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<p>
The PerlDancer Advent Calendar is a community-driven project that aims 
to showcase the Dancer Perl web framework.
</p>

<p>
Each day of December until Christmas, one article about Dancer. Stay tuned for new moves!
</p>

<ul id="sidebar-items">
<li>
    <h3>About Dancer</h3>
    <ul class="links">
        <li><a href="http://www.perldancer.org/">Dancer homepage</a></li>
        <li><a href="http://twitter.com/PerlDancer">Official Twitter</a></li>
        <li><a href="http://github.com/PerlDancer/Dancer">Dancer on GitHub</a></li>
        <li><a href="http://github.com/PerlDancer/Dancer2">Dancer 2 on GitHub</a></li>
        <li><a class="feed" href="/feed/2018">RSS</a></li>
    </ul>
</li>
</ul>
</div>


<div id="content">
<div class="pod-document"><h1><a name="logging_with_dancer2__logger__log4perl"></a>Logging with Dancer2::Logger::Log4perl</h1>

<h2><a name="history"></a>History</h2>

<p>It's late summer 2018, and there has been a hole in the logging ecosystem of Dancer2 for some time 
now - we've been missing a Log4perl plugin. There are certainly a lot of great logging options 
available (<code>Dancer2::Logger::LogAny</code>, <code>Dancer2::Logger::Syslog</code>, and <code>Dancer2::Logger::LogReport</code> 
to name a few), and a couple of them even include appenders for Log4perl. In looking at our own
needs, however, they all seemed to be overkill.</p>
<p>At <code>$work</code>, everything is based on Log4perl (and, in our client-facing Java apps, Log4j), so 
there has never been need for another logger. We'd been happily using <code>Dancer::Logger::Log4perl</code>
for years, and wanted to continue to use something familar to us. Before investing some time in
writing our own Log4perl plugin for Dancer2, we expanded our search beyond metacpan, and were
rewarded in doing so.</p>
<p>We stumbled across a project from Ryan Larscheidt and Jon Miner at the University of Wisconsin.
They wrote a Log4perl plugin for Dancer2 with the intent of releasing it, but as their priorities
and projects shifted, it never made its way to CPAN. I used to go to MadMongers (Madison Perl Mongers),
so before long, I was able to track them down and they gave me their blessing to release it.</p>
<p>A little packaging and a few tests later, <code>Dancer2::Logger::Log4perl</code> was on its way to CPAN!</p>
<h2><a name="configuration"></a>Configuration</h2>

<p>First, we need to create a basic Log4perl configuration. Create a new Dancer2 application (I'll call
mine <code>TestLog4perl</code>), and in the application directory create a <i>log4perl.conf</i> file with the following:</p>
<pre class="prettyprint">log4perl.rootLogger              = DEBUG, LOG1

log4perl.appender.LOG1           = Log::Log4perl::Appender::File
log4perl.appender.LOG1.filename  = logs/mylog.log
log4perl.appender.LOG1.mode      = append
log4perl.appender.LOG1.layout    = Log::Log4perl::Layout::PatternLayout
log4perl.appender.LOG1.layout.ConversionPattern = %d %p %m %n</pre>

<p>The first one defines our root application logger. The first parameter after the equals sign says
what is the minimum level we should log. Since we are saying the minimum log level should be <code>DEBUG</code>,
any messages from Dancer2 itself, and anything logged at the <code>core</code> level will be ignored.</p>
<p>After the minimum log level is a comma-separated list of appenders to write to. For now, we will create a
single appender named <code>LOG1</code> (we will see how to add a second appender below).  This will write
to a file in the <i>logs/</i> directory named <i>mylog.log</i>, using the <a href="https://metacpan.org/pod/Log::Log4perl::Appender::File">Log::Log4perl::Appender::File</a>
appender. When the app is started, it will append to an existing log file of that name (or create the file
if necessary), and will write to it with a format specified by <a href="https://metacpan.org/pod/Log::Log4perl::Layout::PatternLayout">Log::Log4perl::Layout::PatternLayout</a>.</p>
<p>Each appender can have its own configuration directives. Please consult the pod for each appender for a list
of its configuration parameters.</p>
<p>Next, we have to tell our application that we are using <code>Dancer2::Logger::Log4perl</code> as our preferred logger.
Edit your <i>environments/development.yml</i> file, and comment out the <code>logger: "console"</code> line. Replace it
with the following:</p>
<pre class="prettyprint">logger: log4perl
log: core
engines:
   logger:
      log4perl:
         config_file: log4perl.conf</pre>

<p>This tells Dancer2 to use <code>Dancer2::Logger::Log4perl</code> as its logging engine, and to send all levels of message to
it. Finally, Log4perl should look for its configuration file in the root application directory in a file called
<i>log4perl.conf</i>.</p>
<h2><a name="usage"></a>Usage</h2>

<p>Using Log4perl is simple: use the logging keywords you are already familiar with in Dancer:</p>
<pre class="prettyprint">get '/' =&gt; sub {
    debug "I'M IN UR INDEX";
    template 'index' =&gt; { 'title' =&gt; 'TestLog4perl' };
};</pre>

<p>Start your application and visit <code>http://localhost:5000/</code>. You will see the following in your <i>logs/mylog.log</i> file:</p>
<pre class="prettyprint">2018/12/18 21:36:02 DEBUG I'M IN UR INDEX</pre>

<h2><a name="hey__i_can_t_see_my_log_messages_on_the_screen_"></a>Hey, I can't see my log messages on the screen!</h2>

<p>That's because we didn't add a screen appender! With Log4perl, adding another appender is easy. Let's
add another section to our <i>log4perl.conf</i> file:</p>
<pre class="prettyprint">log4perl.appender.SCREEN         = Log::Log4perl::Appender::Screen
log4perl.appender.SCREEN.stderr  = 0
log4perl.appender.SCREEN.layout  = Log::Log4perl::Layout::PatternLayout
log4perl.appender.SCREEN.layout.ConversionPattern = %m %n</pre>

<p>This creates another appender named <code>SCREEN</code>. We then need to tell our root logger to use this appender
as well:</p>
<pre class="prettyprint">log4perl.rootLogger = DEBUG, LOG1, SCREEN</pre>

<p>Now, restart your application, and visit a route that has logging installed, and you will see your log
message not only goes to the <i>logs/mylog.log</i> file, but also displays on the console running your
application. Easy!</p>
<h2><a name="some_gotchas"></a>Some Gotchas</h2>

<p>There are a couple of important nuances you should be aware of:</p>
<ul>
<li><a name="item_Environment_configuration_replaces_logging_configuration"></a><b>Environment configuration replaces logging configuration</b>
<p>If you put your logging configuration in <i>config.yml</i> rather than one of your environment-specific
configuration files (such as <i>environments/development.yml</i>), you stand a good chance of not using
the logging configuration you think you are using. The default configuration file for the development
environment, for example, logs only to the console. If you put your Log4perl configuration in <i>config.yml</i>
and don't change your development configuration file, your Log4perl configuration will be passed over
for the default console logger.</p>
<p>From my own experience, <b>always</b> configure your logger in your environment-specific configuration, unless
you use the same configuration across all environments (I don't endorse this practice).</p>
</li>
<li><a name="item_Core_level_messages_are_passed_as_log_level_trace__but_will_not_be_passed_unless_Dancer2_s_log_level_is_core_"></a><b>Core level messages are passed as log level trace, but will not be passed unless Dancer2's log level is core.</b>
<p>Since <code>core</code> doesn't have a good corresponding level in Log4perl, <code>core</code> level messages are sent 
over to Log4perl at the <code>trace</code> log level. This <b>only</b> happens when you set Dancer2's log level in your
<i>config.yml</i> file to <code>core</code> however. So your preferred log level setting is respected, even if <code>core</code> level 
messages have to be reported at a different level.</p>
</li>
<li><a name="item__code_log__code__should_be_set_a_lower_priority_than_the_lowest_priority_as_set_in_your_Log4perl_configuration_"></a><b><code>log</code> should be set a lower priority than the lowest priority as set in your Log4perl configuration.<...
<p>If it isn't, the log messages will not be passed to Log4perl.</p>
</li>
</ul>
<h2><a name="conclusion"></a>Conclusion</h2>

<p>If Log4perl is all the logging you need in your Dancer2 applications, then <code>Dancer2::Logger::Log4perl</code> is well worth
a look. It gives you much of the functionality available to Log4perl while using the logging syntax built into Dancer2.
This article should give you sufficient grounding to get going with Log4perl in your Dancer2 applications.</p>
<h2><a name="further_reading"></a>Further Reading</h2>

<ul>
<li><a name="item__a_href__https___metacpan_org_pod_Dancer2__Logger__Log4perl__Dancer2__Logger__Log4perl__a_"></a><b><a href="https://metacpan.org/pod/Dancer2::Logger::Log4perl">Dancer2::Logger::Log4perl</a></b>
</li>
<li><a name="item__a_href__https___metacpan_org_pod_Log__Log4perl__Log4perl__a_"></a><b><a href="https://metacpan.org/pod/Log::Log4perl">Log4perl</a></b>
</li>
<li><a name="item__a_href__https___www_perl_com_pub_2002_09_11_log4perl_html___Log4Perl_Tutorial__a_"></a><b><a href="https://www.perl.com/pub/2002/09/11/log4perl.html/">Log4Perl Tutorial</a></b>
</li>
</ul>
<h2><a name="author"></a>Author</h2>

<p>This article has been written by Jason Crome (CromeDome) for the Perl Dancer 
Advent Calendar 2018.</p>
<h2><a name="copyright"></a>Copyright</h2>

<p>No copyright retained. Enjoy.</p>
<p>Jason A. Crome</p>
</div>