Azure Function Log4Net configuration

This post was inspired by the couple of hours lost from my life yesterday while I figured out how to get Apache Log4Net and Azure Application Insights working in an Azure Function built with .Net Core 2.X.

After extensive searching I found a couple of relevant blog posts but these had complex approaches and I wanted to keep the churn in the codebase I was working on to an absolute minimum.

With the different versions of the libraries involved (Late March 2019) this was what worked for me so YMMV. To provide the simplest possible example I have created a TimerTrigger which logs information via Log4Net to Azure Application Insights.

Initially the Log4Net configuration wasn’t loaded because its location is usually configured in the AssemblyInfo.cs file and .Net Core 2.x code doesn’t have one.

// You can specify all the values or you can default the Build and Revision Numbers
// by using the '*' as shown below:
// [assembly: AssemblyVersion("1.0.*")]
[assembly: AssemblyVersion("")]
[assembly: AssemblyFileVersion("")]
[assembly: log4net.Config.XmlConfigurator]

I figured I would have to manually load the Log4Net configuration and had to look at the file system of machine running the function to figure out where the Log4Net XML configuration file was getting copied to.

The “Copy to output directory” setting is important

Then I had to get the Dependency Injection (DI) framework to build an ExecutionContext for me so I could get the FunctionAppDirectory to combine with the Log4Net config file name. I used Path.Combine which is more robust and secure than manually concatenating segments of a path together.

    Copyright ® 2019 March devMobile Software, All Rights Reserved
    MIT License
namespace ApplicationInsightsAzureFunctionLog4NetClient
	using System;
	using System.IO;
	using System.Reflection;
	using log4net;
	using log4net.Config;
	using Microsoft.ApplicationInsights.Extensibility;
	using Microsoft.Azure.WebJobs;

	public static class ApplicationInsightsTimer
		public static void Run([TimerTrigger("0 */1 * * * *")]TimerInfo myTimer, ExecutionContext executionContext)
			ILog log = log4net.LogManager.GetLogger(System.Reflection.MethodBase.GetCurrentMethod().DeclaringType);

			TelemetryConfiguration.Active.InstrumentationKey = Environment.GetEnvironmentVariable("InstrumentationKey", EnvironmentVariableTarget.Process);

			var logRepository = LogManager.GetRepository(Assembly.GetEntryAssembly());
			XmlConfigurator.Configure(logRepository, new FileInfo(Path.Combine(executionContext.FunctionAppDirectory, "log4net.config")));

			log.Debug("This is a Log4Net Debug message");
			log.Info("This is a Log4Net Info message");
			log.Warn("This is a Log4Net Warning message");
			log.Error("This is a Log4Net Error message");
			log.Fatal("This is a Log4Net Fatal message");


Log4Net logging in Azure Application Insights

The latest code for my Azure Function Log4net to Applications Insights sample along with some samples for other logging platforms is available on GitHub.

Apache Log4net and Application Insights

One of my clients had built a Fintech application which uses Apache log4net and I have just finished moving the logging to Azure Application Insights.

The application used to run on a couple of dedicated servers but its core processing engine is now run on an Azure Virtual Machine Scale Set(VMSS).

The application used to log information to the local file system on each server but with more machines (upto a dozen) and them being started up and shutdown in response to customer demand this wasn’t a viable approach.

// Copyright (c) 2018, devMobile Software
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
using System;
using log4net;
using Microsoft.ApplicationInsights.Extensibility;

namespace devMobile.Azure.ApplicationInsightsLog4NetClient
   class Program
      public static ILog log = log4net.LogManager.GetLogger(System.Reflection.MethodBase.GetCurrentMethod().DeclaringType);

      static void Main(string[] args)
         if (args.Length != 1)
            Console.WriteLine("Command line argument InstrumentationKey missing");
         TelemetryConfiguration.Active.InstrumentationKey = args[0];

         log.Info("This is Log4net");
         log.Debug("This is a Debug message");
         log.Info("This is a Info message");
         log.Warn("This is a Warning message");
         log.Error("This is an Error message");
         log.Fatal("This is a Fatal message");

         new Microsoft.ApplicationInsights.TelemetryClient().Flush();

Sample code ApplicationInsightsLog4NetClient