Application Insights - Stop tracking 404 errors

By default Application Insights will log every 404 error in your web app as an error. I think this is a good default, but what if you don’t want to see these 404 errors?

There are 2 options to solve this:

• Through a Telemetry Processor
• Through a Telemetry Initializer

Telemetry Processor

A telemetry processor gives you direct control over what is included or excluded from the telemetry stream.

	public class 404TelemetryFilter : ITelemetryProcessor
	{
	private readonly ITelemetryProcessor _next;
	public 404TelemetryFilter(ITelemetryProcessor next)
	{
	_next = next;
	}
	public void Process(ITelemetry item)
	{
	// Ignore item if we got a 404 response, this is often expected
	if (item is RequestTelemetry reqTel)
	if (reqTel.ResponseCode.Equals("404", StringComparison.OrdinalIgnoreCase))
	return;
	_next.Process(item);
	}
	}

view raw 404TelemetryFilter.cs hosted with ❤ by GitHub

We can register our new TelemetryProcessor by using the AddApplicationInsightsTelemetryProcessor extension method on IServiceCollection, as shown below:

	public void ConfigureServices(IServiceCollection services)
	{
	services.AddApplicationInsightsTelemetry();
	services.AddApplicationInsightsTelemetryProcessor<404TelemetryFilter>();
	}

view raw Startup.cs hosted with ❤ by GitHub

Telemetry Initializer

Telemetry initializers allow you to enrich telemetry with additional information and/or to override telemetry properties set by the standard telemetry modules. By default, any request with a response code >= 400 is flagged as failed. But if we want to treat a 404 as a success, we can provide a telemetry initializer that sets the Success property:

	public class 404TelemetryInitializer : ITelemetryInitializer
	{
	public void Initialize(ITelemetry telemetry)
	{
	var requestTelemetry = telemetry as RequestTelemetry;
	// Is this a TrackRequest() ?
	if (requestTelemetry == null) return;
	bool parsed = Int32.TryParse(requestTelemetry.ResponseCode, out code);
	if (!parsed) return;
	if (code == 404 )
	{
	// If we set the Success property, the SDK won't change it:
	requestTelemetry.Success = true;
	}
	}
	}

view raw 404TelemetryInitializer.cs hosted with ❤ by GitHub

We can register the TelemetryInitializer in our Startup.cs:

	public void ConfigureServices(IServiceCollection services)
	{
	services.AddSingleton<ITelemetryInitializer, 404TelemetryInitializer>();
	}

view raw Startup.cs hosted with ❤ by GitHub

Advantage of the Telemetry Initializer is that we still log the 404 event but no longer as an error.

More information: https://docs.microsoft.com/en-us/azure/azure-monitor/app/api-filtering-sampling