Application Insights: Ignore 404 status codes for Web APIs

Application Insights is an awesome monitoring tool, but it considers all 4xx and 5xx HTTP Status Codes as errors, and when writing REST APIs some of these codes have a special meaning and are not errors. A 404 (Not found) response from a REST API usually means there are no results for a given action, not that you have hit a non-existent page.

So, how do we tell Application Insights to ignore those 404s? Simple: we write what is called a Telemetry processor.

	using System;
	using Microsoft.ApplicationInsights.Channel;
	using Microsoft.ApplicationInsights.DataContracts;
	using Microsoft.ApplicationInsights.Extensibility;

	namespace Samples
	{
	public class My404Filter : ITelemetryProcessor
	{
	private ITelemetryProcessor Next { get; set; }

	public My404Filter(ITelemetryProcessor next)
	{
	Next = next;
	}

	public void Process(ITelemetry item)
	{
	// To filter out an item, just return
	if (ShouldIgnoreRequest(item))
	{
	return;
	}

	Next.Process(item);
	}

	private static bool ShouldIgnoreRequest(ITelemetry item)
	{
	if (item.Context.Operation.Name != null)
	{
	var operationName = item.Context.Operation.Name.ToLower();

	if (operationName.StartsWith("get api/getobjectwith404s"))
	{
	var req = item as RequestTelemetry;

	return req != null && req.ResponseCode.Equals("404", StringComparison.OrdinalIgnoreCase);
	}
	return false;
	}
	return false;
	}
	}
	}

view raw

My404Filter.cs

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We then need to add this processor to our Application Insights telemetry configuration, for this we’ll use a helper class:

	using System.Configuration;
	using Microsoft.ApplicationInsights.Extensibility;

	namespace Samples.App_Start
	{
	public static class AppInsightsConfig
	{
	public static void RegisterAppInsights()
	{
	TelemetryConfiguration.Active.InstrumentationKey =
	System.Web.Configuration.WebConfigurationManager.AppSettings["ApplicationInsightsKey"];

	var builder = TelemetryConfiguration.Active.TelemetryProcessorChainBuilder;
	builder.Use(next => new My404Filter(next));
	builder.Build();
	}
	}
	}

view raw

AppInsightsConfig.cs

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And we call it in our global.asax:

	using System.Web;
	using System.Web.Mvc;
	using System.Web.Optimization;
	using System.Web.Routing;
	using Samples.App_Start;

	namespace Samples
	{
	public class MvcApplication : HttpApplication
	{
	protected void Application_Start()
	{
	AppInsightsConfig.RegisterAppInsights();
	AreaRegistration.RegisterAllAreas();
	FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
	RouteConfig.RegisterRoutes(RouteTable.Routes);
	BundleConfig.RegisterBundles(BundleTable.Bundles);
	}
	}
	}