Worker Instance dropped with no warning causing HTTP 5xx errors on our web app

Question

Worker Instance dropped with no warning causing HTTP 5xx errors on our web app.

This happened at 10 AM UTC on 01Mar26 as shown below. Please advise why the instance was dropped and a new one started ? Thank you.

Answer 1

Hello @VJ A,

The dropping of a worker instance without warning, leading to HTTP 5xx errors, can often be attributed to routine maintenance or updates performed on the Azure App Service. During planned maintenance, the update mechanism may swap in a new worker instance while allowing the old instance to complete its ongoing requests.

This process can sometimes result in minimal downtime if the application takes longer to start or if it fails to start altogether, which can lead to HTTP 5xx errors being returned to users. Specifically, if the application fails to start on the new worker instance, it can cause a longer downtime until corrective actions are taken, such as manually restarting the application on that instance.

Additionally, HTTP 5xx errors indicate that the server encountered an error while processing the request. To troubleshoot these errors, you can check the Application Event Logs for details about the error, including the call stack, and capture memory dumps if necessary to analyze the root cause of the failure

Troubleshoot 4xx and 5xx HTTP errors

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