Most useful NLog configurations

Question

What are the best or most useful configurations for logging with NLog? (These can be simple or complex, as long as they're useful.)

I'm thinking of examples like automatically rolling over log files at a certain size, changing the layout (log message) whether or not there is an exception, escalating the log level once an error has occurred, etc.

Here are some links:

• NLog Demo
• Examples in the source

Solution 1

Some of these fall into the category of general NLog (or logging) tips rather than strictly configuration suggestions.

Here are some general logging links from here at SO (you might have seen some or all of these already):

log4net vs. Nlog

Logging best practices

What's the point of a logging facade?

Why do loggers recommend using a logger per class?

Use the common pattern of naming your logger based on the class Logger logger = LogManager.GetCurrentClassLogger(). This gives you a high degree of granularity in your loggers and gives you great flexibility in the configuration of the loggers (control globally, by namespace, by specific logger name, etc).

Use non-classname-based loggers where appropriate. Maybe you have one function for which you really want to control the logging separately. Maybe you have some cross-cutting logging concerns (performance logging).

If you don't use classname-based logging, consider naming your loggers in some kind of hierarchical structure (maybe by functional area) so that you can maintain greater flexibility in your configuration. For example, you might have a "database" functional area, an "analysis" FA, and a "ui" FA. Each of these might have sub-areas. So, you might request loggers like this:

Logger logger = LogManager.GetLogger("Database.Connect");
Logger logger = LogManager.GetLogger("Database.Query");
Logger logger = LogManager.GetLogger("Database.SQL");
Logger logger = LogManager.GetLogger("Analysis.Financial");
Logger logger = LogManager.GetLogger("Analysis.Personnel");
Logger logger = LogManager.GetLogger("Analysis.Inventory");

And so on. With hierarchical loggers, you can configure logging globally (the "*" or root logger), by FA (Database, Analysis, UI), or by subarea (Database.Connect, etc).

Loggers have many configuration options:

<logger name="Name.Space.Class1" minlevel="Debug" writeTo="f1" /> 
<logger name="Name.Space.Class1" levels="Debug,Error" writeTo="f1" /> 
<logger name="Name.Space.*" writeTo="f3,f4" />
<logger name="Name.Space.*" minlevel="Debug" maxlevel="Error" final="true" /> 

See the NLog help for more info on exactly what each of the options means. Probably the most notable items here are the ability to wildcard logger rules, the concept that multiple logger rules can "execute" for a single logging statement, and that a logger rule can be marked as "final" so subsequent rules will not execute for a given logging statement.

Use the GlobalDiagnosticContext, MappedDiagnosticContext, and NestedDiagnosticContext to add additional context to your output.

Use "variable" in your config file to simplify. For example, you might define variables for your layouts and then reference the variable in the target configuration rather than specify the layout directly.

  <variable name="brief" value="${longdate} | ${level} | ${logger} | ${message}"/>
  <variable name="verbose" value="${longdate} | ${machinename} | ${processid} | ${processname} | ${level} | ${logger} | ${message}"/>
  <targets>
    <target name="file" xsi:type="File" layout="${verbose}" fileName="${basedir}/${shortdate}.log" />
    <target name="console" xsi:type="ColoredConsole" layout="${brief}" />
  </targets>

Or, you could create a "custom" set of properties to add to a layout.

  <variable name="mycontext" value="${gdc:item=appname} , ${mdc:item=threadprop}"/>
  <variable name="fmt1withcontext" value="${longdate} | ${level} | ${logger} | [${mycontext}] |${message}"/>
  <variable name="fmt2withcontext" value="${shortdate} | ${level} | ${logger} | [${mycontext}] |${message}"/>

Or, you can do stuff like create "day" or "month" layout renderers strictly via configuration:

  <variable name="day" value="${date:format=dddd}"/>
  <variable name="month" value="${date:format=MMMM}"/>
  <variable name="fmt" value="${longdate} | ${level} | ${logger} | ${day} | ${month} | ${message}"/>
  <targets>
    <target name="console" xsi:type="ColoredConsole" layout="${fmt}" />
  </targets>

You can also use layout renders to define your filename:

  <variable name="day" value="${date:format=dddd}"/>
  <targets>
    <target name="file" xsi:type="File" layout="${verbose}" fileName="${basedir}/${day}.log" />
  </targets>

If you roll your file daily, each file could be named "Monday.log", "Tuesday.log", etc.

Don't be afraid to write your own layout renderer. It is easy and allows you to add your own context information to the log file via configuration. For example, here is a layout renderer (based on NLog 1.x, not 2.0) that can add the Trace.CorrelationManager.ActivityId to the log:

  [LayoutRenderer("ActivityId")]
  class ActivityIdLayoutRenderer : LayoutRenderer
  {
    int estimatedSize = Guid.Empty.ToString().Length;

    protected override void Append(StringBuilder builder, LogEventInfo logEvent)
    {
      builder.Append(Trace.CorrelationManager.ActivityId);
    }

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

Tell NLog where your NLog extensions (what assembly) like this:

  <extensions>
    <add assembly="MyNLogExtensions"/>
  </extensions>

Use the custom layout renderer like this:

  <variable name="fmt" value="${longdate} | ${ActivityId} | ${message}"/>

Use async targets:

<nlog>
  <targets async="true">
    <!-- all targets in this section will automatically be asynchronous -->
  </targets>
</nlog>

And default target wrappers:

<nlog>  
  <targets>  
    <default-wrapper xsi:type="BufferingWrapper" bufferSize="100"/>  
    <target name="f1" xsi:type="File" fileName="f1.txt"/>  
    <target name="f2" xsi:type="File" fileName="f2.txt"/>  
  </targets>  
  <targets>  
    <default-wrapper xsi:type="AsyncWrapper">  
      <wrapper xsi:type="RetryingWrapper"/>  
    </default-wrapper>  
    <target name="n1" xsi:type="Network" address="tcp://localhost:4001"/>  
    <target name="n2" xsi:type="Network" address="tcp://localhost:4002"/>  
    <target name="n3" xsi:type="Network" address="tcp://localhost:4003"/>  
  </targets>  
</nlog>

where appropriate. See the NLog docs for more info on those.

Tell NLog to watch and automatically reload the configuration if it changes:

<nlog autoReload="true" /> 

There are several configuration options to help with troubleshooting NLog

<nlog throwExceptions="true" />
<nlog internalLogFile="file.txt" />
<nlog internalLogLevel="Trace|Debug|Info|Warn|Error|Fatal" />
<nlog internalLogToConsole="false|true" />
<nlog internalLogToConsoleError="false|true" />

See NLog Help for more info.

NLog 2.0 adds LayoutRenderer wrappers that allow additional processing to be performed on the output of a layout renderer (such as trimming whitespace, uppercasing, lowercasing, etc).

Don't be afraid to wrap the logger if you want insulate your code from a hard dependency on NLog, but wrap correctly. There are examples of how to wrap in the NLog's github repository. Another reason to wrap might be that you want to automatically add specific context information to each logged message (by putting it into LogEventInfo.Context).

There are pros and cons to wrapping (or abstracting) NLog (or any other logging framework for that matter). With a little effort, you can find plenty of info here on SO presenting both sides.

If you are considering wrapping, consider using Common.Logging. It works pretty well and allows you to easily switch to another logging framework if you desire to do so. Also if you are considering wrapping, think about how you will handle the context objects (GDC, MDC, NDC). Common.Logging does not currently support an abstraction for them, but it is supposedly in the queue of capabilities to add.

Solution 2

Treating exceptions differently

We often want to get more information when there is an exception. The following configuration has two targets, a file and the console, which filter on whether or not there is any exception info. (EDIT: Jarek has posted about a new method of doing this in vNext.)

The key is to have a wrapper target with xsi:type="FilteringWrapper" condition="length('${exception}')>0"

<nlog xmlns="http://www.nlog-project.org/schemas/NLog.mono2.xsd"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      autoReload="true"
      internalLogLevel="Warn"
      internalLogFile="nlog log.log"
      >
    <variable name="VerboseLayout" 
              value="${longdate} ${level:upperCase=true} ${message}  
                    (${callsite:includSourcePath=true})"            />
    <variable name="ExceptionVerboseLayout"  
              value="${VerboseLayout} (${stacktrace:topFrames=10})  
                     ${exception:format=ToString}"                  />

    <targets async="true">
        <target name="file" xsi:type="File" fileName="log.log"
                layout="${VerboseLayout}">
        </target>

        <target name="fileAsException"  
                xsi:type="FilteringWrapper" 
                condition="length('${exception}')>0">
            <target xsi:type="File"  
                    fileName="log.log"  
                    layout="${ExceptionVerboseLayout}" />
        </target>

        <target xsi:type="ColoredConsole"
                name="console"
                layout="${NormalLayout}"/>

        <target xsi:type="FilteringWrapper"  
                condition="length('${exception}')>0"  
                name="consoleException">
            <target xsi:type="ColoredConsole" 
                    layout="${ExceptionVerboseLayout}" />
        </target>
    </targets>

    <rules>
        <logger name="*" minlevel="Trace" writeTo="console,consoleException" />
        <logger name="*" minlevel="Warn" writeTo="file,fileAsException" />
    </rules>

</nlog>

Solution 3

Apparently, you can now use NLog with Growl for Windows.

<?xml version="1.0" encoding="utf-8" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

    <extensions>
        <add assembly="NLog.Targets.GrowlNotify" />
    </extensions>

    <targets>
        <target name="growl" type="GrowlNotify" password="" host="" port="" />
    </targets>

    <rules>
        <logger name="*" minLevel="Trace" appendTo="growl"/>
    </rules>

</nlog>

Solution 4

Configure NLog via XML, but Programmatically

What? Did you know that you can specify the NLog XML directly to NLog from your app, as opposed to having NLog read it from the config file? Well, you can. Let's say that you have a distributed app and you want to use the same configuration everywhere. You could keep a config file in each location and maintain it separately, you could maintain one in a central location and push it out to the satellite locations, or you could probably do a lot of other things. Or, you could store your XML in a database, get it at app startup, and configure NLog directly with that XML (maybe checking back periodically to see if it had changed).

  string xml = @"<nlog>
                   <targets>
                     <target name='console' type='Console' layout='${message}' />
                   </targets>

                   <rules>
                     <logger name='*' minlevel='Error' writeTo='console' />
                   </rules>
                 </nlog>";

  StringReader sr = new StringReader(xml);
  XmlReader xr = XmlReader.Create(sr);
  XmlLoggingConfiguration config = new XmlLoggingConfiguration(xr, null);
  LogManager.Configuration = config;
  //NLog is now configured just as if the XML above had been in NLog.config or app.config

  logger.Trace("Hello - Trace"); //Won't log
  logger.Debug("Hello - Debug"); //Won't log
  logger.Info("Hello - Info");   //Won't log
  logger.Warn("Hello - Warn");   //Won't log
  logger.Error("Hello - Error"); //Will log
  logger.Fatal("Hello - Fatal"); //Will log

  //Now let's change the config (the root logging level) ...
  string xml2 = @"<nlog>
                  <targets>
                     <target name='console' type='Console' layout='${message}' />
                   </targets>

                   <rules>
                     <logger name='*' minlevel='Trace' writeTo='console' />
                   </rules>
                 </nlog>";

  StringReader sr2 = new StringReader(xml2);
  XmlReader xr2 = XmlReader.Create(sr2);
  XmlLoggingConfiguration config2 = new XmlLoggingConfiguration(xr2, null);
  LogManager.Configuration = config2;

  logger.Trace("Hello - Trace"); //Will log
  logger.Debug("Hello - Debug"); //Will log
  logger.Info("Hello - Info");   //Will log
  logger.Warn("Hello - Warn");   //Will log
  logger.Error("Hello - Error"); //Will log
  logger.Fatal("Hello - Fatal"); //Will log

I'm not sure how robust this is, but this example provides a useful starting point for people that might want to try configuring like this.

Solution 5

Logging different levels depending on whether or not there is an error

This example allows you to get more information when there is an error in your code. Basically, it buffers messages and only outputs those at a certain log level (e.g. Warn) unless a certain condition is met (e.g. there has been an error, so the log level is >= Error), then it will output more info (e.g. all messages from log levels >= Trace). Because the messages are buffered, this lets you gather trace information about what happened before an Error or ErrorException was logged - very useful!

I adapted this one from an example in the source code. I was thrown at first because I left out the AspNetBufferingWrapper (since mine isn't an ASP app) - it turns out that the PostFilteringWrapper requires some buffered target. Note that the target-ref element used in the above-linked example cannot be used in NLog 1.0 (I am using 1.0 Refresh for a .NET 4.0 app); it is necessary to put your target inside the wrapper block. Also note that the logic syntax (i.e. greater-than or less-than symbols, < and >) has to use the symbols, not the XML escapes for those symbols (i.e. > and <) or else NLog will error.

app.config:

<?xml version="1.0"?>
<configuration>
    <configSections>
        <section name="nlog" type="NLog.Config.ConfigSectionHandler, NLog"/>
    </configSections>

    <nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          throwExceptions="true" internalLogToConsole="true" internalLogLevel="Warn" internalLogFile="nlog.log">
        <variable name="appTitle" value="My app"/>
        <variable name="csvPath" value="${specialfolder:folder=Desktop:file=${appTitle} log.csv}"/>

        <targets async="true">
            <!--The following will keep the default number of log messages in a buffer and write out certain levels if there is an error and other levels if there is not. Messages that appeared before the error (in code) will be included, since they are buffered.-->
            <wrapper-target xsi:type="BufferingWrapper" name="smartLog">
                <wrapper-target xsi:type="PostFilteringWrapper">
                    <!--<target-ref name="fileAsCsv"/>-->
                    <target xsi:type="File" fileName="${csvPath}"
                    archiveAboveSize="4194304" concurrentWrites="false" maxArchiveFiles="1" archiveNumbering="Sequence"
                    >
                        <layout xsi:type="CsvLayout" delimiter="Tab" withHeader="false">
                            <column name="time" layout="${longdate}" />
                            <column name="level" layout="${level:upperCase=true}"/>
                            <column name="message" layout="${message}" />
                            <column name="callsite" layout="${callsite:includeSourcePath=true}" />
                            <column name="stacktrace" layout="${stacktrace:topFrames=10}" />
                            <column name="exception" layout="${exception:format=ToString}"/>
                            <!--<column name="logger" layout="${logger}"/>-->
                        </layout>
                    </target>

                     <!--during normal execution only log certain messages--> 
                    <defaultFilter>level >= LogLevel.Warn</defaultFilter>

                     <!--if there is at least one error, log everything from trace level--> 
                    <when exists="level >= LogLevel.Error" filter="level >= LogLevel.Trace" />
                </wrapper-target>
            </wrapper-target>

        </targets>

        <rules>
            <logger name="*" minlevel="Trace" writeTo="smartLog"/>
        </rules>
    </nlog>
</configuration>

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