How to apply different layouts to the same target in NLog?

Question

NLog allows me to use SplitGroup to log my messages to several targets. I'd like to use this feature to log each message to a common, user-specific and date-specific logs at once:

<variable name="commonLog" value="${logDir}\Common.log" />
<variable name="username" value="${identity:fSNormalize=true:authType=false:isAuthenticated=false}" />
<variable name="userLog" value="${logDir}\ByUser\${username}.log" />
<variable name="dateLog" value="${logDir}\ByDate\${shortdate}.log" />

<target name="logFiles" xsi:type="SplitGroup">
  <target xsi:type="File" fileName="${commonLog}" layout="${myLayout}" />
  <target xsi:type="File" fileName="${userLog}" layout="${myLayout}" />
  <target xsi:type="File" fileName="${dateLog}" layout="${myLayout}" />
</target>

This is great, but I also want to use different layouts for different levels of severity . For example, errorLayout would include exception information and insert [!] marker so I could later highlight errors in log viewers like BareTail :

<variable name="stamp" value="${date} ${username} ${logger}" />

<variable name="debugLayout" value="${stamp} ... ${message}" />
<variable name="infoLayout" value="${stamp} [i] ${message}" /> 
<variable name="warnLayout" value="${stamp} [!] ${message}" />
<variable name="errorLayout"
   value="${warnLayout}${newline}${pad:padding=10:inner=${exception:format=ToString}}" />

<!-- logFiles target -->

<rules>
  <logger name="*" level="Debug" writeTo="logFiles" layout="debugLayout"  />
  <logger name="*" level="Info" writeTo="logFiles" layout="infoLayout" />
  <logger name="*" level="Warn" writeTo="logFiles" layout="warnLayout" />
  <logger name="*" level="Error" writeTo="logFiles" layout="errorLayout" />
</rules>

This code assumes Error s always come with exceptions and Warning s don't but that's not the point.

The problem is this configuration is wrong . It won't work because logger does not have layout attribute. It's defined for target only.

Layout which is being used must be declared by targets themselves but I see no means of specifying different layouts for different severity levels.

For now, I had to copy-paste the same configuration code four times just to have four different layout s for same set of files:

<targets>
  <target name="logFilesDebug" xsi:type="SplitGroup">
    <target xsi:type="File" fileName="${commonLog}" layout="${debugLayout}" />
    <target xsi:type="File" fileName="${userLog}" layout="${debugLayout}" />
    <target xsi:type="File" fileName="${dateLog}" layout="${debugLayout}" />
  </target>

  <target name="logFilesInfo" xsi:type="SplitGroup">
    <target xsi:type="File" fileName="${commonLog}" layout="${infoLayout}" />
    <target xsi:type="File" fileName="${userLog}" layout="${infoLayout}" />
    <target xsi:type="File" fileName="${dateLog}" layout="${infoLayout}" />
  </target>

  <target name="logFilesWarn" xsi:type="SplitGroup">
    <target xsi:type="File" fileName="${commonLog}" layout="${warnLayout}" />
    <target xsi:type="File" fileName="${userLog}" layout="${warnLayout}" />
    <target xsi:type="File" fileName="${dateLog}" layout="${warnLayout}" />
  </target>

  <target name="logFilesError" xsi:type="SplitGroup">
    <target xsi:type="File" fileName="${commonLog}" layout="${errorLayout}" />
    <target xsi:type="File" fileName="${userLog}" layout="${errorLayout}" />
    <target xsi:type="File" fileName="${dateLog}" layout="${errorLayout}" />
  </target>
</targets>

<rules>
  <logger name="*" level="Debug" writeTo="logFilesDebug"  />
  <logger name="*" level="Info" writeTo="logFilesInfo" />
  <logger name="*" level="Warn" writeTo="logFilesWarn" />
  <logger name="*" level="Error" writeTo="logFilesError" />
</rules>

This just hurts my eyes.
Is there any better way to do this and avoid duplication?

Answer 1

An alternate solution is to use the when condition in the layout.

target.Layout = "${longdate}|[${level}]|${logger}|${message}${onexception:inner=|${exception}${when:when=(level > LogLevel.Warn):inner=|[!] ${exception:format=ToString:innerFormat=Message:maxInnerExceptionLevel=5} }}"

I wanted to just provide the exception message when anything less than error. When there was an error I wanted full stack trace.

Answer 2

I'm not sure, but I think that you are probably stuck with the duplication. You want 4 different layouts to be used on the same file and you want 3 different files. One target requires one Layout. So, if you only wanted to log to 1 file, you would still have to define 4 Targets, each pointing to the same file and each with its own Layout. I don't think that NLog has a more convenient way to associate multiple Layouts with a Target and then choosing one Layout based on the content of the logging message.

Depending on exactly what you want to achieve with your formats, you might be able to reduce the duplication somewhat by writing a custom LayoutRenderer. In your example, you show that the Debug layout has "..." in it, Info has [i], Warn has [!], and Error has Warn + exception. You could write a LayoutRenderer that adds the special marker, depending on what the level of the message is. That way, you would roll Debug, Info, and Warn all into one Layout and Error would retain its own Layout.

For example:

Something like this for a custom LayoutRenderer (based on NLog 1.0 refresh, not 2.0):

  [LayoutRenderer("LevelMarkerLayoutRenderer")]   
  class LevelMarkerLayoutRenderer : LayoutRenderer   
  {     
    int estimatedSize = 3;      
    protected override void Append(StringBuilder builder, LogEventInfo logEvent)
    {       
      string marker;
      switch (logEvent.Level)
      {
        case Debug:
          marker = "...";
          break;
        case Info:
          marker = "[i]";
          break;
        case Warn:
          marker = "[!]";
          break;
        case Error:
          marker = "[!]";
          break;
        case Fatal:
          marker = "[!]";
          break;
        default:
          marker = "?";
      }

      builder.Append(marker);     
    }      

    protected override int GetEstimatedBufferSize(LogEventInfo logEvent)     
    {       
      return estimatedSize;     
    }
  } 

Now you could configure two Layouts: "normal", and "error".

Something like:

<variable name="stamp" value="${date} ${username} ${logger}" />

<variable name="normal" value="${stamp} ${LevelMarkerLayoutRenderer} ${message}" />
<variable name="error"
   value="${warnLayout}${newline}${pad:padding=10:inner=${exception:format=ToString}}" />

You could probably even create a custom LayoutRenderer to handle exceptions. If no exception, don't output anything. If exception, concatentate newline, padding, and the exception string.

If you had a "conditional" exception layout renderer, then you could have just one layout that might look like this:

<variable name="normal" value="${stamp} ${LevelMarkerLayoutRenderer} ${message} ${ConditionalExceptionLayoutRenderer}" />

Most of the time, ConditionalExceptionLayoutRenderer would yield null because there would not be an exception.

Hope this helps.

Answer 3

If you want different NLog Layout depending on LogLevel (or some other condition), then you can use NLog Conditions:

<nlog throwConfigExceptions="true">
  <variable name="infoLayout" value="${longdate} ${logger} ${message}" />
  <variable name="errorLayout" value="${longdate} ${logger} ${message} ${exception}" />
  <variable name="defaultLayout" value="${when:when=level<=LogLevel.Info:inner=${infoLayout}:else=${errorLayout}}" />
    
  <targets>
    <target xsi:type="file" name="logfile" fileName="App.Log" layout="${defaultLayout}" />
  </target>

  <rules>
    <logger name="*" minlevel="Debug" writeTo="logfile"  />
  </rules>
</nlog>

See also: https://github.com/NLog/NLog/wiki/When-Layout-Renderer

See also: https://github.com/NLog/NLog/wiki/OnException-Layout-Renderer