Creating XML based templates in log4net.

Motivation

I decided to use log4net for a recent project I had been working on. There is a concept of loggers and appenders. Loggers are composed of appenders and a log threshold. Appenders are consumers of logging information and provide specific implementations (eg. file, email, event log, database). You can configure the loggers and appenders either in the application configuration or at runtime.

Configuring logging in the application configuration provides the most flexibility. It is great being able to change settings on the fly, especially when it is running in a production environment and redeploying the build is out of the question. Although this approach comes at the expense of having a lot of information in your application configuration for the loggers and appenders. No big deal though if you just have to configure once.

Why Templates?

I had quite a bit of projects with a lot of redundant logging configuration information in each one's application configuration file. Much of the information had a standard form that we wanted uniform across our different projects too (eg. log file name conventions, event log setup, email format). Also, if we updated the logging appender configuration for a new standard, we would need to do it to every project's application configuration file. This is where templates came into play.

Writing the Code

To cut down the amount of configuration information needed to start a new project with logging and make the configuration more uniform where needed, we offloaded it into the code and left the rest in the application configuration file like so.

<log4net xsi:noNamespaceSchemaLocation="log4net.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

<logger name="LoggerTemplate">
  <appender name="SmtpAppenderTemplate" type="log4net.Appender.SmtpAppender">
    <to value="peter@initech.com" />
    <from value="system@initech.com" />
    <smtpHost value="mail.initech.com" />
    <username value="peter" />
    <password value="abc123" />
  </appender>
</logger>

<root>
  <level value="INFO" />
</root>

</log4net>

We don't use the logger directly but rather as a template for our root logger. Now we just need to craft a method to consume the template and create the root appenders at runtime.

/// <summary>
/// Get appenders matching the logger template name and use them to populate the root appenders at runtime.
/// </summary>
/// <param name="loggerTemplateName">The logger template name found in the application configuration.</param>
public static void ConfigureRootFromTemplate(string loggerTemplateName)
{
  ILog logTemplate = LogManager.GetLogger(loggerTemplateName);

  if (logTemplate == null)
  {
    throw new ArgumentException(
      String.Format(
        "Logger template {0} not found in log4net configuration. Make sure there is an " +
        "logger in the log4net configuration with the name {0}.",
        loggerTemplateName),
        "loggerTemplateName");
  }

  IAppender[] appenderTemplates = logTemplate.Logger.Repository.GetAppenders();
  var smtpAppenderTemplate = appenderTemplates.FirstOrDefault(a => a is SmtpAppender) as SmtpAppender;

  if (smtpAppenderTemplate == null)
  {
    throw new ArgumentException(
      String.Format(
        "SmtpAppender template not found in log4net configuration. Make sure there is an " +
        "SmtpAppender in the log4net {0} logger.",
        loggerTemplateName),
        "loggerTemplateName");
  }

  // Can repeat the above pattern with other appenders as well.
  // Create appenders using the template information from above.
  
  AddAppendersToRootAndConfigure(
    new AppenderCollection 
 {
      // Put your created appenders here.
    });
}

private static void AddAppendersToRootAndConfigure(AppenderCollection appenders)
{
  // Get the log repository.
  var hierarchy = (Hierarchy)log4net.LogManager.GetRepository();
  // Get the root logger.
  Logger rootLogger = hierarchy.Root;
  foreach (var appender in appenders)
  {
    // Add all the appenders and activate.
    rootLogger.AddAppender(appender);
    ((AppenderSkeleton)appender).ActivateOptions();
  }
  // Flag root logger as configured with new appender information.
  rootLogger.Hierarchy.Configured = true;
}

Then just call the configuration method at the application's startup.

class Program
{
  /// 

  /// The main entry point for the application.
  /// 
  static void Main()
  {
    // Other startup configuration code.

    log4net.Config.XmlConfigurator.Configure(); // Load the application configuration information.
    Log.ConfigureRootFromTemplate("LoggerTemplate");

    // More startup configuration code.
  }

Considerations

I don't recommend using this approach in all cases. It definitely cuts down the amount of application configuration needed but at the cost of information hiding, since it has been moved to code. Also, it may not be obvious to an uninitiated developer what your application configuration is doing, especially since this template approach is not encoded into the structure of log4net's XML. Although, if you have many projects and need to effect changes to logging across them, this may be a good solution for you.

More Reading & Resources

• C&C Inc: Logging Best Practices
• beefycode: Log4Net Recommended Practices pt 1: Your Code
• JayWay: A nice, basic log4net setup
• Stack Overflow: Why do loggers recommend using a logger per class?
• Stack Overflow: log4net: Configure to ignore messages from a specific class
• Stack Overflow: Best way to dynamically set an appender file path