OnLogging Event

Remarks

• A working demo is in project JSNLogDemo_Log4Net_LoggingEventHandlers

The Logging event is raised by JSNLog just before it sends a log message to the server side logging package (events tutorial).

This allows you to:

• Access the cookies, request headers, etc. of the log request sent from the client;
• Modify the log message, severity and logger name before they are logged;
• Stop the message from being logged at all.

Note that a simpler but less powerful way to add information to your log messages is via the serverSideMessageFormat attribute of the JSNLog configuration element.

Creating and adding a logging event handler

Because you need to create and add the handler only once in the life of the application, you will want to add your code to the Application_Start handler in the Global.asax.cs file:

// Global.asax.cs

public class MvcApplication : System.Web.HttpApplication
{
    protected void Application_Start()
    {
        ...
    }

    ...
}

Your handler has to match the LoggingHandler delegate, as defined by JSNLog. This means your method takes a LoggingEventArgs parameter and returns void:

// Global.asax.cs

using JSNLog;

public class MvcApplication : System.Web.HttpApplication
{
    protected void Application_Start()
    {
        ...

        LoggingHandler loggingHandler =
            (LoggingEventArgs loggingEventArgs) =>
            {
            ...
            };

        // Add the new handler to the logging event.
        JavascriptLogging.OnLogging += loggingHandler;

    }

    ...
}

Now you can add code to stop messages from being logged:

// Global.asax.cs

using JSNLog;

public class MvcApplication : System.Web.HttpApplication
{
    protected void Application_Start()
    {
        ...

        LoggingHandler loggingHandler =
            (LoggingEventArgs loggingEventArgs) =>
            {
                if (loggingEventArgs.FinalMessage.Contains("this will be suppressed"))
                {
                    loggingEventArgs.Cancel = true;
                    return;
                }
            };

        // Add the new handler to the logging event.
        JavascriptLogging.OnLogging += loggingHandler;

    }

    ...
}

Or add for example all request headers to every log message:

// Global.asax.cs

using JSNLog;

public class MvcApplication : System.Web.HttpApplication
{
    protected void Application_Start()
    {
        ...

        LoggingHandler loggingHandler =
            (LoggingEventArgs loggingEventArgs) =>
            {
                Dictionary<string, string> logRequestHeaders = loggingEventArgs.LogRequest.Headers;
                string logRequestHeadersString =
                string.Join(" | ", logRequestHeaders.Select(m => m.Key + ":" + m.Value).ToArray());

                // Add string with headers to the log message that will be sent to the logging package
                loggingEventArgs.FinalMessage += " | " + logRequestHeadersString;
            };

        // Add the new handler to the logging event.
        JavascriptLogging.OnLogging += loggingHandler;

    }

    ...
}

LoggingEventArgs

An object of this type will be passed to your event handler.

Definition

public class LoggingEventArgs : FinalLogData
{
    public bool Cancel { get; set; }
}
    
public class FinalLogData
{
    public ILogRequest LogRequest { get; }

    public string FinalLogger { get; set; }
    public Level FinalLevel { get; set; }
    public string FinalMessage { get; set; }

    public string ServerSideMessageFormat { get; set; }
}

Properties

ILogRequest

Describes the original log request received from the client.

Definition

public interface ILogRequest
{
    string Message { get; }
    string Logger { get; }
    string Level { get; }
    DateTime UtcDate { get; }
    string JsonMessage { get; }

    string UserAgent { get; }
    string UserHostAddress { get; }
    string RequestId { get; }
    string Url { get; }

    Dictionary<string, string> QueryParameters { get; }
    Dictionary<string, string> Cookies { get; }
    Dictionary<string, string> Headers { get; }
}

Properties

UserHostAddress

When JSNLog works out the IP address where the page request was received (that is, the IP address of the user's browser), this may not be as simple as looking at the source address in the request.

If your web server sits behind a load balancer, the source of the final request to the web server is not the browser, but the load balancer. The request may also have been passed on through intermediate proxies, causing the same issue.

The most common (but non-standard) solution to this is that proxies and load balancers (such as AWS' Elastic Load Balancer) send an X-Forwarded-For request header with the IP addresses of the browser and any proxies that the request passed through, except for the final source address. JSNLog uses this request header to work out the IP address of the actual browser and the proxies and/or load balancer that the request passed through.

In that case, the "IP address" is actually a string with a comma separated list of IP addresses. First is the IP address of the browser itself, then intermediate proxies (in the order in which they were reached) and then the IP address of the load balancer if there is one:

Browser IP address, Proxy 1, ..., Proxy N [, Load Balancer]