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+ <a name="top">
+ </a>
+ <a name="main"></a><div class="section"><h2 id="main">Apache log4net™ Frequently Asked Questions</h2>
+ <a name="information"></a><div class="section"><h2 id="information">Information</h2>
+
+ <a name="what-is-log4net"></a><div class="section"><h2 id="what-is-log4net">What is log4net?</h2>
+ <p>
+ log4net is a tool to help the programmer output log statements to a variety of
+ output targets.
+ </p>
+ <p>
+ In case of problems with an application, it is helpful to enable logging so
+ that the problem can be located. With log4net it is possible to enable logging at
+ runtime without modifying the application binary. The log4net package is designed
+ so that log statements can remain in <i>production</i> code without incurring a
+ high performance cost. It follows that the speed of logging (or rather not
+ logging) is crucial.
+ </p>
+ <p>
+ At the same time, log output can be so voluminous that it quickly becomes
+ overwhelming. One of the distinctive features of log4net (and common to all of
+ the log4x libraries) is the notion of <i>hierarchical
+ loggers</i>. Using these loggers it is possible to selectively control
+ which log statements are output at arbitrary granularity.
+ </p>
+ <p>
+ log4net is designed with two distinct goals in mind: speed and flexibility. There
+ is a tight balance between these two requirements.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="reliability"></a><div class="section"><h2 id="reliability">Is log4net a reliable logging system?</h2>
+ <p>
+ No. log4net is not reliable. It is a best-effort and <i>fail-stop</i> logging system.
+ </p>
+ <p>
+ By fail-stop, we mean that log4net will not throw unexpected exceptions at
+ run-time potentially causing your application to crash. <b>If for any reason, log4net
+ throws an uncaught exception</b> (except for <span class="code">ArgumentException</span> and
+ <span class="code">ArgumentNullException</span> which may be thrown), <b>please send an email
+ to the <a class="externalLink" href="mailto:log4net-user@logging.apache.org">
+ log4net-user@logging.apache.org</a> mailing list</b>. Uncaught exceptions
+ are handled as serious bugs requiring immediate attention.
+ </p>
+ <p>
+ Moreover, log4net will not revert to <span class="code">System.Console.Out</span>
+ or <span class="code">System.Console.Error</span> when its designated
+ output stream is not opened, is not writable or becomes full. This avoids
+ corrupting an otherwise working program by flooding the user's terminal because
+ logging fails. However, log4net will output a single message to
+ <span class="code">System.Console.Error</span> and <span>System.Diagnostics.Trace</span>
+ indicating that logging can not be performed.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="prerequisites"></a><div class="section"><h2 id="prerequisites">What are the prerequisites for log4net?</h2>
+ <p>
+ log4net runs on many different frameworks and each framework has its own requirements.
+ As a rule of thumb you will need an ECMA-335 compliant CLI runtime, for example,
+ the Microsoft® .NET runtime 1.0 (1.0.3705) or 1.1 (1.1.4322).
+ </p>
+ <p>
+ Not all frameworks are created equal and some features have been excluded from
+ some of the builds. See the <a href="framework-support.html">Framework Support</a>
+ document for more information.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="examples"></a><div class="section"><h2 id="examples">Is there example code for using log4net?</h2>
+ <p>
+ There is a directory containing examples in <span class="code">log4net\examples</span>.
+ The examples are broken down by framework.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="features"></a><div class="section"><h2 id="features">What are the features of log4net?</h2>
+ <ul>
+ <li>
+ log4net is optimized for speed.</li>
+ <li>
+ log4net is based on a named logger hierarchy.</li>
+ <li>
+ log4net is fail-stop but not reliable.</li>
+ <li>
+ log4net is thread-safe.</li>
+ <li>
+ log4net is not restricted to a predefined set of facilities.</li>
+ <li>
+ Logging behavior can be set at runtime using a configuration file.
+ Configuration files are in XML format.</li>
+ <li>
+ log4net is designed to handle exceptions from the start.</li>
+ <li>
+ log4net can direct its output to many sinks including: a file, the console, the NT EventLog or even e-mail.</li>
+ <li>
+ log4net categorizes logging into levels: DEBUG, INFO, WARN, ERROR and FATAL.</li>
+ <li>
+ The format of the log output can be easily changed by implementing a new layout class.</li>
+ <li>
+ The target of the log output as well as the writing strategy can be altered by
+ writing a new appender class.</li>
+ <li>
+ log4net supports multiple output appenders per logger.</li>
+ </ul>
+ <p>
+ See the <a href="../features.html">features</a> overview document for more information on the features of log4net.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="thread-safe"></a><div class="section"><h2 id="thread-safe">Is log4net thread-safe?</h2>
+ <p>
+ Yes, log4net is thread-safe.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="output"></a><div class="section"><h2 id="output">What does log output look like?</h2>
+ <p>
+ The log output can be customized in many ways. Moreover, one can completely
+ override the output format by implementing one's own <span class="code">ILayout</span>
+ </p>
+ <p>
+ Here is an example output using <span class="code">PatternLayout</span> with the conversion
+ pattern <span class="code">%timestamp [%thread] %-5level %logger{2} %ndc - %message%newline</span>
+ </p>
+ <div class="source"><pre>
+176 [main] INFO examples.Sort - Populating an array of 2 elements in reverse order.
+225 [main] INFO examples.SortAlgo - Entered the sort method.
+262 [main] DEBUG SortAlgo.OUTER i=1 - Outer loop.
+276 [main] DEBUG SortAlgo.SWAP i=1 j=0 - Swapping intArray[0] = 1 and intArray[1] = 0
+290 [main] DEBUG SortAlgo.OUTER i=0 - Outer loop.
+304 [main] INFO SortAlgo.DUMP - Dump of integer array:
+317 [main] INFO SortAlgo.DUMP - Element [0] = 0
+331 [main] INFO SortAlgo.DUMP - Element [1] = 1
+343 [main] INFO examples.Sort - The next log statement should be an error message.
+346 [main] ERROR SortAlgo.DUMP - Tried to dump an uninitialized array.
+467 [main] INFO examples.Sort - Exiting main method.</pre></div>
+ <p>
+ The first field is the number of milliseconds elapsed since the start of the
+ program. The second field is the thread outputting the log statement. The third
+ field is the level of the log statement. The fourth field is the rightmost
+ two components of the name of the logger making the log request. The fifth field (just
+ before the '-') is the <i>nested diagnostic context</i> (<span class="code">NDC</span>). Note the
+ nested diagnostic context may be empty as in the first two statements. The text
+ after the '-' is the message of the statement.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="loggers"></a><div class="section"><h2 id="loggers">What are Loggers?</h2>
+ <p>
+ The logger concept lies at the heart of log4net's configuration. Loggers are organized into a
+ hierarchy and give the programmer <i>run-time</i> control on which logging statements
+ are printed or not.
+ </p>
+ <p>
+ Loggers are assigned levels through the configuration of log4net. A log statement is
+ routed through to the appender depending on its level <i>and</i> its logger.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="contributing"></a><div class="section"><h2 id="contributing">Why should I donate my extensions to log4net back to the project?</h2>
+ <p>
+ Contrary to the GNU Public License (GPL) the Apache Software License does not
+ make any claims over your extensions. By extensions, we mean totally new code
+ that invokes existing log4net code. <i>You are free to do whatever you wish with
+ your proprietary log4net extensions.</i> In particular, you may choose to
+ never release your extensions to the wider public. For details see the
+ <a class="externalLink" href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.
+ </p>
+ <p>
+ We are very careful not to unnecessarily change the log4net client API so that newer log4net
+ releases are backward compatible with previous versions. We are a lot less
+ scrupulous with the internal log4net API. Thus, if your extension is designed to
+ work with the internals of a specific log4net version, then when the next release
+ of log4net comes out, you will probably need to adapt your proprietary extensions to the
+ new release. Thus, you will be forced to spend precious resources in order to
+ keep up with log4net changes. This is commonly referred to as the "stupid-tax".
+ By donating the code and making it part of the standard distribution, you save
+ yourself the unnecessary maintenance work.
+ </p>
+ <p>
+ If your extensions are useful then someone will eventually write an extension
+ providing the same or very similar functionality. Your development effort will
+ be wasted.
+ </p>
+ <p>
+ Unless the proprietary log4net extension is business critical, there is little
+ reason for not donating your extensions back to the project.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="contributing-guidelines"></a><div class="section"><h2 id="contributing-guidelines">What should I keep in mind when contributing code?</h2>
+ <ol style="list-style-type: decimal">
+ <li>
+ Stick to the existing indentation style even if you hate it.
+ <p>
+ Alternating between indentation styles makes it hard to understand the source
+ code. Make it hard on yourself but easier on others.
+ </p>
+ </li>
+ <li>
+ <b>Thoroughly test your code.</b>
+ <p>
+ There is nothing more irritating than finding the bugs in debugging (i.e. logging) code.
+ </p>
+ </li>
+ <li>
+ Keep it simple, small and fast.
+ <p>
+ It's all about the application not about logging.
+ </p>
+ </li>
+ <li>
+ Did I mention sticking with the indentation style?</li>
+ </ol>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="bug-fixes"></a><div class="section"><h2 id="bug-fixes">How fast do bugs in log4net get fixed?</h2>
+ <p>
+ As fast as they get reported ;-)
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="history"></a><div class="section"><h2 id="history">What is the history of log4net?</h2>
+ <p>
+ log4net is a port of the popular <a class="externalLink" href="http://logging.apache.org/log4j/">Apache log4j™</a> logging library.
+ The initial port was done in June 2001, since then we have tried to remain in the
+ spirit of the original log4j. See the log4net <a href="../history.html">history</a> page for more details.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="download"></a><div class="section"><h2 id="download">Where can I find the latest distribution of log4net?</h2>
+ <p>
+ The log4net <a class="externalLink" href="http://logging.apache.org/log4net/">home page</a> is a good place to start.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+ <a name="two-snks"></a><div class="section"><h2 id="two-snks">Why are there two different strong name keys?</h2>
+ <p>
+ Starting with log4net 1.2.11 there are two
+ different binary distributions,
+ <tt>oldkey</tt> and <tt>newkey</tt>.
+ </p>
+
+ <p>
+ The <tt>oldkey</tt> distribution contains
+ assemblies signed with the same strong name key
+ that was used to sign the assemblies of log4net
+ 1.2.10 and earlier. This strong name key is only
+ available to log4net developers.
+ </p>
+
+ <p>
+ The <tt>newkey</tt> distribution contains
+ assemblies signed with the strong name key
+ available from <a class="externalLink" href="https://svn.apache.org/repos/asf/logging/log4net/trunk/log4net.snk">log4net's
+ svn area</a> or inside the source distribution.
+ Everybody can create assemblies that have the same
+ strong name.
+ </p>
+
+ <p>
+ For open source projects it is important that you
+ can create your own patched version of a product
+ and use it instead of the official release. This
+ is something that is now possible if the
+ <tt>newkey</tt> is used throughout.
+ </p>
+
+ <p>
+ The <tt>oldkey</tt> distribution is mostly
+ only for people who work with third-party
+ dependencies that require one of the earlier
+ releases of log4net and can't be recompiled to use
+ the new strong name. If you start a new project
+ or can recompile all your dependencies we strongly
+ recommend you use the <tt>newkey</tt>
+ assemblies.
+ </p>
+
+ <p>
+ Note that the "new" strong name no longer provides
+ any kind of authenticity. If you want to be sure
+ you have the "real" Apache log4net, download the binary
+ release from one of the mirrors and verify the PGP
+ signature.
+ </p>
+
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+ </div>
+
+ <a name="configuration"></a><div class="section"><h2 id="configuration">Configuration</h2>
+
+ <a name="dynamic"></a><div class="section"><h2 id="dynamic">How can I change log behavior at runtime?</h2>
+ <p>
+ Logging behavior can be set using configuration files which are parsed at runtime.
+ Using configuration files the programmer can define loggers and set their
+ levels.
+ </p>
+ <p>
+ Configuration files are specified in XML. See <span class="code">log4net.Config.XmlConfigurator</span>
+ for more details.
+ </p>
+ <p>
+ See the various <span class="code">log4net.Layout</span> and <span class="code">log4net.Appender</span>
+ components for specific configuration options.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="runtime-disable"></a><div class="section"><h2 id="runtime-disable">How do I completely disable all logging at runtime?</h2>
+ <p>
+ Setting the <span class="code">Threshold</span> on the Hierarchy to Level OFF will disable all
+ logging from that Hierarchy. This can be done in the log4net configuration file
+ by setting the "threshold" attribute on the log4net configuration element to "OFF".
+ For example:
+ </p>
+ <div class="source"><pre>
+<log4net threshold="OFF" /></pre></div>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+ <a name="appender-options"></a><div class="section"><h2 id="appender-options">What are the configurable options for an appender?</h2>
+ <p>
+ log4net uses public properties to configure components such as
+ Appenders, Layouts, Loggers etc.
+ </p>
+ <p>
+ Thus, any writable public property in on the appender corresponds to a
+ configurable option. For example, in <span class="code">RollingFileAppender</span> the
+ <span class="code">public int MaxSizeRollBackups { set; }</span> property corresponds to
+ the <span class="code">MaxSizeRollBackups</span> option.
+ </p>
+ <p>
+ Layouts options are also defined by their writable properties. Same goes for most
+ other log4net components.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="appender-level"></a><div class="section"><h2 id="appender-level">Is it possible to direct log output to different appenders by level?</h2>
+ <p>
+ Yes it is. Setting the <span class="code">Threshold</span> option of any appender extending
+ <span class="code">AppenderSkeleton</span>, (most log4net appenders extend
+ <span class="code">AppenderSkeleton</span>) will filter out all log events
+ with a <i>lower</i> level than the value of the threshold option.
+ </p>
+ <p>
+ For example, setting the threshold of an appender to DEBUG will also allow INFO,
+ WARN, ERROR and FATAL messages to log along with DEBUG messages. (DEBUG is the
+ lowest level). This is usually acceptable as there is little use for DEBUG
+ messages without the surrounding INFO, WARN, ERROR and FATAL messages.
+ Similarly, setting the threshold of an appender to ERROR will filter out DEBUG,
+ INFO and WARN messages but not ERROR or FATAL messages.
+ </p>
+ <p>
+ This policy usually best encapsulates what the user actually wants to do, as
+ opposed to her mind-projected solution.
+ </p>
+ <p>
+ If you must filter events by exact level match, then you can attach a
+ <span class="code">LevelMatchFilter</span> to any appender to filter out logging
+ events by exact level match.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="config-reload"></a><div class="section"><h2 id="config-reload">Is there a way to get log4net to automatically reload a configuration file if it changes?</h2>
+ <p>
+ Yes. The <span class="code">XmlConfigurator</span> supports automatic
+ reloading through the <span class="code">ConfigureAndWatch</span> APIs. See the API
+ documentation for more details.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="appender-assembly"></a><div class="section"><h2 id="appender-assembly">Can I load an appender from another assembly?</h2>
+ <p>
+ Yes. When specifying the type in the configuration file you can give the assembly
+ qualified name of the type. For example:
+ </p>
+ <div class="source"><pre>
+<appender name="..." type="MyNamespace.MyAppender, MyAssembly"></pre></div>
+ <p>
+ The .NET runtime will try to locate the assembly called <i>MyAssembly</i>.
+ How .NET locates assemblies is beyond the scope of this FAQ.
+ </p>
+ <p>
+ When loading an assembly from the GAC the fully qualified assembly name,
+ including the version, culture and public key must be specified. This is
+ in the standard syntax supported by <span class="code">System.Type.GetType</span>.
+ See the next FAQ on how to get the version and public key for an assembly.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="assembly-public-key"></a><div class="section"><h2 id="assembly-public-key">How do I get the Public Key for an assembly?</h2>
+ <p>
+ The fully qualified name for an assembly includes the version, culture and
+ public key. The public key is derived from the strong name used to identify
+ the publisher. When referencing an assembly from the GAC the fully qualified
+ name must be used. To get the version, culture and public key you can use a
+ tool like the excellent .NET Reflector from Lutz Roeder available from
+ <a class="externalLink" href="http://www.aisto.com/roeder/dotnet">http://www.aisto.com/roeder/dotnet</a>.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="layout-header-xml-newlines"></a><div class="section"><h2 id="layout-header-xml-newlines">How do I insert newlines into the layout header?</h2>
+ <p>
+ Newlines in the config file need to be escaped using an XML numeric character reference.
+ The sequence that represents a CR LF is &#13; &#10;. The following example adds
+ a header and footer to the output each followed by a newline.
+ </p>
+ <div class="source"><pre>
+<layout type="log4net.Layout.PatternLayout">
+ <header value="[Header]&#13;&#10;" />
+ <footer value="[Footer]&#13;&#10;" />
+ <conversionPattern value="%date [%thread] %-5level %logger - %message%newline" />
+</layout></pre></div>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="pattern-string"></a><div class="section"><h2 id="pattern-string">How do I use a pattern to set the value of a string property?</h2>
+ <p>
+ Log4net supports a pattern syntax for setting string properties similar to the
+ <span class="code">PatternLayout</span> used to format the output messages.
+ This pattern syntax can be used by specifying <span class="code">type="log4net.Util.PatternString"</span>
+ on the string property in the config file. This tells the config parser to pass the
+ value to the <span class="code">PatternString</span> type before converting the result
+ to a string. For details on the patterns supported see the <a href="sdk/log4net.Util.PatternString.html">
+ PatternString SDK Reference</a>.
+ </p>
+ <p>
+ The following example sets the file name for a <span class="code">FileAppender</span> to include the
+ current process id by specifying the <span class="code">%processid</span> pattern in the
+ <span class="code">File</span> property.
+ </p>
+ <div class="source"><pre>
+<appender name="LogFileAppender" type="log4net.Appender.FileAppender">
+ <file type="log4net.Util.PatternString" value="log-file-[%processid].txt" />
+ <layout type="log4net.Layout.PatternLayout" value="%date [%thread] %-5level %logger - %message%newline" />
+</appender></pre></div>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ </div>
+
+ <a name="implementing"></a><div class="section"><h2 id="implementing">Implementing Logging</h2>
+
+ <a name="naming"></a><div class="section"><h2 id="naming">Are there any suggested ways for naming loggers?</h2>
+ <p>
+ Yes, there are.
+ </p>
+ <p>
+ You can name logging loggers by <b>locality</b>. It turns out that
+ instantiating a logger in each class, with the logger name equal to the
+ fully-qualified name of the class, is a useful and straightforward approach of
+ defining loggers. This approach has many benefits:
+ </p>
+ <ul>
+ <li>
+ It is very simple to implement.</li>
+ <li>
+ It is very simple to explain to new developers.</li>
+ <li>
+ It automatically mirrors your application's own modular design.</li>
+ <li>
+ It can be further refined at will.</li>
+ <li>
+ Printing the logger automatically gives information on the locality of the
+ log statement.</li>
+ </ul>
+ <p>
+ However, this is not the only way for naming loggers. A common alternative
+ is to name loggers by <b>functional areas</b>. For example, the
+ "database" logger, "remoting" logger, "security" logger, or the "XML"
+ logger.
+ </p>
+ <p>
+ You may choose to name loggers by functionality and subcategorize by
+ locality, as in "DATABASE.MyApp.MyClass" or
+ "DATABASE.MyApp.MyModule.MyOtherClass".
+ </p>
+ <p>
+ <i>You are totally free in choosing the names of your loggers.</i> The
+ log4net package merely allows you to manage your names in a hierarchy. However,
+ it is your responsibility to define this hierarchy.
+ </p>
+ <p>
+ <b>Note:</b> by naming loggers by locality one tends to name things by
+ functionality, since in most cases the locality relates closely to
+ functionality.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="static-class-name"></a><div class="section"><h2 id="static-class-name">How do I get the fully-qualified name of a class in a static block?</h2>
+ <p>
+ You can easily retrieve the fully-qualified name of a class in a static block
+ for class <span class="code">X</span>, with the statement <span class="code">typeof(X).Name</span>.
+ Note that <span class="code">X</span> is the class name and span an instance.
+ However because the <span class="code">LogManager.GetLogger</span> method is overloaded
+ to take an instance of <span class="code">Type</span> as well as <span class="code">string</span>
+ usually only the type of the class is required.
+ </p>
+ <p>
+ Here is the suggested usage template:
+ </p>
+ <div class="source"><pre>
+public class Foo
+{
+ private static readonly ILog log = LogManager.GetLogger(typeof(Foo));
+ ... other code
+}</pre></div>
+ <p>
+ An equivalent and more portable solution, though slightly longer, is to use the declaring type
+ of the static constructor.
+ </p>
+ <div class="source"><pre>
+public class Foo
+{
+ private static readonly ILog log = LogManager.GetLogger(System.Reflection.MethodBase.GetCurrentMethod().DeclaringType);
+ ... other code
+}</pre></div>
+ <p>
+ <b>Note:</b> the .NET Compact Framework 1.0 does not support <span class="code">System.Reflection.MethodBase.GetCurrentMethod()</span>.
+ </p>
+
+ <p>
+ <b>Note:</b> the two forms are only equivalent
+ if <span class="code">Foo</span> is not a
+ generic class. For a generic class <span class="code">Foo<T></span> the variant
+ using <span class="code">typeof</span> generates
+ a different logger for each different type
+ parameter <span class="code">T</span> while the
+ variant using reflection generates the same
+ logger for all <span class="code">T</span>s.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="perf-not-logging"></a><div class="section"><h2 id="perf-not-logging">What is the fastest way of (not) logging?</h2>
+ <p>
+ For some logger <span class="code">log</span>, writing,
+ </p>
+ <div class="source"><pre>
+log.Debug("Entry number: " + i + " is " + entry[i]);</pre></div>
+ <p>
+ incurs the cost of constructing the message parameter, that is converting both
+ integer <span class="code">i</span> and <span class="code">entry[i]</span> to
+ a string, and concatenating intermediate strings. This, regardless of whether
+ the message will be logged or not.
+ </p>
+ <p>
+ If you are worried about speed, then write
+ </p>
+ <div class="source"><pre>
+if(log.IsDebugEnabled)
+{
+ log.Debug("Entry number: " + i + " is " + entry[i]);
+}</pre></div>
+ <p>
+ This way you will not incur the cost of parameter construction if debugging is
+ disabled for logger <span class="code">log</span>. On the other hand, if the logger is
+ debug enabled, you will incur the cost of evaluating whether the logger is
+ enabled or not, twice: once in <span class="code">IsDebugEnabled</span> and once in <span class="code">Debug</span>.
+ This is an insignificant overhead since evaluating a logger takes less than
+ 1% of the time it takes to actually log a statement.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="perf-not-logging2"></a><div class="section"><h2 id="perf-not-logging2">What is REALLY the FASTEST way of (not) logging?</h2>
+ <p>
+ So you don't think that the previous FAQ is really the fastest way
+ of not logging? Well there is a faster way but it does have some
+ drawbacks. Starting from:
+ </p>
+ <div class="source"><pre>
+if(log.IsDebugEnabled)
+{
+ log.Debug("Entry number: " + i + " is " + entry[i]);
+}</pre></div>
+ <p>
+ It is possible to further eliminate the calls to <span class="code">IsDebugEnabled</span>
+ so that the call is only made once per logger. If you are using one logger
+ for each class then you can store the enabled state for the logger in a static
+ variable in the class and then test against this variable:
+ </p>
+ <div class="source"><pre>
+public class FastLogger
+{
+ private static readonly ILog log = LogManager.GetLogger(typeof(FastLogger));
+ private static readonly bool isDebugEnabled = log.IsDebugEnabled;
+
+ public void MyMethod()
+ {
+ if(isDebugEnabled)
+ {
+ log.Debug("Entry number: " + i + " is " + entry[i]);
+ }
+ }
+}</pre></div>
+ <p>
+ So why exactly is this faster? Well to start with the <span class="code">IsDebugEnabled</span>
+ is not called for each log statement, it is called once per logger. Furthermore as the
+ <span class="code">isDebugEnabled</span> variable is <span class="code">private static readonly</span>
+ the JIT compiler can at <i>run-time</i> optimize out the <span class="code">if</span> test altogether.
+ This means that at runtime the JIT compiler won't even compile the logging statements into native code, i.e.
+ all the logging just disappears.
+ </p>
+ <p>
+ So what is the downside to using this? Well one of the clever features of log4net is that
+ you can change the logging configuration while your program is running. If you need to
+ investigate an issue in your application, you don't have to stop the application, setup the
+ logging and restart the application, you can change the logging configuration and the
+ log4net will reload it (see <span class="code">XmlConfigurator.ConfigureAndWatch</span> APIs for more
+ information). However if the JIT has compiled out all of the logging statements
+ then they are gone and you can't get them back by reloading the configuration file. Effectively
+ this means that the logging configuration can only be set when the application loads and
+ it cannot be changed at runtime. It is up to you to decide if you need ultimate speed or need
+ to be able to reload the logging configuration while the application is running.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="multiple-files"></a><div class="section"><h2 id="multiple-files">Can the outputs of multiple client request go to different log files?</h2>
+ <p>
+ Many developers are confronted with the problem of distinguishing the log
+ output originating from the same class but different client requests. They come
+ up with ingenious mechanisms to fan out the log output to different files. In
+ most cases, this is not the right approach.
+ </p>
+ <p>
+ It is simpler to use a context property or stack (<span class="code">ThreadContext</span>).
+ Typically, one would <span class="code">ThreadContext.Properties["ID"] = "XXX"</span>
+ client specific information, such as the client's hostname, ID or any other
+ distinguishing information when starting to handle the client's request.
+ Thereafter, log output will automatically include the context data
+ so that you can distinguish logs from different client requests even if they
+ are output to the same file.
+ </p>
+ <p>
+ See the <span class="code">ThreadContext</span> and the <span class="code">PatternLayout</span> classes for more
+ information.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="remove-logger"></a><div class="section"><h2 id="remove-logger">Logger instances seem to be create only. Why isn't there a method to remove logger instances?</h2>
+ <p>
+ It is quite nontrivial to define the semantics of a "removed" logger which is
+ still referenced by the user.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="single-file"></a><div class="section"><h2 id="single-file">How do I get multiple process to log to the same file?</h2>
+ <p>
+ Before you even start trying any of the
+ alternatives provided, ask yourself whether you
+ really need to have multiple processes log to the
+ same file, then don't do it ;-).
+ </p>
+
+ <p>
+ FileAppender offers pluggable locking models for
+ this usecase but all existing implementations have
+ issues and drawbacks.
+ </p>
+
+ <p>
+ By default the <span class="code">FileAppender</span> holds an
+ exclusive write lock on the log file while it
+ is logging. This prevents other processes from
+ writing to the file. This model is known to
+ break down with (at least on some versions of)
+ Mono on Linux and log files may get corrupted
+ as soon as another process tries to access the
+ log file.
+ </p>
+
+ <p>
+ <span class="code">MinimalLock</span> only
+ acquires the write lock while a log is being
+ written. This allows multiple processes to
+ interleave writes to the same file, albeit with
+ a considerable loss in performance.
+ </p>
+ <p>
+ <span class="code">InterProcessLock</span>
+ doesn't lock the file at all but synchronizes
+ using a system wide Mutex. This will only work
+ if all processes cooperate (and use the same
+ locking model). The acquisition and release of a
+ Mutex for every log entry to be written will
+ result in a loss of performance, but the Mutex
+ is preferable to the use of MinimalLock.
+ </p>
+ <p>
+ If you use <span class="code">RollingFileAppender</span> things
+ become even worse as several process may try to
+ start rolling the log file concurrently. <span class="code">RollingFileAppender</span>
+ completely ignores the locking model when
+ rolling files, rolling files is simply not
+ compatible with this scenario.
+ </p>
+ <p>
+ A better alternative is to have your processes
+ log to <span class="code">RemotingAppenders</span>. Using
+ the <span class="code">RemoteLoggingServerPlugin</span>
+ (or <span class="code">IRemoteLoggingSink</span>) a
+ process can receive all the events and log
+ them to a single log file. One of the
+ examples shows how to use the <span class="code">RemoteLoggingServerPlugin</span>.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="distributed"></a><div class="section"><h2 id="distributed">If I have many processes across multiple hosts (possibly across multiple time zones) logging to the same file using the RemotingAppender, what happens to timestamps?</h2>
+ <p>
+ The timestamp is created when the logging event is created. That is so say,
+ when the <span class="code">Debug</span>, <span class="code">Info</span>,
+ <span class="code">Warn</span>, <span class="code">Error</span>
+ or <span class="code">Fatal</span> method is invoked. This is unaffected by the time at
+ which they may arrive at a remote server. Since the timestamps are
+ transmitted in UTC format by the <span class="code">RemotingAppender</span>,
+ they all appear in the same time zone as
+ the host creating the logfile. Since the clocks of various machines may not be
+ synchronized, this may account for time interval inconsistencies between events
+ generated on different hosts.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="first-log"></a><div class="section"><h2 id="first-log">When should I log my first message?</h2>
+ <p>
+ The simple answer is as soon as possible. The long answer is more complex.
+ </p>
+ <p>
+ If you are configuring log4net programmatically, i.e. by calling the
+ <span class="code">XmlConfigurator.Configure</span> method then you should do so
+ before you begin logging and it is reasonable to do this very soon after application
+ start.
+ </p>
+ <p>
+ If you are configuring log4net by specifying assembly level attributes on
+ your assembly then the configuration will be loaded once the first call to
+ the <span class="code">LogManager.GetLogger</span> is made. It is necessary
+ that the first call to <span class="code">LogManager.GetLogger</span> made
+ during the process (or AppDomain) is made from the assembly that has the
+ configuration attributes. Log4net will look only once and only on the first
+ calling assembly for the configuration attributes.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ </div>
+
+ <a name="customization"></a><div class="section"><h2 id="customization">Customization</h2>
+
+ <a name="custom-output"></a><div class="section"><h2 id="custom-output">Can the log output format be customized?</h2>
+ <p>
+ Yes. You can implement the <span class="code">log4net.Layout.ILayout</span>
+ interface to create you own customized log format, or you can extend the
+ <span class="code">LayoutSkeleton</span> class which provides a default
+ implementation of the <span class="code">ILayout</span> interface.
+ Appenders can be parameterized to use the layout of your choice.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="custom-appender"></a><div class="section"><h2 id="custom-appender">Can I write a custom appender?</h2>
+ <p>
+ Yes. You can implement the <span class="code">log4net.Appender.IAppender</span>
+ interface to create you own customized appender. We recommend that you extend the
+ <span class="code">log4net.Appender.AppenderSkeleton</span> class rather than
+ starting from scratch. You should implement your custom code in a assembly
+ separate from the log4net assembly. To get started it is worth looking at the
+ source of the <span class="code">log4net.Appender.TraceAppender</span> as an
+ example of the minimum amount of code required to get an appender working.
+ </p>
+ <p>
+ To configure log4net to use your custom appender you need to specify the
+ assembly qualified name of the appender type in the config file. For
+ example:
+ </p>
+ <div class="source"><pre>
+<appender name="..." type="MyNamespace.MyAppender, MyAssembly"></pre></div>
+ <p>
+ The .NET runtime will try to locate the assembly called <i>MyAssembly</i>.
+ How .NET locates assemblies is beyond the scope of this FAQ.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+ </div>
+
+ <a name="troubleshooting"></a><div class="section"><h2 id="troubleshooting">Troubleshooting</h2>
+
+ <a name="internalDebug"></a><div class="section"><h2 id="internalDebug">How do I enable log4net internal debugging?</h2>
+ <p>
+ There are 2 different ways to enable internal debugging in log4net.
+ These are listed below. The preferred method is to specify
+ the <span class="code">log4net.Internal.Debug</span> option in the application's
+ config file.
+ </p>
+ <ul>
+ <li>
+ <p>
+ Internal debugging can also be enabled by setting a value in the application's
+ configuration file (not the log4net configuration file, unless the log4net config
+ data is embedded in the application's config file). The <span class="code">log4net.Internal.Debug</span>
+ application setting must be set to the value <span class="code">true</span>.
+ For example:
+ </p>
+ <div class="syntax"><div class="code"><pre>
+<?xml version="1.0" encoding="utf-8" ?>
+<configuration>
+ <appSettings>
+ <add key="log4net.Internal.Debug" value="true"/>
+ </appSettings>
+</configuration></pre></div></div>
+ <p>
+ This setting is read immediately on startup an will cause all internal
+ debugging messages to be emitted.
+ </p>
+ </li>
+ <li>
+ <p>
+ To enable log4net's internal debug programmatically you need
+ to set the <span class="code">log4net.Util.LogLog.InternalDebugging</span>
+ property to <span class="code">true</span>. Obviously the sooner this
+ is set the more debug will be produced.
+ </p>
+ </li>
+ </ul>
+ <p>
+ Internal debugging messages are written to the console and to the
+ <span class="code">System.Diagnostics.Trace</span>
+ system. If the application does not have a console the messages logged
+ there will be lost. Note that an application can redirect the console
+ stream by setting the <span class="code">System.Console.Out</span>. The
+ Trace system will by default send the message to an attached debugger
+ (where the messages will appear in the output window). If the process
+ does not have a debugger attached then the messages are sent to the
+ system debugger. A utility like DebugView from
+ <a class="externalLink" href="http://www.sysinternals.com">http://www.sysinternals.com</a>
+ may be used to capture these messages.
+ </p>
+ <p>
+ As log4net internal debug messages are written to the <span class="code">System.Diagnostics.Trace</span>
+ system it is possible to redirect those messages to a local file. You can define
+ a trace listener by adding the following to your application's .config file:
+ </p>
+ <div class="syntax"><div class="code"><pre>
+<configuration>
+ ...
+
+ <system.diagnostics>
+ <trace autoflush="true">
+ <listeners>
+ <add
+ name="textWriterTraceListener"
+ type="System.Diagnostics.TextWriterTraceListener"
+ initializeData="C:\tmp\log4net.txt" />
+ </listeners>
+ </trace>
+ </system.diagnostics>
+
+ ...
+</configuration></pre></div></div>
+ <p>
+ Make sure that the process running your application has permission
+ to write to this file.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="trouble-EventLog"></a><div class="section"><h2 id="trouble-EventLog">Why doesn't the EventLogAppender work?</h2>
+ <p>
+ If you are not getting events delivered to the event log this usually indicates
+ a permissions problem. Basically if the event log does not exist the EventLogAppender
+ tries to create it, but you need local administrator permissions to create event logs
+ (just to write into the right bit of the registry). You don't need administrator
+ permissions to log to an existing event log, but it must exist. If you are using the
+ event log from a web application or service using the event log can be a little tricky.
+ </p>
+ <p>
+ A web application will run as the user account ASPNET. This account deliberately has
+ few permissions to reduce the chances of someone hacking into the web server. While the
+ account has permission to write to the event log it does not have permission to create
+ event sources (registry create and write access), which are needed to write to the event log.
+ </p>
+ <p>
+ There are a couple of solutions:
+ </p>
+ <ol style="list-style-type: decimal">
+ <li>
+ <p>
+ Make the ASPNET user a member of the Administrators group. This will work because the
+ user will then have the required permissions. This is <b>not recommended</b>
+ for production use.
+ </p>
+ </li>
+ <li>
+ <p>
+ As the event source only needs to be created once for the machine, create an installer
+ and configure it to create the event source.
+ The installer will need to be run as Administrator (don't they all). See
+ <span class="code">System.Diagnostics.EventLogInstaller</span> in the Microsoft .NET
+ Framework SDK for an example of how to create a simple event log installer.
+ </p>
+ </li>
+ </ol>
+ <p>
+ There is a Microsoft Knowledge Base article that covers this issue and how to resolve
+ it. <a class="externalLink" href="http://support.microsoft.com/default.aspx?scid=kb;en-us;329291">
+ PRB: "Requested Registry Access Is Not Allowed" Error Message When ASP.NET
+ Application Tries to Write New EventSource in the EventLog</a>.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="trouble-file-perm"></a><div class="section"><h2 id="trouble-file-perm">Why can't I log to a FileAppender from a web application?</h2>
+ <p>
+ The web application runs as a special user account on the web server
+ called ASPNET. This account has restricted permissions to protect the
+ web server from attacks. By default this account may not have permission
+ to write to the file system. Make sure that the ASPNET account has
+ permission to create and write to files in the directory chosen for
+ logging.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="trouble-service"></a><div class="section"><h2 id="trouble-service">Why doesn't the logging in my service work?</h2>
+ <p>
+ A windows service runs as a user account specified in the services
+ control panel. This account may have restricted permissions, make
+ sure that the account has permission to create and write to files
+ in the directory chosen for logging.
+ </p>
+ <p>
+ A windows service is launched by windows. The current directory in
+ a service is set to the windows system directory (e.g.
+ <span class="code">C:\Windows\System32</span>). If you are loading
+ the configuration file from the current directory then be aware
+ that this path will not be the location of your assemblies.
+ The best way to get the path to your assemblies is to use
+ <span class="code">AppDomain.BaseDirectory</span>.
+ Note that the log4net internals never use the current directory.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="trouble-db"></a><div class="section"><h2 id="trouble-db">I am having trouble using the AdoNetAppender to connect to my database?</h2>
+ <p>
+ For details on the different ways in which ADO.NET can connect to a database see:
+ <a class="externalLink" href="http://msdn.microsoft.com/library/en-us/cpguide/html/cpconadonetconnections.asp">Connecting to a Data Source Using ADO.NET</a>.
+ </p>
+ <p>
+ If you need to use ODBC to connect to your database then please note that the
+ ADO.NET ODBC drivers are not included in the standard .NET framework redistributable.
+ You can download the drivers from microsoft download at:
+ <a class="externalLink" href="http://www.microsoft.com/downloads/details.aspx?FamilyID=6ccd8427-1017-4f33-a062-d165078e32b1">ODBC .NET Data Provider</a>.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+
+ <a name="report-bugs"></a><div class="section"><h2 id="report-bugs">How do I report bugs?</h2>
+ <p>
+ First make sure it really is a bug and not a
+ usage error. When in doubt, ask on the <a href="../mail-lists.html">log4net-user mailing
+ list</a> first.
+ </p>
+ <p>
+ If you have identified a bug, please report it
+ via our <a href="../issue-tracking.html">Issue
+ Tracker</a>. You may want to check it hasn't
+ been reported before by searching the existing
+ issues.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+ <a name="log-early"></a><div class="section"><h2 id="log-early">log4net doesn't log when built in RELEASE mode</h2>
+ <p>
+ If you use attributes to configure log4net then
+ the order by which assemblies are loaded may
+ determine whether you attributes are used or
+ not. Assembly load order may be different in
+ DEBUG and RELEASE mode.
+ </p>
+
+ <p>
+ As stated in <a href="manual/configuration.html#attributes">the
+ manual</a> the attribute will only be read for
+ the first assembly that tries to use log4net.
+ So it is important that you obtain your
+ <tt>ILog</tt> instance as early as possible.
+ </p>
+
+ <p>
+ For a command line application "as early as
+ possible" probably is the class holding the
+ <tt>Main</tt> method, for a Web-Application
+ it would be your <tt>Global.asax</tt> class
+ and for a Windows Service it would be the class
+ deriving from <tt>ServiceBase</tt>.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+ </div>
+
+ <a name="misc"></a><div class="section"><h2 id="misc">Miscellaneous</h2>
+
+ <a name="vsnet-add-reference"></a><div class="section"><h2 id="vsnet-add-reference">How do I make log4net appear in the Visual Studio Add References dialog?</h2>
+ <p>
+ There is a good discussion of this topic on Robert McLaws blog:
+ <a class="externalLink" href="http://weblogs.asp.net/rmclaws/archive/2003/11/15/37743.aspx">Building a Better Server Control Experience, Part 2</a>.
+ </p>
+ </div>
+ <p><a href="#top">Back to Top</a></p>
+
+ </div>
+
+ </div>
+
+
+
+ </div>
+ </div>
+ <div class="clear">
+ <hr/>
+ </div>
+ <div id="footer">
+ <div class="xright">
+ <div class="xright">Copyright © 2004-2012
+ <a href="http://www.apache.org">Apache Software Foundation</a>.
+
+
+ Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache Software License, Version 2.0</a>.</div><br />
+ <div class="xright">Apache log4net, Apache, log4net, the Apache feather logo, the Apache Logging Services project logo and the Built by Maven logo are trademarks of The Apache Software Foundation.</div>
+ <div class="clear">
+ </div>
+ </div>
+ </body>
+</html>
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