error Handling Policies**:
                    
                       In APIM, add or review the <on-error> section in policies to capture and log more details about 500 errors, which can reveal underlying issues like SSL/TLS failures.
                       
                       **Restart and Scale**:
                       
                          Restart the Function App and APIM instance as a quick fix for transient issues. If under load, scale out your Function App plan.
                          

If these don't resolve it, enable Application Insights for deeper diagnostics or open a support ticket with Azure (include trace IDs from errors for faster help). Share more details like your region, exact error traces, or Function triggers, and I can refine this advice—hopefully, it's something simple like a config drift!

Best Regards,

Jerald Felix