We had an issue today where requests to our Azure App Service were extraordinarily slow. According to our app service metrics, requests were being handled in around 15 milliseconds, however clients were seeing requests take half a minute. Clearly this was something related to the network. Our service is behind an Azure Application Gateway, though nothing in Azure that I could find would show me the end-to-end request time and where the bottleneck was. After doing some testing on my own, I found that my initial requests were instant but then subsequently slowed. This was the tip-off. When you see a request slow over time, it’s an indication that some adaptive firewall is sitting in the middle and, after some initial traffic, has seen something it doesn’t like and has decided to start interfering with the traffic. Hunting around, I found the firewall rule enabling the firewall’s inspection of the body of requests. After disabling that, it’s been smooth sailing.

When I was initially trying to find the source of the problem, I went through Microsoft and Azure’s own troubleshooting guide where it ran checks on my software and made suggestions. Its “documents from the web that might help you” were no help at all.

Shockingly, “Hollywood: Where are they now?” didn’t help me fix my Azure App Service problems.