In general, HTTP 4xx responses indicate client issues. However, when creating a distributed web service with Azure Application Gateway, some number of 4xx responses are expected. Blue Matador watches the Response Status metric with the HttpStatusGroup=4xx dimension for the number of 4xx responses returned by your application gateway, and creates events when this metric is anomalous. A sudden spike in the number of 4xx responses can be the result of the following:
HTTP 4xx responses usually result from problems specific to your application, but the general process for troubleshooting the problem is the same. First, enable and check access logs for your application gateway. Look for the endpoint or endpoints that are returning the 4xx status codes. Next, scan the logs emitted by that endpoint (with the log management solution your organization uses) for exceptions or other errors that could be resulting in 4xx responses. Finally, fix the issue and deploy the solution.