Azure .Net Core App Service fails to start 503 error

Was playing with a POC app recently. I pushed out a web deploy to an Azure App Service. Everything built and deployed without issue. The app fired up and nothing just the vanilla error message returned. I checked the site logs via Kudu and nothing jumped out. Tried another deploy and no dice. So where do we go from here?

Enable stdout

As I was in Kudu I navigated to the apps web.config and edited it to enable stdoutlogs.

I refreshed the site a couple of times to generate some log activity then opened the log file and realised that the SQL DB the app was talking to wasn’t enabled for Azure services. A quick SQL config and all was good.

Other causes of this error:

• Badly formed appsettings.json file
• New key value in appsettings.json without a corresponding app setting in Azure.